Today helicopters are quite reliable. However
emergencies do occur, whether a result of mechanical
failure or pilot error. By having a thorough knowledge
of the helicopter and its systems, you will be able to
more readily handle the situation. In addition, by
knowing the conditions that can lead to an
emergency, many potential accidents can be avoided.
AUTOROTATION
In a helicopter, an autorotation is a descending maneu-
ver where the engine is disengaged from the main rotor
system and the rotor blades are driven solely by the
upward flow of air through the rotor. In other words, the
engine is no longer supplying power to the main rotor.
The most common reason for an autorotation is an
engine failure, but autorotations can also be performed
in the event of a complete tail rotor failure, since there
is virtually no torque produced in an autorotation. If
altitude permits, they can also be used to recover from
settling with power. If the engine fails, the freewheel-
ing unit automatically disengages the engine from the
main rotor allowing the main rotor to rotate freely.
Essentially, the freewheeling unit disengages anytime
the engine r.p.m. is less than the rotor r.p.m.
At the instant of engine failure, the main rotor blades
are producing lift and thrust from their angle of attack
and velocity. By immediately lowering collective pitch,
which must be done in case of an engine failure, lift and
drag are reduced, and the helicopter begins an immedi-
ate descent, thus producing an upward flow of air
through the rotor system. This upward flow of air
through the rotor provides sufficient thrust to maintain
rotor r.p.m. throughout the descent. Since the tail rotor
is driven by the main rotor transmission during autoro-
tation, heading control is maintained as in normal flight.
Several factors affect the rate of descent in autorota-
tion; density altitude, gross weight, rotor r.p.m., and
airspeed. Your primary control of the rate of descent is
airspeed. Higher or lower airspeeds are obtained with
the cyclic pitch control just as in normal flight.
In theory, you have a choice in the angle of descent
varying from a vertical descent to maximum range,
which is the minimum angle of descent. Rate of descent
is high at zero airspeed and decreases to a minimum at
approximately 50 to 60 knots, depending upon the par-
ticular 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 energy stored
in the rotating blades is used to decrease the rate of
descent and make a soft landing. 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.
Each type of helicopter has a specific airspeed at which
a power-off glide is most efficient. The best airspeed is
the one which combines the greatest glide range with
the slowest rate of descent. The specific airspeed is
somewhat different for each type of helicopter, yet
certain factors affect all configurations in the same
manner. For specific autorotation airspeeds for a partic-
ular helicopter, refer to the FAA-approved rotorcraft
flight manual.
The specific airspeed for autorotations is established
for each type of helicopter on the basis of average
weather and 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 air-
speed in the descent. For autorotations 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 to existing
conditions, you can achieve approximately the same
glide angle in any set of circumstances and estimate the
touchdown point.
When making turns during an autorotation, generally
use cyclic control only. Use of antitorque pedals to
assist or speed the turn causes loss of airspeed and
downward pitching of the nose. When an autorotation
is initiated, sufficient antitorque pedal pressure should
be used to maintain straight flight and prevent yawing.
This pressure should not be changed to assist the turn.
Use collective pitch control to manage rotor r.p.m. If
rotor r.p.m. builds too high during an autorotation, raise
the collective sufficiently to decrease r.p.m. back to the
normal operating range. If the r.p.m. begins decreasing,
you have to again lower the collective. Always keep
the rotor r.p.m. within the established range for your
helicopter. During a turn, rotor r.p.m. increases due to
the increased back cyclic control pressure, which
induces a greater airflow through the rotor system. The
r.p.m. builds rapidly and can easily exceed the maxi-
mum limit if not controlled by use of collective. The
tighter the turn and the heavier the gross weight, the
higher the r.p.m.
To initiate an autorotation, other than in a low hover,
lower the collective pitch control. This holds true
whether performing a practice autorotation or in the
event of an in-flight engine failure. This reduces the
pitch of the main rotor blades and allows them to
continue turning at normal r.p.m. During practice
autorotations, maintain the r.p.m. in the green arc
with the throttle while lowering collective. Once the
collective is fully lowered, reduce engine r.p.m. by
decreasing the throttle. This causes a split of the
engine and rotor r.p.m. needles.
STRAIGHT-IN AUTOROTATION
A straight-in autorotation implies an autorotation from
altitude with no turns. The speed at touchdown and the
resulting ground run depends on the rate and amount of
flare. The greater the degree of flare and the longer it is
held, the slower the touchdown speed and the shorter
the ground run. The slower the speed desired at touch-
down, the more accurate the timing and speed of the
flare must be, especially in helicopters with low inertia
rotor systems.
