When performing slope takeoff and landing maneu-
vers, 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 to 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 reduc-
tion, at a rate less than approximately full up to full down
in two seconds, is adequate to stop the rolling motion.
Take care, however, not to dump collective at too high a
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 to the upslope side, reducing collective too
fast may create a high roll rate in the opposite direction.
When the upslope skid/wheel hits the ground, the
dynamics of the motion can cause the helicopter to
bounce off the upslope skid/wheel, and the inertia can
cause the helicopter to roll about the downslope ground
contact point and over on its side. [Figure 11-8]
The collective should not be pulled suddenly to get air-
borne, as a large and abrupt rolling moment in the
opposite direction could occur. Excessive application
of collective can result in the upslope skid rising suffi-
ciently to exceed lateral cyclic control limits. This
movement may be uncontrollable. If the helicopter
develops a roll rate with one skid/wheel on the ground,
the helicopter can roll over on its side.
PRECAUTIONS
The following lists several areas to help you avoid
dynamic rollover.
1. Always practice hovering autorotations into the
wind, but never when the wind is gusty or over
10 knots.
2. When hovering close to fences, sprinklers,
bushes, runway/taxi lights, or other obstacles that
could catch a skid, use extreme caution.
3. Always use a two-step liftoff. Pull in just enough
collective pitch control to be light on the skids
and feel for equilibrium, then gently lift the
helicopter into the air.
4. When practicing hovering maneuvers close to
the ground, make sure you hover high enough to
have adequate skid clearance with any obsta-
cles, especially when practicing sideways or
rearward flight.
5. When the wind is coming from the upslope direc-
tion, less lateral cyclic control will be available.
6. Tailwind conditions should be avoided when
conducting slope operations.
7. When the left skid/wheel is upslope, less lateral
cyclic control is available due to the translating
tendency of the tail rotor. (This is true for
counter-rotating rotor systems)
8. If passengers or cargo are loaded or unloaded, the
lateral cyclic requirement changes.
9. 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 center of gravity, 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, if the upslope
skid/wheel starts to leave the ground before the
downslope skid/wheel, smoothly and gently
Tail Rotor Thrust
Slope
Horizontal
Area ofCritical Rollover
Full Opposite Cyclic Limit □
to Prevent Rolling Motion
Figure 11-8. Downslope rolling motion.
lower the collective and check to see if the
downslope skid/wheel is caught on something.
Under these conditions vertical ascent is the only
acceptable method of liftoff.
12. During flight operations on a floating platform, if
the platform is pitching/rolling while attempting to
land or takeoff, the result could be dynamic rollover.
LOW G CONDITIONS AND MAST
BUMPING
For cyclic control, small helicopters depend primarily
on tilting the main rotor thrust vector to produce
control moments about the aircraft center of gravity
(CG), causing the helicopter to roll or pitch in the
desired direction. Pushing the cyclic control forward
abruptly from either straight-and-level flight or after a
climb can put the helicopter into a low G (weightless)
flight condition. In forward flight, when a push-over is
performed, the angle of attack and thrust of the rotor is
reduced, causing a low G or weightless flight condi-
tion. During the low G condition, the lateral cyclic has
little, if any, effect because the rotor thrust has been
reduced. Also, in a counter-clockwise rotor system (a
clockwise system would be the reverse), there is no
main rotor thrust component to the left to counteract
the tail rotor thrust to the right, and since the tail rotor
is above the CG, the tail rotor thrust causes the helicop-
ter to roll rapidly to the right, If you attempt to stop the
right roll by applying full left cyclic before regaining
main rotor thrust, the rotor can exceed its flapping
limits and cause structural failure of the rotor shaft due
to mast bumping, or it may allow a blade to contact the
airframe. [Figure 11-9]
Since a low G condition could have disastrous results,
the best way to prevent it from happening is to avoid the
conditions where it might occur. This means avoiding
turbulence as much as possible. If you do encounter
turbulence, slow your forward airspeed and make small
control inputs. If turbulence becomes excessive,
consider making a precautionary landing. To help pre-
vent turbulence induced inputs, make sure your cyclic
arm is properly supported. One way to accomplish this
is to brace your arm against your leg. Even if you are
not in turbulent conditions, you should avoid abrupt
movement of the cyclic and collective.
