10-5
1 2 3 4
5
Figure 10-3. Rapid deceleration or quick stop.
between 25 and 40 feet, depending upon the manufacturer’s
recommendations, accelerate to the desired entry speed,
which is approximately 45 knots for most training helicopters
(position 2). The altitude chosen should be high enough to
avoid danger to the tail rotor during the flare, but low enough
to stay out of the hazardous areas of that helicopter’s height-
velocity diagram throughout the maneuver. In addition, this
altitude should be low enough that the helicopter can be
brought to a hover during the recovery.
At position 3, initiate the deceleration by applying aft cyclic
to reduce forward groundspeed. Simultaneously, lower the
collective, as necessary, to counteract any climbing tendency.
The timing must be exact. If too little collective is taken out
for the amount of aft cyclic applied, the helicopter climbs. If
too much downward collective is applied, the helicopter will
descend. A rapid application of aft cyclic requires an equally
rapid application of down collective. As collective is lowered,
apply proper antitorque pedal pressure to maintain heading,
and adjust the throttle to maintain rpm. The G loading on the
rotor system depends on the pitch-up attitude. If the attitude is
too high, the rotor system may stall and cause the helicopter
to impact the surface.
After attaining the desired speed (position 4), initiate the
recovery by lowering the nose and allowing the helicopter
to descend to a normal hovering height in level flight and
zero groundspeed (position 5). During the recovery, increase
collective pitch, as necessary, to stop the helicopter at normal
hovering height, adjust the throttle to maintain rpm, and apply
proper antitorque pedal pressure, as necessary, to maintain
heading. During the maneuver, visualize rotating about the
tail rotor’s horizontal axis until a normal hovering height is
reached.
Common Errors
1. Initiating the maneuver by lowering the collective
without aft cyclic pressure to maintain altitude.
2. Initially applying aft cyclic stick too rapidly, causing
the helicopter to balloon (climb).
3. Failing to effectively control the rate of deceleration
to accomplish the desired results.
4. Allowing the helicopter to stop forward motion in a
tail-low attitude.
5. Failing to maintain proper rotor rpm.
6. Waiting too long to apply collective pitch (power)
during the recovery, resulting in an overtorque
situation when collective pitch is applied rapidly.
7. Failing to maintain a safe clearance over the terrain.
8. Using antitorque pedals improperly, resulting in erratic
heading changes.
9. Using an excessively nose-high attitude.
Steep Approach
A steep approach is used primarily when there are obstacles
in the approach path that are too high to allow a normal
approach. A steep approach permits entry into most confined
areas and is sometimes used to avoid areas of turbulence
around a pinnacle. An approach angle of approximately 13°
to 15° is considered a steep approach. [Figure 10-4] Caution
must be exercised to avoid the parameters for vortex ring
state (20–100 percent of available power applied, airspeed
of less than 10 knots, and a rate of descent greater than 300
feet per minute (fpm)). For additional information on vortex
ring state (formerly referenced as settling-with-power), refer
to Chapter 11, Helicopter Emergencies and Hazards.
10-6
1
2
3
4
15° Approach angle
Figure 10-4. Steep approach to a hover.
Technique
On final approach, maintain track with the intended
touchdown point and into the wind as much as possible at the
recommended approach airspeed [Figure 10-4, position 1].
When intercepting an approach angle of 13° to 15°, begin
the approach by lowering the collective sufficiently to start
the helicopter descending down the approach path and
decelerating (position 2). Use the proper antitorque pedal for
trim. Since this angle is steeper than a normal approach angle,
reduce the collective more than that required for a normal
approach. Continue to decelerate with slight aft cyclic and
smoothly lower the collective to maintain the approach angle.
The intended touchdown point may not always be visible
throughout the approach, especially when landing to a hover.
Pilots must learn to cue in to other references that are parallel
to the intended landing area that will help them maintain
ground track and position.
Constant management of approach angle and airspeed is
essential to any approach. Aft cyclic is required to decelerate
sooner than with a normal approach, and the rate of closure
becomes apparent at a higher altitude. Maintain the approach
angle and rate of descent with the collective, rate of closure
with the cyclic, and trim with antitorque pedals.
The helicopter should be kept in trim just prior to loss of
effective translational lift (approximately 25 knots). Below
100 feet above ground level (AGL), the antitorque pedals
should be adjusted to align the helicopter with the intended
touchdown point. Visualize the location of the tail rotor
behind the helicopter and fly the landing gear to 3 feet above
the intended landing point. In small confined areas, the pilot
must precisely position the helicopter over the intended
landing area. Therefore, the approach must stop at that point.
Loss of effective translational lift occurs higher in a steep
approach (position 3), requiring an increase in the collective
to prevent settling, and more forward cyclic to achieve
the proper rate of closure. Once the intended landing area
is reached, terminate the approach to a hover with zero
groundspeed (position 4). If the approach has been executed
properly, the helicopter will come to a halt at a hover altitude
of 3 feet over the intended landing point with very little
additional power required to hold the hover.
The pilot must remain aware that any wind effect is lost once
the aircraft has descended below the barriers surrounding a
confined area, causing the aircraft to settle more quickly.
Additional power may be needed on a strong wind condition
as the helicopter descends below the barriers.
Common Errors
1. Failing to maintain proper rpm during the entire
approach.
2. Using collective improperly in maintaining the
selected angle of descent.
3. Failing to make antitorque pedal corrections to
compensate for collective pitch changes during the
approach.
4. Slowing airspeed excessively in order to remain on
the proper angle of descent.
5. Failing to determine when effective transla tional lift
is being lost.
6. Failing to arrive at hovering height and attitude, and
zero groundspeed almost simultaneously.
7. Utilizing low rpm in transition to the hover at the end
of the approach.
8. Using too much aft cyclic close to the surface, which
may result in the tail rotor striking the sur face.
9. Failure to align landing gear with direction of travel
no later than beginning of loss of translational lift.
Shallow Approach and Running/Roll-On
Landing
Use a shallow approach and running landing when a
high-density altitude, a high gross weight condition, or
some combination thereof, is such that a normal or steep
approach cannot be made because of insufficient power
to hover. [Figure 10-5] To compensate for this lack of
power, a shallow approach and running landing makes
use of translational lift until surface contact is made. If
flying a wheeled helicopter, a roll-on landing can be used
10-7
1 2 3 4
5° Approach angle
Figure 10-5. Shallow approach and running landing.
to minimize the effect of downwash. The glide angle for a
shallow approach is approximately 3° to 5°. This angle is
similar to the angle used on an instrument landing system
(ILS) approach. Since the helicopter is sliding or rolling
to a stop during this maneuver, the landing area should
be smooth, and the landing gear must be aligned with the
direction of travel to prevent dynamic rollover and must be
long enough to accomplish this task. After landing, ensure
that the pitch of the rotor blades is not too far aft as the main
rotor blades could contact the tailboom.
Technique
A shallow approach is initiated in the same manner as the
normal approach except that a shallower angle of descent is
maintained. The power reduction to initiate the desired angle
of descent is less than that for a normal approach since the
angle of descent is less (position 1).
As the collective is lowered, maintain heading with proper
antitorque pedal pressure and rpm with the throttle. Maintain
approach airspeed until the apparent rate of closure appears
to be increasing. Then, begin to slow the helicopter with aft
cyclic (position 2).
As in normal and steep approaches, the primary control
for the angle and rate of descent is the collective, while the
cyclic primarily controls the groundspeed. However, there
must be a coordination of all the con trols for the maneuver
to be accomplished successfully. The helicopter should
arrive at the point of touchdown at or slightly above effective
translational lift. Since translational lift diminishes rapidly
at slow airspeeds, the deceleration must be coordinated
smoothly, at the same time keeping enough lift to prevent
the helicopter from settling abruptly.
Just prior to touchdown, place the helicopter in a level
attitude with the cyclic, and maintain heading with the
antitorque pedals. Use the cyclic to keep the direction of
travel and ground track identical (position 3). Allow the
helicopter to descend gently to the surface in a straight- and-
level attitude, cushioning the landing with the collective.
After surface contact, move the cyclic slightly forward to
ensure clearance between the tail boom and the rotor disk.
Use the cyclic to maintain the surface track (position 4).
A pilot normally holds the collective stationary until the
helicopter stops; however, to get more braking action, lower
the collective slightly.
Keep in mind that, due to the increased ground friction when
the collective is lowered or if the landing is being executed
to a rough or irregular surface, the helicopter may come to
an abrupt stop and the nose might pitch forward. Exercise
caution not to correct this pitching movement with aft cyclic,
which could result in the rotor making contact with the tail
boom. An abrupt stop may also cause excessive transmission
movement resulting in the transmission contacting its mount.
During the landing, maintain normal rpm with the throttle
and directional control with the antitorque pedals.
For wheeled helicopters, use the same technique except
after landing, lower the collective, neutralize the controls,
and apply the brakes, as necessary, to slow the helicopter.
Do not use aft cyclic when bringing the helicopter to a stop.
Common Errors
1. Assuming excessive nose-high attitude to slow the
helicopter near the surface.
2. Utilizing insufficient collective and throttle to cushion
a landing.
3. Failure to maintain heading resulting in a turning or
pivoting motion.
4. Failure to add proper antitorque pedal as collec tive is
added to cushion landing, resulting in a touchdown
while the helicopter is moving sideward.
5. Failure to maintain a speed that takes advantage of
effective translational lift.
6. Touching down at an excessive groundspeed for the
existing conditions. (Some helicopters have maximum
touchdown groundspeeds.)
7. Failure to touch down in the appropriate attitude
necessary for a safe landing. Appropriate attitude is
based on the type of helicopter and the landing gear
installed.
8. Failure to maintain proper rpm during and after
touchdown.
9. Maintaining poor alignment with direction of travel
during touchdown.
Slope Operations
Prior to conducting any slope operations, be thoroughly
familiar with the characteristics of dynamic rollover and
mast bumping, which are discussed in Chapter 11, Helicopter
10-8
1 2 3 4
Figure 10-6. Slope landing.
Emergencies and Hazards. The approach to a slope is similar
to the approach to any other landing area. During slope
operations, make allowances for wind, barriers, and forced
landing sites in case of engine failure. Since the slope may
constitute an obstruction to wind passage, anticipate turbulence
and downdrafts.
Slope Landing
A pilot usually lands a helicopter across the slope rather than
with the slope. Landing with the helicopter facing down
the slope or downhill is not recommended because of the
possibility of striking the tail rotor on the surface.
Technique
Refer to Figure 10-6. At the termination of the approach, if
necessary, move the helicopter slowly toward the slope, being
careful not to turn the tail upslope. Position the helicopter
across the slope at a stabilized hover headed into the wind
over the intended landing spot (frame 1). Downward pressure
on the collective starts the helicopter descending. As the
upslope skid touches the ground, hesitate momentarily in a
level attitude, then apply slight lateral cyclic in the direction
of the slope (frame 2). This holds the skid against the slope
while the pilot continues lowering the downslope skid with
the col lective. As the collective is lowered, continue to move
the cyclic toward the slope to maintain a fixed position (frame
3) The slope must be shallow enough to hold the helicopter
against it with the cyclic during the entire landing. A slope of
5° is recommended maximum for training in most helicopters.