TECHNIQUE
Refer to figure 11-1 (position 1). From level flight at
the manufacturer’s recommended airspeed, between
500 to 700 feet AGL, and heading into the wind,
smoothly, but firmly lower the collective pitch control
to the full down position, maintaining r.p.m. in the
green arc with throttle. Coordinate the collective move-
ment with proper antitorque pedal for trim, and apply
aft cyclic control to maintain proper airspeed. Once the
collective is fully lowered, decrease throttle to ensure a
clean split of the needles. After splitting the needles,
readjust the throttle to keep engine r.p.m. above
normal idling speed, but not high enough to cause
rejoining of the needles. The manufacturer often
recommends the proper r.p.m.
At position 2, adjust attitude with cyclic control to
obtain the manufacturer’s recommended autorotation
or best gliding speed. Adjust collective pitch control, as
necessary, to maintain rotor r.p.m. in the green arc. Aft
cyclic movements cause an increase in rotor r.p.m.,
which is then controlled by a small increase in collec-
tive pitch control. Avoid a large collective pitch
increase, which results in a rapid decay of rotor r.p.m.,
and leads to “chasing the r.p.m.” Avoid looking straight
down in front of the aircraft. Continually cross-check
attitude, trim, rotor r.p.m., and airspeed.
At approximately 40 to 100 feet above the surface, or
at the altitude recommended by the manufacturer (posi-
tion 3), begin the flare with aft cyclic control to reduce
forward airspeed and decrease the rate of descent.
Maintain heading with the antitorque pedals. Care must
be taken in the execution of the flare so that the cyclic
control is not moved rearward so abruptly as to cause
the helicopter to climb, nor should it be moved so
slowly as to not arrest the descent, which may allow
the helicopter to settle so rapidly that the tail rotor
strikes the ground. When forward motion decreases to
the desired groundspeed, which is usually the slowest
possible speed (position 4), move the cyclic control
forward to place the helicopter in the proper attitude
for landing.
The altitude at this time should be approximately 8 to
15 feet AGL, depending on the altitude recommended
by the manufacturer. Extreme caution should be used
to avoid an excessive nose high and tail low attitude
below 10 feet. At this point, if a full touchdown landing
is to be made, allow the helicopter to descend vertically
(position 5). Increase collective pitch, as necessary, to
check the descent and cushion the landing. Additional
antitorque pedal is required to maintain heading as col-
lective pitch is raised due to the reduction in rotor
r.p.m. and the resulting reduced effect of the tail rotor.
Touch down in a level flight attitude.
A power recovery can be made during training in lieu
of a full touchdown landing. Refer to the section on
power recoveries for the correct technique.
Figure 11-1. Straight-in autorotation.
After touchdown and after the helicopter has come to a
complete stop, 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. Rather, by lowering the collective
slightly during the ground run, more weight is placed
on the undercarriage, slowing the helicopter.
COMMON ERRORS
1. Failing to use sufficient antitorque pedal when
power is reduced.
2. Lowering the nose too abruptly when power is
reduced, thus placing the helicopter in a dive.
3. Failing to maintain proper rotor r.p.m. during
the descent.
4. Application of up-collective pitch at an excessive
altitude resulting in a hard landing, loss of
heading control, and possible damage to the tail
rotor and to the main rotor blade stops.
5. Failing to level the helicopter.
POWER RECOVERY FROM PRACTICE
AUTOROTATION
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
At approximately 8 to 15 feet above the ground,
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 r.p.m. 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 propor-
tion to the increase in collective pitch, a loss of rotor
r.p.m. results. Use sufficient collective pitch to stop the
descent and coordinate proper antitorque pedal
pressure to maintain heading. When a landing is to be
made following the power recovery, bring the helicop-
ter to a hover at normal hovering altitude and then
descend to a landing.
If a go-around is to be made, the cyclic control should
be moved forward to resume forward flight. In transi-
tioning from a practice autorotation to a go-around,
exercise care to avoid an altitude-airspeed combination
that would place the helicopter in an unsafe area of its
height-velocity diagram.
COMMON ERRORS
1. Initiating recovery too late, requiring a rapid appli-
cation of controls, resulting in overcontrolling.
2. Failing to obtain and maintain a level attitude
near the surface.