If you do find yourself in a low G condition, which
can be recognized by a feeling of weightlessness
and an uncontrolled roll to the right, you should imme-
diately and smoothly apply aft cyclic. Do not attempt
to correct the rolling action with lateral cyclic. By
applying aft cyclic, you will load the rotor system,
which in turn produces thrust. Once thrust is restored,
left cyclic control becomes effective, and you can roll
the helicopter to a level attitude.
LOW ROTOR RPM AND BLADE STALL
As mentioned earlier, low rotor r.p.m. during an
autorotation might result in a less than successful
maneuver. However, if you let rotor r.p.m. decay to the
point where all the rotor blades stall, the result is usu-
ally fatal, especially when it occurs at altitude. The
danger of low rotor r.p.m. and blade stall is greatest in
small helicopters with low blade inertia. It can occur
in a number of ways, such as simply rolling the throt-
tle the wrong way, pulling more collective pitch than
power available, or when operating at a high density
altitude.
When the rotor r.p.m. drops, the blades try to maintain
the same amount of lift by increasing pitch. As the
pitch increases, drag increases, which requires more
power to keep the blades turning at the proper r.p.m.
When power is no longer available to maintain r.p.m.,
and therefore lift, the helicopter begins to descend.
This changes the relative wind and further increases
the angle of attack. At some point the blades will stall
unless r.p.m. is restored. If all blades stall, it is almost
impossible to get smooth air flowing across the
blades.
Even though there is a safety factor built into most hel-
icopters, anytime your rotor r.p.m. falls below the green
arc, and you have power, simultaneously add throttle
and lower the collective. If you are in forward flight,
gently applying aft cyclic loads up the rotor system and
helps increase rotor r.p.m. If you are without power,
immediately lower the collective and apply aft cyclic.
RECOVERY FROM LOW ROTOR RPM
Under certain conditions of high weight, high tempera-
ture, or high density altitude, you might get into a
situation where the r.p.m. is low even though you are
using maximum throttle. This is usually the result of
Figure 11-9. In a low G condition, improper corrective action
could lead to the main rotor hub contacting the rotor mast.
The contact with the mast becomes more violent with each
successive flapping motion. This, in turn, creates a greater
flapping displacement. The result could be a severely
damaged rotor mast, or the main rotor system could sepa-
rate from the helicopter.
the main rotor blades having an angle of attack that has
created so much drag that engine power is not suffi-
cient to maintain or attain normal operating r.p.m.
If you are in a low r.p.m. situation, the lifting power of
the main rotor blades can be greatly diminished. As soon
as you detect a low r.p.m. condition, immediately apply
additional throttle, if available, while slightly lowering
the collective. This reduces main rotor pitch and drag. As
the helicopter begins to settle, smoothly raise the collec-
tive to stop the descent. At hovering altitude you may
have to repeat this technique several times to regain nor-
mal operating r.p.m. This technique is sometimes called
“milking the collective.” When operating at altitude, the
collective may have to be lowered only once to regain
rotor speed. The amount the collective can be lowered
depends on altitude. When hovering near the surface,
make sure the helicopter does not contact the ground as
the collective is lowered.
Since the tail rotor is geared to the main rotor, low main
rotor r.p.m. may prevent the tail rotor from producing
enough thrust to maintain directional control. If pedal
control is lost and the altitude is low enough that a
landing can be accomplished before the turning rate
increases dangerously, slowly decrease collective pitch,
maintain a level attitude with cyclic control, and land.