However, additional training to the manufacturer’s
limitations may be required. Consult the Rotorcraft Flight
Manual (RFM) or Pilot’s Operating Handbook (POH) for
the specific limitations of the helicopter being flown.
Be aware of any abnormal vibration or mast bumping that
signals maximum cyclic deflection. If helicopter mast
moment or slope limits are reached before the helicopter
is firmly on the ground, return the helicopter to a hover.
Select a new area with a lesser degree of slope. In most
helicopters with a counterclockwise rotor system, landings
can be made on steeper slopes when holding the cyclic to the
right. When landing on slopes using left cyclic, some cyclic
input must be used to overcome the translating tendency.
If wind is not a factor, consider the drifting tendency when
determining landing direction.
After the downslope skid is on the surface, reduce the
collective to full down, and neutralize the cyclic and pedals
(frame 4). Normal operating rpm should be maintained
until the full weight of the helicopter is on the landing gear.
This ensures adequate rpm for immediate takeoff in case the
helicopter starts sliding down the slope. Use antitorque pedals
as necessary throughout the landing for heading control.
Before reducing the rpm, move the cyclic control as neces sary
to check that the helicopter is firmly on the ground.
Common Errors
1. Failing to consider wind effects during the approach
and landing.
2. Failing to maintain proper rpm throughout the entire
maneuver.
3. Failure to maintain heading resulting in a turning or
pivoting motion.
4. Turning the tail of the helicopter into the
slope.
5. Lowering the downslope skid or wheel too rapidly.
6. Applying excessive cyclic control into the slope,
causing mast bumping.
Slope Takeoff
A slope takeoff is basically the reverse of a slope land ing.
[Figure 10-7] Conditions that may be associated with the
slope, such as turbulence and obstacles, must be considered
during the takeoff. Planning should include suitable forced
landing areas.
10-9
1 2 3
Figure 10-7. Slope takeoff.
Technique
Begin the takeoff by increasing rpm to the normal range with
the collective full down. Then, move the cyclic toward the
slope (frame 1). Holding the cyclic toward the direction of
the slope causes the downslope skid to rise as the pilot slowly
raises the collective (frame 2). As the skid comes up, move
the cyclic as necessary to maintain a level attitude in relation
to the horizon. If properly coordinated, the helicopter should
attain a level attitude as the cyclic reaches the neutral position.
At the same time, use antitorque pedal pressure to maintain
heading and throttle to maintain rpm. With the helicopter
level and the cyclic centered, pause momentarily to verify
everything is correct, and then gradually raise the collective
to complete the liftoff (frame 3). After reaching a hover,
avoid hitting the ground with the tail rotor by not turning the
helicopter tail upslope and gaining enough altitude to ensure
the tail rotor is clear. If an upslope wind exists, execute a
crosswind takeoff and then make a turn into the wind after
clearing the ground with the tail rotor.
Common Errors
1. Failing to adjust cyclic control to keep the heli copter
from sliding down slope.
2. Failing to maintain proper rpm.
3. Holding excessive cyclic into the slope as the down
slope skid is raised.
4. Failure to maintain heading, resulting in a turning or
pivoting motion.
5. Turning the tail of the helicopter into the slope during
takeoff.
Confined Area Operations
A confined area is an area where the flight of the heli copter
is limited in some direction by terrain or the presence of
obstructions, natural or manmade. For example, a clearing
in the woods, a city street, a road, a building roof, etc., can
each be regarded as a confined area. The helicopter pilot
has added responsibilities when conducting operations
from a confined area that airplanes pilots do not. He or she
assumes the additional roles of the surveyor, engineer, and
manager when selecting an area to conduct operations. While
airplane pilots generally operate from known pre-surveyed
and improved landing areas, helicopter pilots fly into areas
never used before for helicopter operations. Generally,
takeoffs and landings should be made into the wind to obtain
maximum airspeed with minimum groundspeed. The pilot
should begin with as nearly accurate an altimeter setting as
possible to determine the altitude.
There are several things to consider when operating in
confined areas. One of the most important is maintaining
a clearance between the rotors and obstacles forming the
confined area. The tail rotor deserves special considera tion
because, in some helicopters, it is not always visible from
the cabin. This not only applies while making the approach,
but also while hovering. Another consider ation is that wires
are especially difficult to see; however, their supporting
devices, such as poles or towers, serve as an indication of
their presence and approximate height. If any wind is present,
expect some turbulence. [Figure 10-8]
Something else to consider is the availability of forced
landing areas during the planned approach. Think about
the possibility of flying from one alternate landing area to
another throughout the approach, while avoiding unfavorable
areas. Always leave a way out in case the landing cannot be
completed, or a go-around is necessary.
During the high reconnaissance, the pilot needs to formulate
a takeoff plan as well. The heights of obstacles need to be
determined. It is not good practice to land in an area and
then determine that insufficient power exists to depart.
Generally, more power is required to take off than to land
so the takeoff criteria is most crucial. Fixing the departure
azimuth or heading on the compass is a good technique to
use. This ensures that the pilot is able to take off over the
preselected departure path when it is not visible while sitting
in the confined area.
10-10
WIND
Figure 10-8. If the wind velocity is 10 knots or greater, expect updrafts on the windward side and downdrafts on the lee side of obstacles.
Plan the approach with these factors in mind, but be ready to alter plans if the wind speed or direction changes.
Approach
A high reconnaissance should be completed before ini tiating
the confined area approach. Start the approach phase using
the wind and speed to the best possible advantage. Keep in
mind areas suitable for forced land ing. It may be necessary to
choose a crosswind approach that is over an open area, then
one directly into the wind that is over trees. If these conditions
exist, consider the possibility of making the initial phase of
the approach crosswind over the open area and then turn ing
into the wind for the final portion of the approach.
Always operate the helicopter as close to its normal capabilities
as possible, taking into consideration the situation at hand. In
all confined area operations, with the exception of a pinnacle
operation (see next section, Takeoff), the angle of descent
should be no steeper than necessary to clear any barrier with
the tail rotor in the approach path and still land on the selected
spot. The angle of climb on takeoff should be normal, or not
steeper than necessary to clear any bar rier. Clearing a barrier by
a few feet and maintaining normal operating rpm, with perhaps
a reserve of power, is better than clearing a barrier by a wide
mar gin but with a dangerously low rpm and no power reserve.
Always make the landing to a specific point and not to some
general area. This point should be located well forward,
away from the approach end of the area. The more confined
the area is, the more essential it is that the helicopter land
precisely at a definite point. Keep this point in sight during
the entire final approach.
When flying a helicopter near obstacles, always consider
the tail rotor. A safe angle of descent over bar riers must be
established to ensure tail rotor clearance of all obstructions.
After coming to a hover, avoid turning the tail into obstructions.
Takeoff
A confined area takeoff is considered an altitude over
airspeed maneuver where altitude gain is more important to
airspeed gain. Before takeoff, make a reconnaissance from
the ground or cockpit to determine the type of takeoff to
be performed, to determine the point from which the take-
off should be initiated to ensure the maximum amount of
available area, and finally, how to maneuver the helicopter
best from the landing point to the proposed take off position.
If wind conditions and available area permit, the heli-
copter should be brought to a hover, turned around, and
hovered forward from the landing position to the take off
position. Under certain conditions, sideward flight to the
takeoff position may be preferred, but rearward flight may
be necessary, stopping often while moving to check on the
location of obstacles relative to the tail rotor.
When planning the takeoff, consider the direction of the wind,
obstructions, and forced landing areas. To help fly up and
over an obstacle, form an imaginary line from a point on the
leading edge of the helicopter to the highest obstacle to be
cleared. Fly this line of ascent with enough power to clear
the obstacle by a safe distance. After clearing the obstacle,
maintain the power setting and accelerate to the normal climb
speed. Then, reduce power to the normal climb power setting.
Common Errors
1. Failure to perform, or improper performance of, a high
or low reconnaissance.
2. Approach angle that is too steep or too shal low for the
existing conditions.
3. Failing to maintain proper rpm.
10-11
Figure 10-9. When flying an approach to a pinnacle or ridgeline,
avoid the areas where downdrafts are present, especially when
excess power is limited. If downdrafts are encountered, it may
become necessary to make an immediate turn away from the
pinnacle to avoid being forced into the rising terrain.
4. Failure to consider emergency landing areas.
5. Failure to select a specific landing spot.
6. Failure to consider how wind and turbulence could
affect the approach.
7. Improper takeoff and climb technique for exist ing
conditions.
8. Failure to maintain safe clearance distance from
obstructions.
Pinnacle and Ridgeline Operations
A pinnacle is an area from which the surface drops away
steeply on all sides. A ridgeline is a long area from which
the surface drops away steeply on one or two sides, such
as a bluff or precipice. The absence of obstacles does not
necessarily decrease the difficulty of pinnacle or ridgeline
operations. Updrafts, downdrafts, and turbulence, together
with unsuitable terrain in which to make a forced landing,
may still present extreme hazards.
Approach and Landing
If there is a need to climb to a pinnacle or ridgeline, do it on
the upwind side, when practicable, to take advantage of any
updrafts. The approach flightpath should be paral lel to the
ridgeline and into the wind as much as possi ble. [Figure 10-9]
Load, altitude, wind conditions, and terrain features
determine the angle to use in the final part of an approach.
As a general rule, the greater the winds are, the steeper the
approach needs to be to avoid turbulent air and downdrafts.
Groundspeed during a pinnacle approach is more difficult to
judge because visual references are farther away than during
approaches over trees or flat terrain. Pilots must continually
perceive the apparent rate of closure by observing the apparent
change in size of the landing zone features. Avoid the
misperception of an increasing rate of closure to the landing
site. The apparent rate of closure should be that of a brisk
walk. If a crosswind exists, remain clear of down-drafts on
the leeward or downwind side of the ridgeline. If the wind
velocity makes the crosswind landing hazardous, it may be
possible to make a low, coordinated turn into the wind just
prior to terminating the approach. When making an approach
to a pinnacle, avoid leeward turbulence and keep the helicopter
within reach of a forced landing area as long as possible.
On landing, take advantage of the long axis of the area when
wind conditions permit. Touchdown should be made in the
forward portion of the area. When approaching to land on
pinnacles, especially manmade areas such as rooftop pads,
the pilot should determine the personnel access pathway to
the helipad and ensure that the tail rotor is not allowed to
intrude into that walkway or zone. Parking or landing with the
tail rotor off the platform ensures personnel safety. Always
per form a stability check prior to reducing rpm to ensure
the landing gear is on firm terrain that can safely support
the weight of the helicopter. Accomplish this by slowly
moving the cyclic and pedals while lowering the collective.
If movement is detected, reposition the aircraft.