3. Failing to coordinate throttle and collective pitch
properly, resulting in either an engine overspeed
or a loss of r.p.m.
4. Failing to coordinate proper antitorque pedal with
the increase in power
AUTOROTATIONS WITH TURNS
A turn, or a series of turns, can be made during an
autorotation in order to land into the wind or avoid
obstacles. The turn is usually made early so that the
remainder of the autorotation is the same as a straight
in autorotation. The most common types are 90° and
180° autorotations. The technique below describes a
180° autorotation.
TECHNIQUE
Establish the aircraft on downwind at recommended
airspeed at 700 feet AGL, parallel to the touchdown area.
In a no wind or headwind condition, establish the ground
track approximately 200 feet away from the touchdown
point. If a strong crosswind exists, it will be necessary to
move your downwind leg closer or farther out. When
abeam the intended touchdown point, reduce
collective, and then split the needles. Apply proper
antitorque pedal and cyclic to maintain proper attitude.
Cross check attitude, trim, rotor r.p.m., and airspeed.
After the descent and airspeed is established, roll into a
180° turn. For training, you should initially roll into a
bank of a least 30°, but no more than 40°. Check your
airspeed and rotor r.p.m. Throughout the turn, it is
important to maintain the proper airspeed and keep the
aircraft in trim. Changes in the aircraft’s attitude and
the angle of bank cause a corresponding change in rotor
r.p.m. Adjust the collective, as necessary, in the turn to
maintain rotor r.p.m. in the green arc.
At the 90° point, check the progress of your turn by
glancing toward your landing area. Plan the second
90 degrees of turn to roll out on the centerline. If you are
too close, decrease the bank angle; if too far out, increase
the bank angle. Keep the helicopter in trim with anti-
torque pedals.
The turn should be completed and the helicopter
aligned with the intended touchdown area prior to pass-
ing through 100 feet AGL. If the collective pitch was
increased to control the r.p.m., it may have to be
lowered on roll out to prevent a decay in r.p.m. Make
an immediate power recovery if the aircraft is not
aligned with the touchdown point, and if the rotor
r.p.m. and/or airspeed is not within proper limits.
From this point, complete the procedure as if it were a
straight-in autorotation.
POWER FAILURE IN A HOVER
Power failures in a hover, also called hovering autoro-
tations, are practiced so that you automatically make
the correct response when confronted with engine
stoppage or certain other emergencies while hovering.
The techniques discussed in this section refer to heli-
copters with a counter-clockwise rotor system and an
antitorque rotor.
TECHNIQUE
To practice hovering autorotations, establish a normal
hovering altitude for the particular helicopter being
used, considering load and atmospheric conditions.
Keep the helicopter headed into the wind and hold
maximum allowable r.p.m.
To simulate a power failure, firmly roll the throttle into
the spring loaded override position, if applicable. 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 head-
ing. 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. Leave the collec-
tive pitch where it is on entry.
Helicopters with low inertia rotor systems will begin to
settle immediately. Keep a level attitude and ensure a
vertical descent with cyclic control while maintaining
heading with the pedals. At approximately 1 foot above
the surface, apply upward collective pitch control, as
necessary, to slow the descent and cushion the landing.
Usually the full amount of collective pitch is required.
As upward collective pitch control is applied, the throt-
tle has to be held in the closed position to prevent the
rotor from re-engaging.
Helicopters with high inertia rotor systems will maintain
altitude momentarily after the throttle is closed. Then, as
the rotor r.p.m. decreases, the helicopter starts to settle.
When the helicopter has settled to approximately 1 foot
above the surface, apply upward collective pitch control
while holding the throttle in the closed position to slow
the descent and cushion the landing. The timing of col-
lective pitch control application, and the rate at which it
is applied, depends upon the particular helicopter being
used, its gross weight, and the existing atmospheric con-
ditions. 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 on the
skids, cease the 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 pitch is a most important
consideration. If it is applied too soon, the remaining
r.p.m. may not be sufficient to make a soft landing. On
the other hand, if collective pitch control is applied too
late, surface contact may be made before sufficient
blade pitch is available to cushion the landing.
COMMON ERRORS
1. Failing to use sufficient proper antitorque pedal
when power is reduced.
2. Failing to stop all sideward or backward move-
ment prior to touchdown.
3. Failing to apply up-collective pitch properly,
resulting in a hard touchdown.