SYSTEM MALFUNCTIONS
The reliability and dependability record of modern
helicopters is very impressive. By following the
manufacturer’s recommendations regarding periodic
maintenance and inspections, you can eliminate most
systems and equipment failures. Most malfunctions or
failures can be traced to some error on the part of the
pilot; therefore, most emergencies can be averted before
they happen. An actual emergency is a rare occurrence.
ANTITORQUE SYSTEM FAILURE
Antitorque failures usually fall into two categories.
One focuses on failure of the power drive portion of the
tail rotor system resulting in a complete loss of anti-
torque. The other category covers mechanical control
failures where the pilot is unable to change or control
tail rotor thrust even though the tail rotor may still be
providing antitorque thrust.
Tail rotor drive system failures include driveshaft fail-
ures, 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 yawing of
the helicopter’s nose. The helicopter yaws to the right
in a counter-clockwise rotor system and to the left in a
clockwise system. This discussion assumes a
helicopter with a counter-clockwise rotor system. The
severity of the yaw 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 yawing to the right. At low power
settings and high airspeeds, the yaw is less severe. High
airspeeds tend to streamline the helicopter and keep it
from spinning.
If a tail rotor failure occurs, power has to 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 will 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 direc-
tional control to allow you to maneuver the helicopter to
a more desirable landing sight. Some of the yaw may be
compensated for by applying slight cyclic control oppo-
site the direction of yaw. This helps in directional
control, but also increases drag. Care must be taken not
to lose too much forward airspeed because the stream-
lining effect diminishes as airspeed is reduced. Also,
more altitude is required to accelerate to the
correct airspeed if an autorotation is entered into at a
low airspeed.
A mechanical control failure limits or prevents con-
trol 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 where 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.
LANDING—STUCK LEFT PEDAL
Be sure to follow the procedures and techniques
outlined in the FAA-approved rotorcraft flight man-
ual for the helicopter you are flying. A stuck left
pedal, such as might be experienced during takeoff or
climb conditions, results in the helicopter’s nose
yawing to the left 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 approach to a
momentary hover at three to four feet above the
surface. Following an analysis, make the landing. If
the helicopter is not turning, simply lower the
helicopter to the surface. If the helicopter is turning
to the right, roll the throttle toward flight idle the
amount necessary to stop the turn as you land. If the
helicopter is beginning to turn left, you should be
able to make the landing prior to the turn rate
becoming excessive. However, if the turn rate
becomes excessive prior to the landing, simply
execute a takeoff 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 or descent with a
running or roll-on landing. The approach profile can
best be described as a steep approach with a flare at the
bottom to slow the helicopter. The power should be low
enough to establish a left yaw during the descent. The
left yaw allows a margin of safety due to the fact that
the helicopter will turn to the right when power is
applied. This allows the momentary use of power at the
bottom of the approach. As you apply power, the heli-
copter rotates to the right and becomes aligned with the
landing area. At this point, roll the throttle to flight idle
and make the landing. The momentary use of power
helps stop the descent and allows additional time for
you to level the helicopter prior to closing the throttle.
If the helicopter is not yawed to the left at the conclusion
of the flare, roll the throttle to flight idle and use the
collective to cushion the touchdown. As with any
running or roll-on landing, use the cyclic to maintain the
ground track. This technique results in a longer ground
run or roll than if the helicopter was yawed to the left.
UNANTICIPATED YAW / LOSS OF TAIL
ROTOR EFFECTIVENESS (LTE)
Unanticipated yaw is the occurrence of an uncom-
manded yaw rate that does not subside of its own
accord and, which, if not corrected, can result in the
loss of helicopter control. This uncommanded yaw rate
is referred to as loss of tail rotor effectiveness (LTE)
and occurs to the right in helicopters with a counter-
clockwise rotating main rotor and to the left in helicop-
ters with a clockwise main rotor rotation. Again, this
discussion covers a helicopter with a counter-clockwise
rotor system and an antitorque rotor.