Takeoff
A pinnacle takeoff is considered an airspeed over altitude
maneuver which can be made from the ground or from a
hover. Since pinnacles and ridgelines are generally higher
than the immediate surrounding terrain, gaining airspeed
on the takeoff is more important than gaining altitude. As
airspeed increases, the departure from the pinnacle becomes
more rapid, and helicopter time in the avoid area of the
height/velocity area decreases. [Figure 11-3] In addition
to covering unfavor able terrain rapidly, a higher airspeed
affords a more favorable glide angle and thus contributes to
the chances of reaching a safe area in the event of a forced
landing. If a suitable forced landing area is not avail able, a
higher airspeed also permits a more effective flare prior to
making an autorotative landing.
On takeoff, as the helicopter moves out of ground effect,
maintain altitude and accelerate to normal climb airspeed.
When normal climb speed is attained, estab lish a normal
climb attitude. Never dive the helicopter down the slope after
clearing the pinnacle.
10-12
Common Errors
1. Failing to perform, or improper performance of, a high
or low reconnaissance.
2. Flying the approach angle too steep or too shal low for
the existing conditions.
3. Failing to maintain proper rpm.
4. Failing to consider emergency landing areas.
5. Failing to consider how wind and turbulence could
affect the approach and takeoff.
6. Failure to maintain pinnacle elevation after takeoff.
7. Failure to maintain proper approach rate of closure.
8. Failure to achieve climb airspeed in timely manner.
Chapter Summary
This chapter described advanced flight maneuvers such
as slope landings, confined area landings, and running
takeoffs. The correlation between helicopter performance
requirements, the environmental factors associated with
different flight techniques, and safety considerations were
also explained to familiarize the pilot with the measures that
can be taken when performing these maneuvers to mitigate
risks. Hazards associated with helicopter flight and certain
aerodynamic considerations were also discussed.
11-1
Introduction
Today, helicopters are quite reliable. However, emergencies
do occur, whether a result of mechanical failure or pilot
error, and should be anticipated. Regardless of the cause, the
recovery needs to be quick and precise. By having a thorough
knowledge of the helicopter and its systems, a pilot is able
to handle the situation more readily. Helicopter emergencies
and the proper recovery procedures should be discussed and,
when possible, practiced in flight. In addition, by knowing
the conditions that can lead to an emergency, many potential
accidents can be avoided.
Helicopter Emergencies and
Hazards
Chapter 11
11-2
Normal Powered Flight Autorotation
Direction of flight
Direction of flight
Figure 11-1. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In
effect, the blades are “gliding” in their rotational plane.
Several factors affect the rate of descent in autorotation:
bank angle, density altitude, gross weight, rotor rpm, trim
condition, and airspeed. The primary ways to control the rate
of descent are with airspeed and rotor rpm. Higher or lower
airspeed is obtained with the cyclic pitch control just as in
normal powered flight. In theory, a pilot has a choice in the
angle of descent, varying, from straight vertical to maximum
horizontal range (which is the minimum angle of descent).
Rate of descent is high at zero airspeed and decreases to a
minimum at approximately 50–60 knots, depending upon the
particular helicopter and the factors just mentioned. As the
airspeed increases beyond that which gives minimum rate
of descent, the rate of descent increases again.
When landing from an autorotation, the only energy available
to arrest the descent rate and ensure a soft landing is the
kinetic energy stored in the rotor blades. Tip weights can
greatly increase this stored energy. A greater amount of
rotor energy is required to stop a helicopter with a high
rate of descent than is required to stop a helicopter that is
descending more slowly. Therefore, autorotative descents
at very low or very high airspeeds are more critical than
those performed at the minimum rate of descent airspeed.
Refer to the height/velocity diagram discussion in Chapter
7, Helicopter Performance.
Each type of helicopter has a specific airspeed and rotor rpm
at which a power-off glide is most efficient. The specific
airspeed is somewhat different for each type of helicopter,
but certain factors affect all configurations in the same
manner. In general, rotor rpm maintained in the low green
area (see Figure 5-3) gives more distance in an autorotation.
Heavier helicopter weights may require more collective to
control rotor rpm. Some helicopters need slight adjustments
to minimum rotor rpm settings for winter versus summer
Autorotation
In a helicopter, an autorotative descent is a power-off
maneuver in which the engine is disengaged from the
main rotor disk and the rotor blades are driven solely by
the upward flow of air through the rotor. [Figure 11-1] In
other words, the engine is no longer supplying power to
the main rotor.
The most common reason for an autorotation is failure of the
engine or drive line, but autorotation may also be performed
in the event of a complete tail rotor failure, since there is
virtually no torque produced in an autorotation. In both
cases, maintenance has often been a contributing factor to the
failure. Engine failures are also caused by fuel contamination
or exhaustion as well resulting in a forced autorotation.
If the engine fails, the freewheeling unit automatically
disengages the engine from the main rotor, allowing it to
rotate freely. Essentially, the freewheeling unit disengages
anytime the engine revolutions per minute (rpm) is less than
the rotor rpm.
At the instant of engine failure, the main rotor blades are
producing lift and thrust from their angle of attack (AOA)
and velocity. By lowering the collective (which must be done
immediately in case of an engine failure), lift and drag are
reduced, and the helicopter begins an immediate descent,
thus producing an upward flow of air through the rotor disk.
This upward flow of air through the rotor disk provides
sufficient thrust to maintain rotor rpm throughout the descent.
Since the tail rotor is driven by the main rotor transmission
during autorotation, heading control is maintained with the
antitorque pedals as in normal flight.
11-3
conditions, and high altitude versus sea level flights. For
specific autorotation airspeed and rotor rpm combinations for
a particular helicopter, refer to the Rotorcraft Flight Manual
(RFM). The specific airspeed and rotor rpm for autorotation
is established for each type of helicopter based on average
weather, calm wind conditions, and normal loading. When
the helicopter is operated with heavy loads in high density
altitude or gusty wind conditions, best performance is
achieved from a slightly increased airspeed in the descent.
For autorotation at low density altitude and light loading,
best performance is achieved from a slight decrease in
normal airspeed. Following this general procedure of fitting
airspeed and rotor rpm to existing conditions, a pilot can
achieve approximately the same glide angle in any set of
circumstances, and thereby estimate the touchdown point
accurately.
It is important that pilots experience autorotations from
various airspeeds. This provides better understanding of
the necessary flight control inputs to achieve the desired
airspeed, rotor rpm and autorotation performance, such
as the maximum glide or minimum descent airspeed. The
decision to use the appropriate airspeed and rotor rpm for
the given conditions should be instinctive to reach a suitable
landing area. The helicopter glide ratio is much less than
that of a fixed-wing aircraft and takes some getting used to.
The flare to land at 80 knots indicated airspeed (KIAS) will
be significantly greater than that from 55 KIAS. Rotor rpm
control is critical at these points to ensure adequate rotor
energy for cushioning the landing.
Use collective pitch control to manage rotor rpm. If rotor rpm
builds too high during an autorotation, raise the collective
sufficiently to decrease rpm back to the normal operating
range, then reduce the collective to maintain proper rotor rpm.
If the collective increase is held too long, the rotor rpm may
decay rapidly. The pilot would have to lower the collective
in order to regain rotor rpm. If the rpm begins decreasing,
the pilot must again lower the collective. Always keep the
rotor rpm within the established recommended range for the
helicopter being flown.
RPM Control
Rotor rpm in low inertia rotor systems has been studied
in simulator flight evaluations which indicate that the
simultaneous application of aft cyclic, down collective,
and alignment with the relative wind (trim) at a wide range
of airspeeds, including cruise airspeeds, is critical for all
operations during the entry of an autorotation. The applicable
Rotorcraft Flight Manual (RFM) should be consulted to
determine the appropriate procedure(s) for safely entering an
autorotation. This is vitally important since the procedure(s)
for safely entering an autorotation may vary with specific
makes and/or models of helicopters. A basic discussion of
the aerodynamics and control inputs for single rotor systems
is in order here.
Helicopter pilots must understand the use of the collective
for rotor rpm control during power off autorotations in a turn.
Upward movement of the collective reduces the rpm and
downward movement increases the rpm. Cyclic movement
is primarily associated with attitude/airspeed control in
powered flight but may not be given the credit appropriate
for rotor rpm control during practice and emergency power
off autorotations. As long as the line of cyclic movement is
parallel with the flight path of the helicopter (trimmed), the
aft movement of the cyclic also creates greater air flow up
through the bottom of the rotor disk and contributes to an
increase in rotor rpm. If the flight path is 10 degrees to the
right of the longitudinal axis of the helicopter, theoretically,
the cyclic should be moved 10 degrees aft and left of the
longitudinal axis to get maximum air up through the rotor
system.
As the pilot lowers the collective in reaction to a loss of
power during cruise flight there may be a tendency for the
nose of the helicopter to pitch down. As a result, the pilot may
tend to lean forward slightly, which delays the application
of simultaneous aft cyclic to prevent the pitch change and
associated loss of rotor rpm. A slight gain in altitude at cruise
airspeed during the power off entry into an autorotation
should not be of great concern as is the case for the execution
of practice or actual quick stops.
Various accident investigations have concluded that, when
faced with a real power failure at cruise airspeed, pilots are
not simultaneously applying down collective, aft cyclic, and
antitorque pedal inputs in a timely manner. Low inertia rotor
systems store less kinetic energy during autorotation and, as
a result, rotor rpm decays rapidly during deceleration and
touchdown. Conversely, less energy is required to regain
safe rotor rpm during autorotation entry and autorotative
descent. The pilot should immediately apply simultaneous
down collective, aft cyclic and trim the helicopter for entry
into an autorotation initiated at cruise airspeed. If rotor rpm
has been allowed to decrease, or has inadvertently decreased
below acceptable limits, an application of aft cyclic may
help rebuild rotor rpm. This application of aft cyclic must
be made at least at a moderate rate and may be combined
with a turn, either left or right, to increase airflow through
the rotor system. This will work to increase rotor rpm. Care
should be maintained to not over-speed the rotor system as
this is attempted.
Risk Management during Autorotation Training
The following sections describe enhanced guidelines for
autorotations during rotorcraft/helicopter flight training,
as stated in Advisory Circular (AC) 61-140. There are
11-4
1
2
3
4
5
Figure 11-2. Straight-in autorotation.
risks inherent in performing autorotations in the training
environment, and in particular the 180-degree autorotation.
This section describes an acceptable means, but not the
only means, of training applicants for a rotorcraft/helicopter
airman certificate to meet the qualifications for various
rotorcraft/helicopter ratings. You may use alternate methods
for training if you establish that those methods meet the
requirements of the Helicopter Flying Handbook (HFH),
FAA practical test standards (PTS), and the Rotorcraft Flight
Manual (RFM).
Straight-In Autorotation
A straight-in autorotation is one made from altitude with
no turns. Winds have a great effect on an autorotation.