4. Failing to touch down in a level attitude.
5. Not rolling the throttle completely to idle.
HEIGHT/VELOCITY DIAGRAM
A height/velocity (H/V) diagram, published by the
manufacturer for each model of helicopter, depicts the
critical combinations of airspeed and altitude should an
engine failure occur. Operating at the altitudes and air-
speeds shown within the crosshatched or shaded areas
of the H/V diagram may not allow enough time for the
critical transition from powered flight to autorotation.
[Figure 11-2]
An engine failure in a climb after takeoff occurring in
section A of the diagram is most critical. During a
climb, a helicopter is operating at higher power settings
and blade angle of attack. An engine failure at this point
causes a rapid rotor r.p.m. decay because the upward
movement of the helicopter must be stopped, then a
descent established in order to drive the rotor. Time is
also needed to stabilize, then increase the r.p.m. to the
normal operating range. The rate of descent must reach
a value that is normal for the airspeed at the moment.
Since altitude is insufficient for this sequence, you end
up with decaying r.p.m., an increasing sink rate, no
deceleration lift, little translational lift, and little
response to the application of collective pitch to cush-
ion the landing.
It should be noted that, once a steady state autorotation
has been established, the H/V diagram no longer
applies. An engine failure while descending through
section A of the diagram, is less critical, provided a safe
landing area is available.
You should avoid the low altitude, high airspeed portion
of the diagram (section B), because your recognition of an
engine failure will most likely coincide with, or shortly
occur after, ground contact. Even if you detect an engine
failure, there may not be sufficient time to rotate the
helicopter from a nose low, high airspeed attitude to one
suitable for slowing, then landing. Additionally, the
altitude loss that occurs during recognition of engine fail-
ure and rotation to a landing attitude, may not leave
enough altitude to prevent the tail skid from hitting the
ground during the landing maneuver.
Basically, if the helicopter represented by this H/V dia-
gram is above 445 feet AGL, you have enough time and
altitude to enter a steady state autorotation, regardless
of your airspeed. If the helicopter is hovering at 5 feet
AGL (or less) in normal conditions and the engine fails,
a safe hovering autorotation can be made. Between
approximately 5 feet and 445 feet AGL, however, the
transition to autorotation depends on the altitude and
airspeed of the helicopter. Therefore, you should
always be familiar with the height/velocity diagram for
the particular model of helicopter you are flying.
THE EFFECT OF WEIGHT VERSUS
DENSITY ALTITUDE
The height/velocity diagram depicts altitude and air-
speed situations from which a successful autorotation
can be made. The time required, and therefore, altitude
necessary to attain a steady state autorotative descent,
is dependent on the weight of the helicopter and the
density altitude. For this reason, the H/V diagram for
some helicopter models is valid only when the helicop-
ter is operated in accordance with the gross weight vs.
density altitude chart. Where appropriate, this chart is
found in the rotorcraft flight manual for the particular
helicopter. [Figure 11-3]
Figure 11-3. Assuming a density altitude of 5,500 feet, the
height/velocity diagram in figure 11-2 would be valid up to a
gross weight of approximately 1,700 pounds. This is found by
entering the graph at a density altitude of 5,500 feet (point A),
then moving horizontally to the solid line (point B). Moving ver-
tically to the bottom of the graph (point C), you find that with the
existing density altitude, the maximum gross weight under
which the height/velocity diagram is applicable is 1,700 pounds.
The gross weight vs. density altitude chart is not
intended as a restriction to gross weight, but as an advi-
sory to the autorotative capability of the helicopter
during takeoff and climb. You must realize, however,
that at gross weights above those recommended by the
gross weight vs. density altitude chart, the H/V diagram
is not restrictive enough.
VORTEX RING STATE (SETTLING WITH
POWER)
V ortex ring state describes an aerodynamic condition
where a helicopter may be in a vertical descent with up
to maximum power applied, and little or no cyclic
authority. The term “settling with power” comes from
the fact that helicopter keeps settling even though full
engine power is applied.
In a normal out-of-ground-effect 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 system and rejoining the air
500□
450□
400□
350□
300□
250□
200□
150□
100□
50□
Smooth Hard Surface.□
Avoid Operation in□
Shaded Areas.