LTE is not related to an equipment or maintenance mal-
function and may occur in all single-rotor helicopters
at airspeeds less than 30 knots. It is the result of the tail
rotor not providing adequate thrust to maintain direc-
tional control, and is usually caused by either certain
wind azimuths (directions) while hovering, or by an
insufficient tail rotor thrust for a given power setting at
higher altitudes.
For any given main rotor torque setting in perfectly
steady air, there is an exact amount of tail rotor thrust
required to prevent the helicopter from yawing either
left or right. This is known as tail rotor trim thrust. In
order to maintain a constant heading while hovering,
you should maintain tail rotor thrust equal to trim thrust.
The required tail rotor thrust is modified by the effects
of the wind. The wind can cause an uncommanded yaw
by changing tail rotor effective thrust. Certain relative
wind directions are more likely to cause tail rotor thrust
variations than others. Flight and wind tunnel tests
have identified three relative wind azimuth regions that
can either singularly, or in combination, create an LTE
conducive environment. These regions can overlap,
and thrust variations may be more pronounced. Also,
flight testing has determined that the tail rotor does not
actually stall during the period. When operating in
these areas at less than 30 knots, pilot workload
increases dramatically.
MAIN ROTOR DISC INTERFERENCE
(285-315°)
Refer to figure 11-10. Winds at velocities of 10 to 30
knots from the left front cause the main rotor
vortex to be blown into the tail rotor by the relative
wind. The effect of this main rotor disc vortex causes
the tail rotor to operated in an extremely turbulent envi-
ronment. During a right turn, the tail rotor experiences
a reduction of thrust as it comes into the area of the
main rotor disc vortex. The reduction in tail rotor thrust
comes from the airflow changes experienced at the tail
rotor as the main rotor disc vortex moves across the tail
rotor disc. The effect of the main rotor disc vortex
initially increases the angle of attack of the tail rotor
blades, thus increasing tail rotor thrust. The increase in
the angle of attack requires that right pedal pressure be
added to reduce tail rotor thrust in order to maintain the
same rate of turn. As the main rotor vortex passes the
tail rotor, the tail rotor angle of attack is reduced. The
reduction in the angle of attack causes a reduction in
thrust and a right yaw acceleration begins. This accel-
eration can be surprising, since you were previously
adding right pedal to maintain the right turn rate. This
thrust reduction occurs suddenly, and if uncorrected,
develops into an uncontrollable rapid rotation about the
mast. When operating within this region, be aware that
the reduction in tail rotor thrust can happen quite
suddenly, and be prepared to react quickly to counter
this reduction with additional left pedal input.
Figure 11-10. Main rotor disc vortex interference.
300ϒ
330ϒ
285ϒ
270ϒ
240ϒ
210ϒ 150ϒ
120ϒ
90ϒ
60ϒ
30ϒ
15 Knots
20 Knots
10 Knots
0ϒ
360ϒ
Region of Disc□
Vortex Interference
315ϒ
WEATHERCOCK STABILITY
(120-240°)
In this region, the helicopter attempts to weathervane
its nose into the relative wind. [Figure 11-11] Unless a
resisting pedal input is made, the helicopter starts a
slow, uncommanded turn either to the right or left
depending upon the wind direction. If the pilot allows a
right yaw rate to develop and the tail of the helicopter
moves into this region, the yaw rate can accelerate
rapidly. In order to avoid the onset of LTE in this
downwind condition, it is imperative to maintain posi-
tive control of the yaw rate and devote full attention to
flying the helicopter.
Figure 11-11. Weathercock stability.
TAIL ROTOR VORTEX RING STATE
(210-330°)
Winds within this region cause a tail rotor vortex ring
state to develop. [Figure 11-12] The result is a non-uni-
form, unsteady flow into the tail rotor. The vortex ring
state causes tail rotor thrust variations, which result in
yaw deviations. The net effect of the unsteady flow is
an oscillation of tail rotor thrust. Rapid and continuous
pedal movements are necessary to compensate for the
rapid changes in tail rotor thrust when hovering in a left
crosswind. Maintaining a precise heading in this region
is difficult, but this characteristic presents no signifi-
cant problem unless corrective action is delayed.