Strong headwinds cause the glide angle to be steeper due
to the slower groundspeed. For example, if the helicopter
is maintaining 60 KIAS and the wind speed is 15 knots,
then the groundspeed is 45 knots. The angle of descent will
be much steeper, although the rate of descent remains the
same. The speed at touchdown and the resulting ground run
depend on the groundspeed and amount of deceleration. The
greater the degree of deceleration, or flare, and the longer
it is held, the slower the touchdown speed and the shorter
the ground run. Caution must be exercised at this point as
the tail rotor will be the component of the helicopter closest
to the ground. If timing is not correct and a landing attitude
not set at the appropriate time, the tail rotor may contact the
ground causing a forward pitching moment of the nose and
possible damage to the helicopter.
A headwind is a contributing factor in accomplishing a slow
touchdown from an autorotative descent and reduces the
amount of deceleration required. The lower the speed desired
at touchdown, the more accurate the timing and speed of the
flare must be, especially in helicopters with low-inertia rotor
disks. If too much collective is applied too early during the
final stages of the autorotation, the kinetic energy may be
depleted, resulting in little or no cushioning effect available.
This could result in a hard landing with corresponding
damage to the helicopter. It is generally better practice to
accept more ground run than a harder landing with minimal
groundspeed. As proficiency increases, the amount of ground
run may be reduced.
Technique (How to Practice)
Refer to Figure 11-2 (position 1). From level flight at
the appropriate airspeed (cruise or the manufacturer’s
recommended airspeed), 500–700 feet above ground level
(AGL), and heading into the wind, smoothly but firmly
lower the collective to the full down position. Use aft cyclic
to prevent a nose low attitude while maintaining rotor rpm
in the green arc with collective. If the collective is in the
full down position, the rotor rpm is then being controlled by
the mechanical pitch stops. During maintenance, the rotor
stops must be set to allow minimum autorotational rpm with
a light loading. This means that collective will still be able
to be reduced even under conditions of extreme reduction of
vertical loading (e.g., very low helicopter weight, at very low-
density altitude). After entering an autorotation, collective
pitch must be adjusted to maintain the desired rotor rpm.
Coordinate the collective movement with proper antitorque
pedal for trim, and apply cyclic control to maintain proper
airspeed. Once the collective is fully lowered, decrease
throttle to ensure a clean split/separation of the needles. This
means that the rotor rpm increases to a rate higher than that of
the engine—a clear indication that the freewheeling unit has
allowed the engine to disconnect. After splitting the needles,
readjust the throttle to keep engine rpm above normal idling
speed, but not high enough to cause rejoining of the needles.
See the RFM for the manufacturer's recommendations for
autorotation rate of descent.
At position 2, adjust attitude with cyclic to obtain the
manufacturer’s recommended autorotation (or best gliding)
speed. Adjust collective as necessary to maintain rotor rpm
in the lower part of the green arc (see page 11-2). Aft cyclic
movements cause an increase in rotor rpm, which is then
controlled by a small increase in collective. Avoid a large
collective increase, which results in a rapid decay of rotor
rpm, and leads to “chasing the rpm.” Avoid looking straight
down in front of the aircraft. Continually crosscheck attitude,
trim, rotor rpm, and airspeed.
At the altitude recommended by the manufacturer (position
3), begin the flare with aft cyclic to reduce forward airspeed
and decrease the rate of descent. Maintain heading with the
antitorque pedals. During the flare, maintain rotor rpm in
11-5
the green range. In the execution of the flare, care must be
taken that the cyclic be moved rearward neither so abruptly
that it causes the helicopter to climb, nor so slowly that it
fails to arrest the descent, which may allow the helicopter
to settle so rapidly that the tail rotor strikes the ground. In
most helicopters, the proper flare attitude is that resulting in a
groundspeed of a slow run. When forward motion decreases
to the desired groundspeed—usually the lowest possible
speed (position 4)—move the cyclic forward to place the
helicopter in the proper attitude for landing.
This action gives the student an idea of airframe attitude to
avoid, because a pilot should never allow ground contact
unless the helicopter is more nose-low than that attitude.
Limiting the flare to that attitude may result in slightly faster
touchdown speeds but will eliminate the possibility of tail
rotor impact on level surfaces.
The landing gear height at this time should be approximately
3–15 feet AGL, depending on the altitude recommended by
the manufacturer. As the apparent groundspeed and altitude
decrease, the helicopter must be returned to a more level
attitude for touchdown by applying forward cyclic. Some
helicopters can be landed on the heels in a slightly nose high
attitude to help decrease the forward groundspeed, whereas
others must land skids or landing gear level, in order to spread
the landing loads equally to all of the landing gear. Extreme
caution should be used to avoid an excessive nose high and
tail low attitude below 10 feet. The helicopter must be close
to the landing attitude to keep the tail rotor from contacting
the surface.
At this point, if a full touchdown landing is to be performed,
allow the helicopter to descend vertically (position 5). This
collective application uses some of the kinetic energy in the
rotor disk to help slow the descent rate of the helicopter.
When the collective is raised, the opposite antitorque pedal
used in powered flight will be needed due to the friction
within the transmission/drive train. Touch down in a level
flight attitude.
Control response with increased pitch angles will be slightly
different than normal. With a decrease in main rotor rpm,
the antitorque authority is reduced (the pedals react more
slowly), requiring larger control inputs to maintain heading
at touchdown.
Some helicopters, such as the Schweitzer 300, have a canted
tail stabilizer. With a canted stabilizer, it is crucial that the
pilot apply the appropriate pedal input at all times during the
autorotation. If not the tailboom tends to swing to the right,
which allows the canted stabilizer to raise the tail. This can
result in a severe nose tuck which is quickly corrected with
right pedal application.
A power recovery can be made during training in lieu of a full
touchdown landing. Refer to the section on power recovery
for the correct technique.
After the helicopter has come to a complete stop after
touchdown, lower the collective pitch to the full-down
position. Do not try to stop the forward ground run with aft
cyclic, as the main rotor blades can strike the tail boom. By
lowering the collective slightly during the ground run, an
increase in weight is placed on the landing carriage, slowing
the helicopter; however, this is dependent on the condition
of the landing surface.
One common error is the holding of the helicopter off the
surface, versus cushioning it onto the surface during an
autorotation. Holding the helicopter in the air by using all of
the rotor rpm kinetic energy usually causes the helicopter to
have a hard landing, which results in the blades flexing down
and contacting the tail boom. The rotor rpm should be used
to cushion the helicopter on to the surface for a controlled,
smooth landing instead of allowing the helicopter to drop
the last few inches.
Common Errors
1. Not understanding the importance of an immediate
entry into autorotation upon powerplant or driveline
failure.
2. Failing to use sufficient antitorque pedal when power
is reduced.
3. Lowering the nose too abruptly when power is
reduced, thus placing the helicopter in a dive.
4. Failing to maintain proper rotor rpm during the
descent.
5. Applying up-collective pitch at an excessive altitude,
resulting in a hard landing, loss of heading control,
and possible damage to the tail rotor and main rotor
blade stops.
6. Failing to level the helicopter or achieve the
manufacturers preferred landing attitude.
7. Failing to minimize or eliminate lateral movement
during ground contact. (Similar for items 8 and 9)
8. Failing to maintain ground track in the air and keeping
the landing gear aligned with the direction of travel
during touchdown and ground contact.
9. Failing (in a practice run) to go around if not within
limits and specified criteria for safe autorotation.
11-6
Autorotation with Turns
Turns (or a series of turns) can be made during autorotation
to facilitate landing into the wind or avoiding obstacles.
Turns during autorotation should be made early so that the
remainder of the autorotation is flown identically to a straight-
in autorotation. The most common turns in an autorotation
are 90 degrees and 180 degrees. The following technique
describes an autorotation with a 180-degree turn.
The pilot establishes the aircraft on a downwind heading
at the recommended airspeed, and parallel to the intended
touchdown point. Then, taking the wind into account, the pilot
establishes the ground track approximately 200 feet laterally
from the desired course line to the touchdown point. In strong
crosswind conditions, the pilot should be prepared to adjust
the downwind leg closer or farther out, as appropriate. The
pilot uses the autorotation entry airspeed recommended by
the RFM. When abeam the intended touchdown point, the
pilot smoothly reduces collective, then reduces power to the
engine to show a split between the rotor rpm and engine rpm
and simultaneously applies appropriate anti-torque pedal
and cyclic to maintain proper attitude/airspeed. Throughout
the autorotation, the pilot should continually crosscheck the
helicopter’s attitude, rotor rpm, airspeed, and verify that the
helicopter is in trim (centered trim ball).
After the descent and autorotation airspeed is established, the
pilot initiates the 180-degree turn. For training operations,
initially roll into a bank of at least 30 degrees, but no more
than 60 degrees. It is important to maintain the proper
airspeed, rotor rpm, and trim (centered trim ball) throughout
the turn. Changes in the helicopter’s attitude and the angle
of bank causes a corresponding change in rotor rpm within
normal limits. Do not allow the nose to pitch up or down
excessively during the maneuver, as it may cause undesirable
rotor rpm excursions.
Pitot-static airspeed indications may be unreliable or lag
during an autorotational turn. The pilot should exercise
caution to avoid using excessive aircraft pitch attitudes and to
avoid chasing airspeed indications in an autorotational turn.
Note: Approaching the 90-degree point, check the position of
the landing area. The second 90 degrees of the turn should
end with a roll-out on a course line to the landing area. If the
helicopter is too close, decrease the bank angle (to increase
the radius of turn); if too far out, increase the bank angle
(to decrease the radius of the turn). A bank angle of no more
than 60 degrees should be encountered during this turn.
Monitor the trim ball (along with one’s kinesthetic sense)
and adjust as necessary with cyclic and anti-torque pedal
to maintain coordinated flight. Prior to passing through
200 feet above ground level (AGL), if landing or making a
surface-level power recovery, the turn should be completed,
and the helicopter aligned with the intended touchdown
area. Upon reaching the course line, set the appropriate
crosswind correction. If the collective pitch was increased
to control the rpm, it may need to be lowered on rollout to
prevent decay in rotor rpm.
This maneuver should be aborted at any point the following
criteria is not met: if the helicopter is not in a stabilized
approach to landing profile (i.e., it is not aligned as close
as possible into the wind with the touchdown point, after
completing the 180-degree turn); if the rotor rpm is not within
limits; if the helicopter is not at a proper attitude/airspeed; or
if the helicopter is not under proper control at 200 feet AGL.
It is essential that the pilot on the controls (or a certificated
flight instructor (CFI), when intervening) immediately abort
the maneuver and execute a smooth power recovery and go-
around. It is important for the CFI who is intervening at this
point to remember that the go-around is a far safer option than
trying to recover lost rotor rpm and reestablish or recover to
the hover or even the preferred hover taxi.
From all entry positions, but particularly true of the
180-degree entry, a primary concern is getting the aircraft
into the course line with as much altitude as possible. Once
the collective has been lowered and the engine set to flight
idle, the helicopter will lose altitude. A delayed turn will
result in a lower altitude when arriving on the course line.
Additionally, an uncoordinated flight condition (trim-ball
not centered) results in an increased sink rate, which may
be unrecoverable if not corrected.