INDICATED AIRSPEED KNOTS□
(CORRECTED FOR INSTRUMENT ERROR)
HEIGHT ABOVE SURFACE - FEET
0 10 20 30 40 50 60 70 80 90 100 110 120
A B
7,000□
6,000□
5,000□
4,000□
3,000
1,500 1,600 1,700 1,800 1,900
GROSS WEIGHT – POUNDS
DENSITY ALTITUDE – FEET
Figure 11-2. By carefully studying the height/velocity
diagram, you will be able to avoid the combinations of alti-
tude and airspeed that may not allow you sufficient time or
altitude to enter a stabilized autorotative descent. You might
want to refer to this diagram during the remainder of the
discussion on the height/velocity diagram.
entering the rotor from the top. This phenomenon is
common to all airfoils and is known as tip vortices. Tip
vortices consume engine power but produce no useful
lift. 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 accelerating 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 disc. This pro-
duces 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 disc. Rotor efficiency is
lost even though power is still being supplied from the
engine. [Figure 11-4]
A fully developed vortex ring state is characterized by
an unstable condition where the helicopter experiences
uncommanded pitch and roll oscillations, has little or
no cyclic authority, and achieves a descent rate, which,
if allowed to develop, may approach 6,000 feet per
minute. It is accompanied by increased levels of
vibration.
A vortex ring state may be entered during any maneu-
ver that places the main rotor in a condition of high
upflow and low forward airspeed. This condition is
sometimes seen during quick-stop type maneuvers or
during recoveries from autorotations. The following
combination of conditions are likely to cause settling in
a vortex ring state:
1. A vertical or nearly vertical descent of at least
300 feet per minute. (Actual critical rate depends
on the gross weight, r.p.m., density altitude, and
other pertinent factors.)
2. The rotor system must be using some of the avail-
able engine power (from 20 to 100 percent).
3. The horizontal velocity must be slower than
effective translational lift.
Some of the situations that are conducive to a settling
with power condition are: attempting to hover out of
ground effect at altitudes above the hovering ceiling of
the helicopter; attempting to hover out of ground effect
without maintaining precise altitude control; or down-
wind and steep power approaches in which airspeed is
permitted to drop to nearly zero.
When recovering from a settling with power condition,
the tendency on the part of the pilot is to first try to stop
the descent by increasing collective pitch. However,
this only results in increasing the stalled area of the
rotor, thus increasing the rate of descent. Since inboard
portions of the blades are stalled, cyclic control is
limited. Recovery is accomplished by increasing
forward speed, and/or partially lowering collective
pitch. In a fully developed vortex ring state, the only
recovery may be to enter autorotation to break the
vortex ring state. When cyclic authority is regained,
you can then increase forward airspeed.
For settling with power demonstrations and training in
recognition of vortex ring state conditions, all maneu-
vers should be performed at an elevation of at least
1,500 feet AGL.
To enter the maneuver, reduce power below hover
power. Hold altitude with aft cyclic until the
airspeed approaches 20 knots. Then allow the sink
rate to increase to 300 feet per minute or more as the
attitude is adjusted to obtain an airspeed of less than
10 knots. When the aircraft begins to shudder, the
application of additional up collective increases the
vibration and sink rate.
Recovery should be initiated at the first sign of vor-
tex ring state by applying forward cyclic to increase
airspeed and simultaneously reducing collective.
The recovery is complete when the aircraft passes
through effective translational lift and a normal
climb is established.
RETREATING BLADE STALL
In forward flight, the relative airflow through the
main rotor disc is different on the advancing and
retreating side. The relative airflow over the advanc-
ing side is higher due to the forward speed of the
Figure 11-4. Vortex ring state.
helicopter, while the relative airflow on the retreat-
ing side is lower. This dissymmetry of lift increases
as forward speed increases.
To generate the same amount of lift across the rotor
disc, the advancing blade flaps up while the retreat-
ing blade flaps down. This causes the angle of attack
to decrease on the advancing blade, which reduces
lift, and increase on the retreating blade, which
increases lift. As the forward speed increases, at
some point the low blade speed on the retreating
blade, together with its high angle of attack, causes a
loss of lift (stall).
Retreating blade stall is a major factor in limiting a
helicopter’s top forward speed (V NE) and can be felt
developing by a low frequency vibration, pitching
up of the nose, and a roll in the direction of the
retreating blade. High weight, low rotor r.p.m., 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 show-
ing a VNE decrease with altitude.
When recovering from a retreating blade stall condi-
tion, moving the cyclic aft only worsens the stall
as aft cyclic produces a flare effect, thus increasing
angles of attack. Pushing forward on the cyclic
also deepens the stall as the angle of attack on the
retreating blade is increased. Correct recovery from
retreating blade stall requires the collective to be
lowered first, which reduces blade angles and thus
angle of attack. Aft cyclic can then be used to slow
the helicopter.