However, high pedal workload, lack of concentration
and overcontrolling can all lead to LTE.
When the tail rotor thrust being generated is less than
the thrust required, the helicopter yaws to the right.
When hovering in left crosswinds, you must concen-
trated on smooth pedal coordination and not allow an
uncontrolled right yaw to develop. If a right yaw rate
is allowed to build, the helicopter can rotate into the
wind azimuth region where weathercock stability then
accelerates the right turn rate. Pilot workload during a
tail rotor vortex ring state is high. Do not allow a right
yaw rate to increase.
Figure 11-12. Tail rotor vortex ring state.
LTE AT ALTITUDE
At higher altitudes, where the air is thinner, tail rotor
thrust and efficiency is reduced. When operating at
high altitudes and high gross weights, especially while
hovering, the tail rotor thrust may not be sufficient to
maintain directional control and LTE can occur. In this
case, the hovering ceiling is limited by tail rotor thrust
and not necessarily power available. In these condi-
tions gross weights need to be reduced and/or
operations need to be limited to lower density altitudes.
REDUCING THE ONSET OF LTE
To help reduce the onset of loss of tail rotor effective-
ness, there are some steps you can follow.
1. Maintain maximum power-on rotor r.p.m. If the
main rotor r.p.m. is allowed to decrease, the anti-
torque thrust available is decreased proportionally.
2. Avoid tailwinds below an airspeed of 30 knots. If
loss of translational lift occurs, it results in an
increased power demand and additional anti-
torque pressures.
3. Avoid out of ground effect (OGE) operations and
high power demand situations below an airspeed
of 30 knots.
4. Be especially aware of wind direction and velocity
when hovering in winds of about 8-12 knots. There
are no strong indicators that translational lift has
been reduced. A loss of translational lift results in
an unexpected high power demand and an
increased antitorque requirement.
Region Where Weathercock□
Stability Can Introduce Y aw Rates
360ϒ
0ϒ
15 Knots
10 Knots
5 Knots
17 Knots
30ϒ
60ϒ
90ϒ
120ϒ
150ϒ
180ϒ
210ϒ
240ϒ
270ϒ
300ϒ
330ϒ
17 Knots
15 Knots
10 Knots
5 Knots
0ϒ
180ϒ
150ϒ
30ϒ
120ϒ
60ϒ
90ϒ
210ϒ
240ϒ
270ϒ
300ϒ
330ϒ
360ϒ
Region of□
Roughness□
Due toTail □
Rotor Vortex□
Ring State
5. Be aware that if a considerable amount of left
pedal is being maintained, a sufficient amount of
left pedal may not be available to counteract an
unanticipated right yaw.
6. Be alert to changing wind conditions, which may
be experienced when flying along ridge lines and
around buildings.
RECOVERY TECHNIQUE
If a sudden unanticipated right yaw occurs, the follow-
ing recovery technique should be performed. Apply full
left pedal while simultaneously moving cyclic control
forward to increase speed. If altitude permits, reduce
power. As recovery is effected, adjust controls for
normal forward flight.
Collective pitch reduction aids in arresting the yaw rate
but may cause an excessive rate of descent. Any large,
rapid increase in collective to prevent ground or
obstacle contact may further increase the yaw rate and
decrease rotor r.p.m. The decision to reduce collective
must be based on your assessment of the altitude
available for recovery.
If the rotation cannot be stopped and ground contact is
imminent, an autorotation may be the best course of
action. Maintain full left pedal until the rotation stops,
then adjust to maintain heading.