During the turn to the course line, the pilot should use a
scan pattern to see outside as well as inside the cockpit. Of
primary importance outside is maintaining the appropriate
descending attitude and a proper turn rate. Essential items to
scan inside are rotor rpm and centered trim ball. Rotor rpm
will build anytime “G” forces are applied to the rotor system.
Usually, this occurs in the turn to the course line and during
the deceleration flare.
Throughout the maneuver, rotor rpm should be maintained
in the range recommended in the RFM. Rotor rpm outside
of the recommended range results in a higher rate of descent
and less glide-ratio. When the rotor rpm exceeds the desired
value as a result of increased G load in the turn, timely
use of up collective will increase the pitch of the blades
and slow the rotor to the desired rpm. In an autorotation,
rotor rpm is the most critical element, as it provides the lift
required to stabilize an acceptable rate of descent and the
energy necessary to cushion the landing. Collective should
be lowered to the full down position to maintain rotor rpm
immediately following a loss of power. However, rapid or
11-7
abrupt collective movement could lead to mast bumping in
some rotorcraft with teetering rotor systems.
Energy is a very important property of all rotating
components, and the kinetic energy stored in the rotor system
is used to cushion the landing. More lift is produced at the
bottom of an autorotation by raising the collective, which
increases the angle of attack of the blades. The rotor rpm will
also rapidly decay at this point and it is essential to properly
time the flare and the final collective pull to fully arrest the
descent and cushion the landing. Upon arriving into the
course line prior to the flare, the scan should focus almost
entirely outside. The scan should include:
• The horizon for attitude, ground track, and nose
alignment;
• the altitude to set the flare and for closure (groundspeed);
and
• the instrument cross-check of airspeed, rotor rpm, and
engine rpm in the descent.
Every autorotational flare will be different depending on the
existing wind conditions, airspeed, density altitude (DA),
and the aircraft gross weight. A pilot operating a helicopter
at a high DA needs to take into account the effects on the
control of the helicopter when recovering from an aborted
autorotation.
Some effects to consider are:
• Higher rate of descent.
• Reduced rotor rpm builds in autorotation.
• Low initial rotor rpm response in autorotation.
• The requirement for a higher flare height.
• Reduced engine power performance.
Common Errors
The following common errors should be prevented:
1. Entering the maneuver at an improper altitude or
airspeed.
2. Entering the maneuver without a level attitude (or not
in coordinated flight).
3. Entering the maneuver and not correcting from the
initial deceleration to a steady state attitude (which
allows excessive airspeed loss in the descent).
4. Improper transition into the descent on entry.
5. Improper use of anti-torque on entry.
6. Failure to establish the appropriate crosswind
correction, allowing the aircraft to drift.
7. Failure to maintain coordinated flight through the tum.
8. Failure to maintain rotor rpm within the RFM
recommended range.
9. Excessive yaw when increasing collective to slow rate
of descent during power recovery autorotations.
10. During power recovery autorotations, a delay in
reapplying power.
11. Initial collective pull either too high or too low.
12. Improper flare (too much or not enough).
13. Flaring too low or too high (AGL).
14. Failure to maintain heading when reapplying power.
15. Not landing with a level attitude.
16. Landing with aircraft not aligned with the direction
of travel.
17. Insufficient collective cushioning during full
autorotations.
18. Abrupt control inputs on touchdown during full
autorotations.
Practice Autorotation with a Power Recovery
A power recovery is used to terminate practice autorotations
at a point prior to actual touchdown. After the power
recovery, a landing can be made or a go-around initiated.
Technique (How to Practice)
At approximately 3–15 feet landing gear height AGL,
depending upon the helicopter being used, begin to level the
helicopter with forward cyclic control. Avoid excessive nose-
high, tail-low attitude below 10 feet. Just prior to achieving
level attitude, with the nose still slightly up, coordinate
upward collective pitch control with an increase in the
throttle to join the needles at operating rpm. The throttle and
collective pitch must be coordinated properly.
If the throttle is increased too fast or too much, an engine
overspeed can occur; if throttle is increased too slowly or too
little in proportion to the increase in collective pitch, a loss of
rotor rpm results. Use sufficient collective pitch to stop the
descent, but keep in mind that the collective pitch application
must be gradual to allow for engine response. Coordinate
proper antitorque pedal pressure to maintain heading. When
a landing is to be made following the power recovery, bring
the helicopter to a hover and then descend to a landing.
In nearly all helicopters, when practicing autorotations with
power recovery, the throttle should be at the flight setting at
the beginning of the flare. As the rotor disk begins to dissipate
its energy, the engine is up to speed as the needles join when
the rotor decreases into the normal flight rpm.
11-8
Helicopters that do not have the throttle control located on
the collective are generally exceptions to basic technique
and require some additional prudence. The autorotation
should be initiated with the power levers left in the “flight,”
or normal, position. If a full touchdown is to be practiced, it
is common technique to move the power levers to the idle
position once the landing area can safely be reached. In most
helicopters, the pilot is fully committed at that point to make
a power-off landing. However, it may be possible to make
a power recovery prior to passing through 100 feet AGL if
the powerplant can recover within that time period and the
instructor is very proficient. The pilot should comply with
the RFM instructions in all cases.
When practicing autorotations to a power recovery, the
differences between reciprocating engines and turbines
may be profound. The reciprocating powerplant generally
responds very quickly to power changes, especially power
increases. Some turbines have delay times depending on
the type of fuel control or governing system installed. Any
reciprocating engine needing turbocharged boost to develop
rated horse power may have significant delays to demands
for increased power, such as in the power recovery. Power
recovery in those helicopters with slower engine response
times must have the engines begin to develop enough power
to rejoin the needles by approximately 100 feet AGL.
If a go-around is to be made, the cyclic control should be
moved forward to resume forward flight. In transition from
a practice autorotation to a go-around, exercise caution to
avoid an altitude-airspeed combination that would place the
helicopter in an unsafe area of its height/velocity diagram.
This is one of the most difficult maneuvers to perform due to
the concentration needed when transitioning from powered
flight to autorotation and then back again to powered flight.
For helicopters equipped with the power control on the
collective, engine power must be brought from flight power
to idle power and then back to a flight power setting. A delay
during any of these transitions can seriously affect rotor rpm
placing the helicopter in a situation that cannot be recovered.
The cyclic must be adjusted to maintain the required
airspeed without power, and then used for the deceleration
flare, followed by the transition to level hovering flight.
Additionally, the cyclic must be adjusted to remove the
compensation for translating tendency. The tail rotor is
no longer needed to produce antitorque thrust until almost
maximum power is applied to the rotor disk for hovering
flight, when the tail rotor must again compensate for the main
rotor torque, which also demands compensation for the tail
rotor thrust and translating tendency.
The pedals must be adjusted from a powered flight anti-
torque trim setting to the opposite trim setting to compensate
for transmission drag and any unneeded vertical fin thrust
countering the now nonexistent torque and then reset to
compensate for the high power required for hovering flight.
All of the above must be accomplished during the 23 seconds
of the autorotation, and the quick, precise control inputs must
be made in the last 5 seconds of the maneuver.
Common Errors
1. Initiating recovery too late, which requires a rapid
application of controls and results in overcontrolling.
2. Failure to obtain and maintain a level attitude near the
surface.
3. Failure to coordinate throttle and collective pitch
properly, which results in either an engine overspeed
or a loss of rotor rpm.
4. Failure to coordinate proper antitorque pedal with the
increase in power.
5. Late engine power engagement causing excessive
temperature or torque, or rpm drop.
6. Failure to go around if not within limits and specified
criteria for safe autorotation.
Practicing Power Failure in a Hover
Power failure in a hover, also called hovering autorotation, is
practiced so that a pilot can automatically make the correct
response when confronted with engine stoppage or certain
other emergencies while hovering. The techniques discussed
in this section are for helicopters with a counterclockwise
rotor disk and an antitorque rotor.
Technique (How to Practice)
To practice hovering autorotation, establish a normal
hovering height (approximately 2–3 feet) for the particular
helicopter being used, considering load and atmospheric
conditions. Keep the helicopter headed into the wind and
hold maximum allowable rpm.
To simulate a power failure, firmly roll the throttle to the
engine idle position. This disengages the driving force of the
engine from the rotor, thus eliminating torque effect. As the
throttle is closed, apply proper antitorque pedal to maintain
heading. Usually, a slight amount of right cyclic control is
necessary to keep the helicopter from drifting to the left, to
compensate for the loss of tail rotor thrust. However, use
cyclic control, as required, to ensure a vertical descent and
a level attitude. Do not adjust the collective on entry.
11-9
Helicopters with low inertia rotor disks settle immediately.
Keep a level attitude and ensure a vertical descent with cyclic
control while maintaining heading with the pedals. Any lateral
movement must be avoided to prevent dynamic rollover. As
rotor rpm decays, cyclic response decreases, so compensation
for the winds will require more cyclic input. At approximately
1 foot AGL, apply upward collective control, as necessary,
to slow the descent and cushion the landing without arresting
the rate of descent above the surface. Usually, the full amount
of collective is required just as the landing gear touches the
surface. As upward collective control is applied, the throttle
must be held in the idle detent position to prevent the engine
from re-engaging. The idle detention position is a ridged stop
position between idle and off in which the idle release button
snaps into, prevent accidental throttle off.
Helicopters with high-inertia rotor disks settle more slowly
after the throttle is closed. In this case, when the helicopter has
settled to approximately 1 foot AGL, apply upward collective
control while holding the throttle in the idle detent position
to slow the descent and cushion the landing. The timing of
collective control application and the rate at which it is applied
depend upon the particular helicopter being used, its gross
weight, and the existing atmospheric conditions. Cyclic control
is used to maintain a level attitude and to ensure a vertical
descent. Maintain heading with antitorque pedals.
When the weight of the helicopter is entirely resting on
the landing gear, cease application of upward collective.
When the helicopter has come to a complete stop, lower the
collective pitch to the full-down position.
The timing of the collective movement is a very important
consideration. If it is applied too soon, the remaining rpm may
not be sufficient to make a soft landing. On the other hand,
if it is applied too late, surface contact may be made before
sufficient blade pitch is available to cushion the landing.
The collective must not be used to hold the helicopter off
the surface, causing a blade stall. Low rotor rpm and ensuing
blade stall can result in a total loss of rotor lift, allowing the
helicopter to fall to the surface and possibly resulting in blade
strikes to the tail boom and other airframe damage such as
landing gear damage, transmission mount deformation, and
fuselage cracking.
Common Errors
1. Failure to use sufficient proper antitorque pedal when
power is reduced.
2. Failure to stop all sideward or backward movement
prior to touchdown.
3. Failure to apply up-collective pitch properly, resulting
in a hard touchdown.
4. Failure to touch down in a level attitude.
5. Failure to roll the throttle completely to idle.
6. Failure to hover at a safe altitude for the helicopter
type, atmospheric conditions, and the level of training/
proficiency of the pilot.
7. Failure to go around if not within limits and specified
criteria for safe autorotation.
Vortex Ring State
Vortex ring state (formerly referenced as settling-with-
power) describes an aerodynamic condition in which a
helicopter may be in a vertical descent with 20 percent up to
maximum power applied, and little or no climb performance.