GROUND RESONANCE
Ground resonance is an aerodynamic phenomenon
associated with fully-articulated rotor systems. It
develops when the rotor blades move out of phase
with each other and cause the rotor disc to become
unbalanced. This condition can cause a helicopter to
self-destruct in a matter of seconds. However, for
this condition to occur, the helicopter must be in
contact with the ground.
If you allow your helicopter to touch down firmly on
one corner (wheel type landing gear is most
conducive for this) the shock is transmitted to the
main rotor system. This may cause the blades to
move out of their normal relationship with each
other. This movement occurs along the drag hinge.
[Figure 11-5]
Figure 11-5. Hard contact with the ground can send a shock
wave to the main rotor head, resulting in the blades of a
three-bladed rotor system moving from their normal 120°
relationship to each other. This could result in something like
122°, 122°, and 116° between blades. When one of the other
landing gear strikes the surface, the unbalanced condition
could be further aggravated.
If the r.p.m. is low, the 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
r.p.m. is in the normal operating range, you should fly the
helicopter off the ground, and allow the blades to auto-
matically realign themselves. You can then make a normal
touchdown. If you lift off and allow 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 semirigid rotor
systems, because there is no drag hinge. In addition,
skid type landing gear are not as prone to ground
resonance as wheel type gear.
DYNAMIC ROLLOVER
A helicopter is susceptible to a lateral rolling tendency,
called dynamic rollover, when lifting off the surface.
For dynamic rollover to occur, some factor has to first
cause the helicopter to roll or pivot around a skid, or
landing gear wheel, until its critical rollover angle is
reached. Then, beyond this point, main rotor thrust con-
tinues the roll and recovery is impossible. 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 around its skid or wheel. This can occur for a
variety of reasons, including the failure to remove a
tiedown or skid securing device, or if the skid or wheel
122ϒ 116ϒ
122ϒ
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 do not use the proper
landing or takeoff technique or while performing slope
operations. Whatever the cause, if the gear or skid
becomes a pivot point, dynamic rollover is possible if
you do not use the proper corrective technique.
Once started, dynamic rollover cannot be stopped by
application of opposite cyclic control alone. For exam-
ple, 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 applying down
collective is the most effective way to stop dynamic
rollover from developing. Dynamic rollover can occur
in both skid and wheel equipped helicopters, and all
types of rotor systems.
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, the critical rollover angle at which recovery
is still possible, is reduced. Other critical conditions
include operating at high gross weights with thrust (lift)
approximately equal to the weight.
Refer to figure 11-6. The following conditions are
most critical for helicopters with counter-clockwise
rotor rotation:
1. right side skid/wheel down, since translating ten-
dency adds to the rollover force.
2. right lateral center of gravity.
3. crosswinds from the left.
4. left yaw inputs.
For helicopters with clockwise rotor rotation, the oppo-
site 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 properly trimmed. For example, if a slow take-
off is attempted and the cyclic is not positioned and
trimmed to account for translating tendency, the critical
recovery angle may be exceeded in less than two sec-
onds. Control can be maintained if you maintain proper
cyclic position and trim, and not allow the helicopter’s
roll and pitch rates to become too great. You should fly
your helicopter into the air smoothly while keeping
movements of pitch, roll, and yaw small, and not allow
any untrimmed cyclic pressures.
NORMAL TAKEOFFS AND LANDINGS
Dynamic rollover is possible even during normal take-
offs and landings on relative level ground, if one wheel
or skid is on the ground and thrust (lift) is approxi-
mately 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 trim 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 to 8°, and full corrective cyclic does
not reduce the angle, the collective should be reduced
to diminish the unstable rolling condition.
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 rising
sufficiently to exceed lateral cyclic control limits, and an
upslope rolling motion can occur. [Figure 11-7]
Pivot Point Bank Angle
Weight
Tip Path Plane Neutral Cyclic
Tip Path Plane Full Left Cyclic
Crosswind
Tail Rotor Thrust
Main□
Rotor□
Thrust
Figure 11-6. Forces acting on a helicopter with right skid on
the ground.
Tail Rotor Thrust
Slope
Horizontal
Area ofCritical Rollover
Full Opposite Cyclic Limit □
to Prevent Rolling Motion
Figure 11-7. Upslope rolling motion.