The previously used term settling-with-power came from the
fact that the helicopter keeps settling even though full engine
power is applied.
In a normal out-of-ground-effect (OGE) hover, the helicopter
is able to remain stationary by propelling a large mass of air
down through the main rotor. Some of the air is recirculated
near the tips of the blades, curling up from the bottom of
the rotor disk and rejoining the air entering the rotor from
the top. This phenomenon is common to all airfoils and is
known as tip vortices. Tip vortices generate drag and degrade
airfoil efficiency. As long as the tip vortices are small, their
only effect is a small loss in rotor efficiency. However, when
the helicopter begins to descend vertically, it settles into its
own downwash, which greatly enlarges the tip vortices. In
this vortex ring state, most of the power developed by the
engine is wasted in circulating the air in a doughnut pattern
around the rotor.
In addition, the helicopter may descend at a rate that exceeds
the normal downward induced-flow rate of the inner blade
sections. As a result, the airflow of the inner blade sections is
upward relative to the disk. This produces a secondary vortex
ring in addition to the normal tip vortices. The secondary
vortex ring is generated about the point on the blade where the
airflow changes from up to down. The result is an unsteady
turbulent flow over a large area of the disk. Rotor efficiency
is lost even though power is still being supplied from the
engine. [Figure 11-3]
A fully developed vortex ring state is characterized by an
unstable condition in which the helicopter experiences
uncommanded pitch and roll oscillations, has little or no
collective authority, and achieves a descent rate that may
approach 6,000 feet per minute (fpm) if allowed to develop.
A vortex ring state may be entered during any maneuver
that places the main rotor in a condition of descending in a
column of disturbed air and low forward airspeed. Airspeeds
11-10
Figure 11-3. Vortex ring state.
that are below translational lift airspeeds are within this
region of susceptibility to vortex ring state aerodynamics.
This condition is sometimes seen during quick-stop type
maneuvers or during recovery from autorotation.
The following combination of conditions is likely to cause
settling in a vortex ring state in any helicopter:
1. A vertical or nearly vertical descent of at least 300
fpm. (Actual critical rate depends on the gross weight,
rpm, density altitude, and other pertinent factors.)
2. The rotor disk must be using some of the available
engine power (20–100 percent).
3. The horizontal velocity must be slower than effective
translational lift.
Situations that are conducive to a vortex ring state condition
are attempting to hover OGE without maintaining precise
altitude control, and approaches, especially steep approaches,
with a tailwind component.
When recovering from a vortex ring state condition, the pilot
tends first to try to stop the descent by increasing collective
pitch. However, this only results in increasing the stalled
area of the rotor, thereby increasing the rate of descent. Since
inboard portions of the blades are stalled, cyclic control
may be limited. The traditional recovery is accomplished
by increasing airspeed, and/or partially lowering collective
to exit the vortex. In most helicopters, lateral cyclic thrust
combined with an increase in power and lateral antitorque
thrust will produce the quickest exit from the hazard. This
technique, known as the Vuichard Recovery (named after the
Swiss examiner from the Federal Office of Civil Aviation
who developed it) recovers by eliminating the descent rate as
opposed to exiting the vortex. If the vortex ring state and the
corresponding descent rate is allowed to progress to what is
called the windmill brake state, the point where the airflow
is completely up through the rotor, the only recovery may
be an autorotation.
Tandem rotor helicopters should maneuver laterally to
achieve clean air in both rotors at the same time.
For vortex ring state demonstrations and training in
recognition and recovery should be performed from a safe
altitude to allow recovery no less than 1000 feet AGL or the
manufacturer’s recommended altitude, whichever is higher.
To enter the maneuver, come to an OGE hover, maintaining
little or no airspeed (any direction), decrease collective
to begin a vertical descent, and as the turbulence begins,
increase collective. Then allow the sink rate to increase to 300
fpm or more as the attitude is adjusted to obtain airspeed of
less than 10 knots. When the aircraft begins to shudder, the
application of additional up collective increases the vibration
and sink rate. As the power is increased, the rate of sink of
the aircraft in the column of air will increase.
If altitude is sufficient, some time can be spent in the
vortices, to enable the pilot to develop a healthy knowledge
of the maneuver. However, helicopter pilots would normally
initiate recovery at the first indication of vortex ring state.
Recovery should be initiated at the first sign of vortex ring
state by applying forward cyclic to increase airspeed and/ or
simultaneously reducing collective. The recovery is complete
when the aircraft passes through effective translational lift
and a normal climb is established.
Common Errors—Traditional Recovery
1. Too much lateral speed for entry into vortex ring state.
2. Excessive decrease of collective.
Common Errors—Vuichard Recovery
1. Excessive lateral cyclic
2. Failure to maintain heading
Retreating Blade Stall
In forward flight, the relative airflow through the main rotor
disk is different on the advancing and retreating side. The
relative airflow over the advancing side is higher due to the
forward speed of the helicopter, while the relative airflow on
the retreating side is lower. This dissymmetry of lift increases
as forward speed increases.
To generate the same amount of lift across the rotor disk,
the advancing blade flaps up while the retreating blade flaps
down. This causes the AOA to decrease on the advancing
11-11
116°
122°
122°
Figure 11-4. Ground resonance.
blade, which reduces lift, and increase on the retreating blade,
which increases lift. At some point as the forward speed
increases, the low blade speed on the retreating blade, and
its high AOA cause a stall and loss of lift.
Retreating blade stall is a factor in limiting a helicopter’s
never-exceed speed (V NE) and its development can be felt
by a low frequency vibration, pitching up of the nose, and
a roll in the direction of the retreating blade. High weight,
low rotor rpm, high density altitude, turbulence and/or
steep, abrupt turns are all conducive to retreating blade stall
at high forward airspeeds. As altitude is increased, higher
blade angles are required to maintain lift at a given airspeed.
Thus, retreating blade stall is encountered at a lower forward
airspeed at altitude. Most manufacturers publish charts and
graphs showing a VNE decrease with altitude.
When recovering from a retreating blade stall condition
caused by high airspeed, moving the cyclic aft only worsens
the stall as aft cyclic produces a flare effect, thus increasing
the AOA. Pushing forward on the cyclic also deepens
the stall as the AOA on the retreating blade is increased.
While the first step in a proper recovery is usually to reduce
collective, RBS should be evaluated in light of the relevant
factors discussed in the previous paragraph and addressed
accordingly. For example, if a pilot at high weight and high
DA is about to conduct a high reconnaissance prior to a
confined area operation where rolling into a steep turn causes
onset of RBS, the recovery is to roll out of the turn. If the
cause is low rotor rpm, then increase the rpm.
Common Errors
1. Failure to recognize the combination of contributing
factors leading to retreating blade stall.
2. Failure to compute VNE limits for altitudes to be flown.
Ground Resonance
Helicopters with articulating rotors (usually designs with
three or more main rotor blades) are subject to ground
resonance, a destructive vibration phenomenon that occurs
at certain rotor speeds when the helicopter is on the ground.
Ground resonance is a mechanical design issue that results
from the helicopter’s airframe having a natural frequency that
can be intensified by an out-of-balance rotor. The unbalanced
rotor disk vibrates at the same frequency (or multiple thereof)
of the airframe’s resonant frequency, and the harmonic
oscillation increases because the engine is adding power
to the system, increasing the magnitude (amplitude) of the
vibrations until the structure or structures fail. This condition
can cause a helicopter to self-destruct in a matter of seconds.
Hard contact with the ground on one corner (and usually
with wheel-type landing gear) can send a shockwave to
the main rotor head, resulting in the blades of a three-blade
rotor disk moving from their normal 120° relationship to
each other. This movement occurs along the drag hinge and
could result in something like 122°, 122°, and 116° between
blades. [Figure 11-4] When another part of the landing gear
strikes the surface, the unbalanced condition could be further
aggravated.
If the rpm is low, the only corrective action to stop ground
resonance is to close the throttle immediately and fully lower
the collective to place the blades in low pitch. If the rpm is in
the normal operating range, fly the helicopter off the ground,
and allow the blades to rephase themselves automatically.
Then, make a normal touchdown. If a pilot lifts off and allows
the helicopter to firmly re-contact the surface before the
blades are realigned, a second shock could move the blades
again and aggravate the already unbalanced condition. This
could lead to a violent, uncontrollable oscillation.
This situation does not occur in rigid or semi-rigid rotor
disks because there is no drag hinge. In addition, skid-type
landing gear is not as prone to ground resonance as wheel-
type landing gear, since the rubber tires' resonant frequency
typically can match that of the spinning rotor, unlike the
condition of a rigid landing gear.
Dynamic Rollover
A helicopter is susceptible to a lateral rolling tendency,
called dynamic rollover, when it is in contact with the surface
11-12
Tail rotor thrust
Tip-path plane neutral cyclic
Tip-path plane full left cyclic
Bank
angle
Pivot point
CG
Weight
Main rotor thrust
Figure 11-5. Forces acting on a helicopter with right skid on the
ground.
during takeoffs or landings. For dynamic rollover to occur,
some factor must first cause the helicopter to roll or pivot
around a skid or landing gear wheel, until its critical rollover
angle is reached. The angle at which dynamic rollover
occurs will vary based on helicopter type. Then, beyond
this point, main rotor thrust continues the roll and recovery
is impossible. After this angle is achieved, the cyclic does
not have sufficient range of control to eliminate the thrust
component and convert it to lift. If the critical rollover angle
is exceeded, the helicopter rolls on its side regardless of the
cyclic corrections made.
Dynamic rollover begins when the helicopter starts to pivot
laterally around its skid or wheel. For dynamic rollover to
occur the following three factors must be present:
1. A rolling moment
2. A pivot point other than the helicopter’s normal CG
3. Thrust greater than weight
This can occur for a variety of reasons, including the failure
to remove a tie down or skid-securing device, or if the skid
or wheel contacts a fixed object while hovering sideward,
or if the gear is stuck in ice, soft asphalt, or mud. Dynamic
rollover may also occur if you use an improper landing or
takeoff technique or while performing slope operations.
Whatever the cause, dynamic rollover is possible if not using
the proper corrective technique.
Once started, dynamic rollover cannot be stopped by
application of opposite cyclic control alone. For example,
the right skid contacts an object and becomes the pivot point
while the helicopter starts rolling to the right. Even with full
left cyclic applied, the main rotor thrust vector and its moment
follows the aircraft as it continues rolling to the right. Quickly
reducing collective pitch is the most effective way to stop
dynamic rollover from developing. Dynamic rollover can
occur with any type of landing gear and all types of rotor disks.
It is important to remember rotor blades have a limited range
of movement. If the tilt or roll of the helicopter exceeds that
range (5–8°), the controls (cyclic) can no longer command a
vertical lift component and the thrust or lift becomes a lateral
force that rolls the helicopter over. When limited rotor blade
movement is coupled with the fact that most of a helicopter’s
weight is high in the airframe, another element of risk is added
to an already slightly unstable center of gravity. Pilots must
remember that in order to remove thrust, the collective must
be lowered as this is the only recovery technique available.
Critical Conditions
Certain conditions reduce the critical rollover angle, thus
increasing the possibility for dynamic rollover and reducing
the chance for recovery. The rate of rolling motion is also
a consideration because, as the roll rate increases, there is
a reduction of the critical rollover angle at which recovery
is still possible. Other critical conditions include operating
at high gross weights with thrust (lift) approximately equal
to the weight.
Refer to Figure 11-5. The following conditions are most
critical for helicopters with counterclockwise rotor rotation:
1. Right side skid or landing wheel down, since
translating tendency adds to the rollover force.
2. Right lateral center of gravity (CG).
3. Crosswinds from the left.
4. Left yaw inputs.
For helicopters with clockwise rotor rotation, the opposite
conditions would be true.
Cyclic Trim
When maneuvering with one skid or wheel on the ground,
care must be taken to keep the helicopter cyclic control
carefully adjusted. For example, if a slow takeoff is attempted
and the cyclic is not positioned and adjusted to account for
translating tendency, the critical recovery angle may be
exceeded in less than two seconds. Control can be maintained
if the pilot maintains proper cyclic position and does not
allow the helicopter’s roll and pitch rates to become too
great. Fly the helicopter into the air smoothly while keeping
movements of pitch, roll, and yaw small; do not allow any
abrupt cyclic pressures.
11-13
Tail rotor thrust
Area of critical rollover
Horizontal
Slope
Full opposite cyclic limit
to prevent rolling motion
Figure 11-6. Upslope rolling motion.
Tail rotor thrust
Area of critical rollover
Horizontal
Slope
Full opposite cyclic limit
to prevent rolling motion
Figure 11-7. Downslope rolling motion.
Normal Takeoffs and Landings
Dynamic rollover is possible even during normal takeoffs and
landings on relatively level ground, if one wheel or skid is on
the ground and thrust (lift) is approximately equal to the weight
of the helicopter. If the takeoff or landing is not performed
properly, a roll rate could develop around the wheel or skid
that is on the ground. When taking off or landing, perform the
maneuver smoothly and carefully adjust the cyclic so that no
pitch or roll movement rates build up, especially the roll rate.
If the bank angle starts to increase to an angle of approximately
5–8°, and full corrective cyclic does not reduce the angle, the
collective should be reduced to diminish the unstable rolling
condition. Excessive bank angles can also be caused by landing
gear caught in a tie down strap, or a tie down strap still attached
to one side of the helicopter. Lateral loading imbalance (usually
outside published limits) is another contributing factor.
Slope Takeoffs and Landings
During slope operations, excessive application of cyclic
control into the slope, together with excessive collective pitch
control, can result in the downslope skid or landing wheel
rising sufficiently to exceed lateral cyclic control limits, and
an upslope rolling motion can occur. [Figure 11-6]
When performing slope takeoff and landing maneuvers, follow
the published procedures and keep the roll rates small. Slowly
raise the downslope skid or wheel to bring the helicopter level,
and then lift off. During landing, first touch down on the
upslope skid or wheel, then slowly lower the downslope skid
or wheel using combined movements of cyclic and collective.
If the helicopter rolls approximately 5–8° to the upslope side,
decrease collective to correct the bank angle and return to level
attitude, then start the landing procedure again.
Use of Collective
The collective is more effective in controlling the rolling
motion than lateral cyclic, because it reduces the main rotor
thrust (lift). A smooth, moderate collective reduction, at a
rate of less than approximately full up to full down in two
seconds, may be adequate to stop the rolling motion. Take
care, therefore, not to dump collective at an excessively high
rate, as this may cause a main rotor blade to strike the fuselage.
Additionally, if the helicopter is on a slope and the roll starts
toward the upslope side, reducing collective too fast may create
a high roll rate in the opposite direction. When the upslope skid
or wheel hits the ground, the dynamics of the motion can cause
the helicopter to bounce off the upslope skid or wheel, and the
inertia can cause the helicopter to roll about the downslope
ground contact point and over on its side. [Figure 11-7]
Under normal conditions on a slope, the collective should
not be pulled suddenly to get airborne because a large and
abrupt rolling moment in the opposite direction could occur.
Excessive application of collective can result in the upslope
skid or wheel rising sufficiently to exceed lateral cyclic
control limits. This movement may be uncontrollable. If the
helicopter develops a roll rate with one skid or wheel on the
ground, the helicopter can roll over on its side.
Precautions
To help avoid dynamic rollover:
1. Always practice hovering autorotations into the wind,
and be wary when the wind is gusty or greater than 10
knots.
2. Use extreme caution when hovering close to fences,
sprinklers, bushes, runway/taxi lights, tiedown cables,
deck nets, or other obstacles that could catch a skid or
wheel. Aircraft parked on hot asphalt overnight might
find the landing gear sunk in and stuck as the ramp
cooled during the evening.
11-14
3. Always use a two-step lift-off. Pull in just enough
collective pitch control to be light on the skids
or landing wheels and feel for equilibrium, then
gently lift the helicopter into the air. 4.
Hover high enough to have adequate skid or landing
wheel clearance from any obstacles when practicing
hovering maneuvers close to the ground, especially
when practicing sideways or rearward flight.
5. Remember that when the wind is coming from
the upslope direction, less lateral cyclic control is
available.
6. Avoid tailwind conditions when conducting slope
operations.
7. Remember that less lateral cyclic control is available
due to the translating tendency of the tail rotor when
the left skid or landing wheel is upslope. (This is true
for counterclockwise rotor disks.)
8. Keep in mind that the lateral cyclic requirement changes
when passengers or cargo are loaded or unloaded.
9. Be aware that if the helicopter utilizes interconnecting
fuel lines that allow fuel to automatically transfer from
one side of the helicopter to the other, the gravitational
flow of fuel to the downslope tank could change the
CG, resulting in a different amount of cyclic control
application to obtain the same lateral result.
10. Do not allow the cyclic limits to be reached. If the
cyclic control limit is reached, further lowering of the
collective may cause mast bumping. If this occurs,
return to a hover and select a landing point with a
lesser degree of slope.
11. During a takeoff from a slope, begin by leveling the
main rotor disk with the horizon or very slightly into
the slope to ensure vertical lift and only enough lateral
thrust to prevent sliding on the slope. If the upslope
skid or wheel starts to leave the ground before the
downslope skid or wheel, smoothly and gently lower
the collective and check to see if the downslope skid or
wheel is caught on something. Under these conditions,
vertical ascent is the only acceptable method of lift-off.
12. Be aware that dynamic rollover can be experienced
during flight operations on a floating platform if the
platform is pitching/rolling while attempting to land
or takeoff. Generally, the pilot operating on floating
platforms (barges, ships, etc.) observes a cycle of seven
during which the waves increase and then decrease to
a minimum. It is that time of minimum wave motion
that the pilot needs to use for the moment of landing
or takeoff on floating platforms. Pilots operating from
floating platforms should also exercise great caution
concerning cranes, masts, nearby boats (tugs) and nets.
Low-G Conditions and Mast Bumping
“G” is an abbreviation for acceleration due to the earth’s
gravity. A person standing on the ground or sitting in an
aircraft in level flight is experiencing one G. An aircraft in a
tight, banked turn with the pilot being pressed into the seat
is experiencing more than one G or high-G conditions. A
person beginning a downward ride in an elevator or riding
down a steep track on a roller coaster is experiencing less
than one G or low-G conditions. The best way for a pilot to
recognize low G is a weightless feeling similar to the start
of a downward elevator ride.
Helicopters rely on positive G to provide much or all of their
response to pilot control inputs. The pilot uses the cyclic
to tilt the rotor disk, and, at one G, the rotor is producing
thrust equal to aircraft weight. The tilting of the thrust
vector provides a moment about the center of gravity to
pitch or roll the fuselage. In a low-G condition, the thrust
and consequently the control authority are greatly reduced.
Although their control ability is reduced, multi-bladed (three
or more blades) helicopters can generate some moment
about the fuselage independent of thrust due to the rotor
hub design with the blade attachment offset from the center
of rotation. However, helicopters with two-bladed teetering
rotors rely entirely on the tilt of the thrust vector for control.
Therefore, low-G conditions can be catastrophic for two-
bladed helicopters.
At lower speeds, such as initiation of a takeoff from hover
or the traditional recovery from vortex ring state, forward
cyclic maneuvers do not cause low G and are safe to perform.
However, an abrupt forward cyclic input or pushover in
a two-bladed helicopter can be dangerous and must be
avoided, particularly at higher speeds. During a pushover
from moderate or high airspeed, as the helicopter noses over,
it enters a low-G condition. Thrust is reduced, and the pilot
has lost control of fuselage attitude but may not immediately
realize it. Tail rotor thrust or other aerodynamic factors will
often induce a roll. The pilot still has control of the rotor disk,
and may instinctively try to correct the roll, but the fuselage
does not respond due to the lack of thrust. If the fuselage is
rolling right, and the pilot puts in left cyclic to correct, the
combination of fuselage angle to the right and rotor disk
angle to the left becomes quite large and may exceed the
clearances built into the rotor hub. This results in the hub
contacting the rotor mast, which is known as mast bumping.
[Figure 11-8] Low-G mast bumping has been the cause of
numerous military and civilian fatal accidents. It was initially
encountered during nap-of-the-earth flying, a very low-
altitude tactical flight technique used by the military where
11-15
Figure 11-8. Result of improper corrective action in a low-G
condition.
the aircraft flies following the contours of the geographical
terrain. The accident sequence may be extremely rapid, and
the energy and inertia in the rotor system can sever the mast
or allow rotor blades to strike the tail or other portions of
the helicopter.
Turbulence, especially severe downdrafts, can also cause a
low-G condition and, when combined with high airspeed,
may lead to mast bumping. Typically, helicopters handle
turbulence better than a light airplane due to smaller
surface area of the rotor blades. During flight in turbulence,
momentary excursions in airspeed, altitude, and attitude are
to be expected. Pilots should respond with smooth, gentle
control inputs and avoid overcontrolling. Most importantly,
pilots should slow down, as mast bumping is less likely at
lower airspeeds.
Pilots can avoid mast bumping accidents as follows:
• Avoid abrupt forward cyclic inputs in two-bladed
helicopters. Airplane pilots may find this a difficult
habit to break because pushing the nose down is an
accepted collision avoidance maneuver in an airplane.
Helicopter pilots would accomplish the same rapid
descent by lowering the collective, and airplane pilots
should train to make this instinctual.
• Recognize the weightless feeling associated with the
onset of low G and quickly take corrective action
before the situation becomes critical.
• Recognize that uncommanded right roll for helicopters
with main rotors which rotate counter-clockwise when
viewed from above indicates that loss of control is
imminent, and immediate corrective action must be
taken.
• Recover from a low-G situation by first gently
applying aft cyclic to restore normal G before
attempting to correct any roll.
• If turbulence is expected or encountered, reduce power
and use a slower than normal cruise speed. Turbulence
(where high rotor flapping angles are already present),
and higher airspeeds (where the controls are more
sensitive) both increase susceptibility to low-G
conditions.
• Use a flight simulator to learn to recognize and
experience low G conditions that result in mast
bumping, its correct recovery technique, and the
consequences of using incorrect recovery actions.
Refer to Chapter 14, Simulation.
Multi-bladed rotors may experience a phenomenon similar
to mast bumping known as droop stop pounding if flapping
clearances are exceeded, but because they retain some control
authority at low G, occurrences are less common than for
teetering rotors.
Low Rotor RPM and Rotor Stall
Rotor rpm is a critically important parameter for all helicopter
operations. Just as airplanes will not fly below a certain
airspeed, helicopters will not fly below a certain rotor
rpm. Safe rotor rpm ranges are marked on the helicopter’s
tachometer and specified in the RFM. If the pilot allows the
rotor rpm to fall below the safe operating range, the helicopter
is in a low rpm situation. If the rotor rpm continues to fall,
the rotor will eventually stall.
Rotor stall should not be confused with retreating blade stall,
which occurs at high forward speeds and over a small portion
of the retreating blade tip. Retreating blade stall causes
vibration and control problems, but the rotor is still very
capable of providing sufficient lift to support the weight of
the helicopter. Rotor stall, however, can occur at any airspeed,
and the rotor quickly stops producing enough lift to support
the helicopter, causing it to lose lift and descend rapidly.
Rotor stall is very similar to the stall of an airplane wing
at low airspeeds. The airplane wing relies on airspeed to
produce the required airflow over the wing, whereas the
helicopter relies on rotor rpm. As the airspeed of the airplane
decreases or the speed of the helicopter rotor slows down, the
AOA of the wing/rotor blade must be increased to support
the weight of the aircraft. At a critical angle (about 15°),
the airflow over the wing or the rotor blade will separate
and stall, causing a sudden loss of lift and increase in drag
(refer to Chapter 2, Aerodynamics of Flight). An airplane
pilot recovers from a stall by lowering the nose to reduce the
AOA and adding power to restore normal airflow over the
wing. However, the falling helicopter is experiencing upward
11-16
airflow through the rotor disk, and the resulting AOA is so
high that even full down collective will not restore normal
airflow. In the helicopter when the rotor stalls, it does not do
so symmetrically because any forward airspeed will produce
a higher airflow on the advancing side than on the retreating
side. This causes the retreating blade to stall first, and its
weight makes it descend as it moves aft while the advancing
blade is climbing as it goes forward. The resulting low aft
blade and high forward blade become a rapid aft tilting of
the rotor disc sometimes referred to as rotor “blow back” or
“flap back.” As the helicopter begins to descend, the upward
flow of air acting on the bottom surfaces of the tail boom
and any horizontal stabilizers tend to pitch the aircraft nose
down. These two effects, combined with any aft cyclic by
the pilot attempting to keep the aircraft level, allow the rotor
blades to blow back and contact the tail boom, in some cases
actually severing the tail boom. Since the tail rotor is geared
to the main rotor, in many helicopters the loss of main rotor
rpm also causes a significant loss of tail rotor thrust and a
corresponding loss of directional control.
Rotor stalls in helicopters are not recoverable. At low altitude,
rotor stall will result in an accident with significant damage
to the helicopter, and at altitudes above approximately 50
feet the accident will likely be fatal. Consequently, early
recognition of the low rotor rpm condition and proper
recovery technique is imperative.
Low rotor rpm can occur during power-off and power-on
operations. During power-off flight, a low rpm situation
can be caused by the failure to quickly lower the collective
after an engine failure or by raising the collective at too
great a height above ground at the bottom of an autorotation.
However, more common are power-on rotor stall accidents.
These occur when the engine is operating normally but the
pilot demands more power than is available by pulling up
too much on the collective. Known as “overpitching,” this
can easily occur at higher density altitudes where the engine
is already producing its maximum horsepower and the pilot
raises the collective. The corresponding increased AOA of
the blades requires more engine horsepower to maintain the
speed of the blades; however, the engine cannot produce any
additional horsepower, so the speed of the blades decreases.
A similar situation can occur with a heavily loaded helicopter
taking off from a confined area. Other causes of a power-on
low rotor rpm condition include the pilot rolling the throttle
the wrong way in helicopters not equipped with a governor
or a governor failure in helicopters so equipped.
As the rpm decreases, the amount of horsepower the engine
can produce also decreases. Engine horsepower is directly
proportional to its rpm, so a 10 percent loss in rpm due
to overpitching, or one of the other scenarios above, will
result in a 10 percent loss in the engine’s ability to produce
horsepower, making recovery even slower and more difficult
than it would otherwise be. With less power from the engine
and less lift from the decaying rotor rpm, the helicopter will
start to settle. If the pilot raises the collective to stop the
settling, the situation will feed upon itself rapidly leading
to rotor stall.
There are a number of ways the pilot can recognize the low
rotor rpm situation. Visually, the pilot can not only see the
rotor rpm indicator decrease but also the change in torque
will produce a yaw; there will also be a noticeable decrease in
engine noise, and at higher airspeeds or in turns, an increase in
vibration. Many helicopters have a low rpm warning system
that alerts the pilot to the low rotor rpm condition.
To recover from the low rotor rpm condition the pilot must
simultaneously lower the collective, increase throttle if
available and apply aft cyclic to maintain a level attitude.
At higher airspeeds, additional aft cyclic may be used to
help recover lost rpm. Recovery should be accomplished
immediately before investigating the problem and must be
practiced to become a conditioned reflex.
System Malfunctions
By following the manufacturer’s recommendations regarding
operating limits and procedures and periodic maintenance
and inspections, many system and equipment failures can
be eliminated. Certain malfunctions or failures can be traced
to some error on the part of the pilot; therefore, appropriate
flying techniques and use of threat and error management
may help to prevent an emergency
Antitorque System Failure
Antitorque failure usually falls into one of two categories.
One is failure of the power drive portion of the tail rotor disk
resulting in a complete loss of antitorque. The other category
covers mechanical control failures prohibiting the pilot from
changing or controlling tail rotor thrust even though the tail
rotor may still be providing antitorque thrust.
Tail rotor drive system failures include driveshaft failures,
tail rotor gearbox failures, or a complete loss of the tail rotor
itself. In any of these cases, the loss of antitorque normally
results in an immediate spinning of the helicopter’s nose. The
helicopter spins to the right in a counterclockwise rotor disk
and to the left in a clockwise system. This discussion is for a
helicopter with a counterclockwise rotor disk. The severity of
the spin is proportionate to the amount of power being used
and the airspeed. An antitorque failure with a high-power
setting at a low airspeed results in a severe spinning to the
11-17
right. At low power settings and high airspeeds, the spin is
less severe. High airspeeds tend to streamline the helicopter
and keep it from spinning.
If a tail rotor failure occurs, power must be reduced in order to
reduce main rotor torque. The techniques differ depending on
whether the helicopter is in flight or in a hover, but ultimately
require an autorotation. If a complete tail rotor failure occurs
while hovering, enter a hovering autorotation by rolling off
the throttle. If the failure occurs in forward flight, enter a
normal autorotation by lowering the collective and rolling
off the throttle. If the helicopter has enough forward airspeed
(close to cruising speed) when the failure occurs, and
depending on the helicopter design, the vertical stabilizer
may provide enough directional control to allow the pilot to
maneuver the helicopter to a more desirable landing sight.
Applying slight cyclic control opposite the direction of yaw
compensates for some of the yaw. This helps in directional
control, but also increases drag. Care must be taken not to
lose too much forward airspeed because the streamlining
effect diminishes as airspeed is reduced. Also, more altitude is
required to accelerate to the correct airspeed if an autorotation
is entered at a low airspeed.
The throttle or power lever on some helicopters is not located
on the collective and readily available. Faced with the loss
of antitorque, the pilot of these models may need to achieve
forward flight and let the vertical fin stop the yawing rotation.
With speed and altitude, the pilot will have the time to set
up for an autorotative approach and set the power control
to idle or off as the situation dictates. At low altitudes, the
pilot may not be able to reduce the power setting and enter
the autorotation before impact.
A mechanical control failure limits or prevents control of tail
rotor thrust and is usually caused by a stuck or broken control
rod or cable. While the tail rotor is still producing antitorque
thrust, it cannot be controlled by the pilot. The amount of
antitorque depends on the position at which the controls jam or
fail. Once again, the techniques differ depending on the amount
of tail rotor thrust, but an autorotation is generally not required.
The specific manufacturer’s procedures should always be
followed. The following is a generalized description of
procedures when more specific procedures are not provided.
Landing—Stuck Left Pedal
A stuck left pedal (high power setting), which might be
experienced during takeoff or climb conditions, results in
the left yaw of the helicopter nose when power is reduced.
Rolling off the throttle and entering an autorotation only
makes matters worse. The landing profile for a stuck left
pedal is best described as a normal-to-steep approach angle to
arrive approximately 2–3 feet landing gear height above the
intended landing area as translational lift is lost. The steeper
angle allows for a lower power setting during the approach
and ensures that the nose remains to the right.
Upon reaching the intended touchdown area and at the
appropriate landing gear height, increase the collective
smoothly to align the nose with the landing direction and
cushion the landing. A small amount of forward cyclic is
helpful to stop the nose from continuing to the right and
directs the aircraft forward and down to the surface. In certain
wind conditions, the nose of the helicopter may remain
to the left with zero to near zero groundspeed above the
intended touchdown point. If the helicopter is not turning,
simply lower the helicopter to the surface. If the nose of the
helicopter is turning to the right and continues beyond the
landing heading, roll the throttle toward flight idle, which is
the amount necessary to stop the turn while landing. Flight
idle is an engine rpm in flight at a given altitude with the
throttle set to the minimum, or idle, position. The flight
idling rpm typically increase with an increase in altitude.
If the helicopter is beginning to turn left, the pilot should
be able to make the landing prior to the turn rate becoming
excessive. However, if the turn rate begins to increase prior
to the landing, simply add power to make a go-around and
return for another landing.
Landing—Stuck Neutral or Right Pedal
The landing profile for a stuck neutral or a stuck right pedal
is a low-power approach terminating with a running or roll-
on landing. The approach profile can best be described as a
shallow to normal approach angle to arrive approximately
2–3 feet landing gear height above the intended landing
area with a minimum airspeed for directional control. The
minimum airspeed is one that keeps the nose from continuing
to yaw to the right.
Upon reaching the intended touchdown area and at the
appropriate landing gear height, reduce the throttle as
necessary to overcome the yaw effect if the nose of the
helicopter remains to the right of the landing heading. The
amount of throttle reduction will vary based on power applied
and winds. The higher the power setting used to cushion the
landing, the more the throttle reduction will be. A coordinated
throttle reduction and increased collective will result in a very
smooth touchdown with some forward groundspeed. If the
nose of the helicopter is to the left of the landing heading,
a slight increase in collective or aft cyclic may be used to
align the nose for touchdown. The decision to land or go
around has to be made prior to any throttle reduction. Using
airspeeds slightly above translational lift may be helpful to