MAIN DRIVE SHAFT FAILURE
The main drive shaft, located between the engine and
the main rotor gearbox, transmits engine power to the
main rotor gearbox. In some helicopters, particularly
those with piston engines, a drive belt is used instead of
a drive shaft. A failure of the drive shaft or belt has the
same effect as an engine failure, because power is no
longer provided to the main rotor, and an autorotation
has to be initiated. There are a few differences,
however, that need to be taken into consideration. If the
drive shaft or belt breaks, the lack of any load on the
engine results in an overspeed. In this case, the throttle
must be closed in order to prevent any further damage.
In some helicopters, the tail rotor drive system
continues to be powered by the engine even if the main
drive shaft breaks. In this case, when the engine
unloads, a tail rotor overspeed can result. If this hap-
pens, close the throttle immediately and enter an
autorotation.
HYDRAULIC FAILURES
Most helicopters, other than smaller piston powered
helicopters, incorporate the use of hydraulic actuators
to overcome high control forces. A hydraulic system
consists of actuators, also called servos, on each flight
control; a pump, which is usually driven by the main
rotor gearbox; and a reservoir to store the hydraulic
fluid. A switch in the cockpit can turn the system off,
although it is left on under normal conditions. A
pressure indicator in the cockpit may be installed to
monitor the system.
An impending hydraulic failure can be recognized by a
grinding or howling noise from the pump or actuators,
increased control forces and feedback, and limited
control movement. The corrective action required is
stated in detail in the appropriate rotorcraft flight
manual. However, in most cases, airspeed needs to be
reduced in order to reduce control forces. The hydraulic
switch and circuit breaker should be checked and
recycled. If hydraulic power is not restored, make a
shallow approach to a running or roll-on landing. This
technique is used because it requires less control force
and pilot workload. Additionally, the hydraulic system
should be disabled, by either pulling the circuit breaker
and/or placing the switch in the off position. The
reason for this is to prevent an inadvertent restoration
of hydraulic power, which may lead to overcontrolling
near the ground.
In those helicopters where the control forces are so
high that they cannot be moved without hydraulic
assistance, two or more independent hydraulic systems
may be installed. Some helicopters use hydraulic accu-
mulators to store pressure that can be used for a short
time while in an emergency if the hydraulic pump fails.
This gives you enough time to land the helicopter with
normal control.
GOVERNOR FAILURE
Governors automatically adjust engine power to main-
tain rotor r.p.m. when the collective pitch is changed. If
the governor fails, any change in collective pitch
requires you to manually adjust the throttle to maintain
correct r.p.m. In the event of a high side governor
failure, the engine and rotor r.p.m. try to increase above
the normal range. If the r.p.m. cannot be reduced and
controlled with the throttle, close the throttle and enter
an autorotation. If the governor fails on the low side,
normal r.p.m. may not be attainable, even if the throttle
is manually controlled. In this case, the collective has
to be lowered to maintain r.p.m. A running or roll-on
landing may be performed if the engine can maintain
sufficient rotor r.p.m. If there is insufficient power,
enter an autorotation.
ABNORMAL VIBRATIONS
With the many rotating parts found in helicopters, some
vibration is inherent. You need to understand the cause
and effect of helicopter vibrations because abnormal
vibrations cause premature component wear and may
even result in structural failure. With experience, you
learn what vibrations are normal versus those that are
abnormal and can then decide whether continued flight
is safe or not. Helicopter vibrations are categorized into
low, medium, or high frequency.
LOW FREQUENCY VIBRATIONS
Low frequency vibrations (100-500 cycles per minute)
usually originate from the main rotor system. The
vibration may be felt through the controls, the airframe,
or a combination of both. Furthermore, the vibration
may have a definite direction of push or thrust. It may
be vertical, lateral, horizontal, or even a combination.
Normally, the direction of the vibration can be deter-
mined by concentrating on the feel of the vibration,
which may push you up and down, backwards and
forwards, or from side to side. The direction of the
vibration and whether it is felt in the controls or the
airframe is an important means for the mechanic
to troubleshoot the source. Some possible causes
could be that the main rotor blades are out of track or
balance, damaged blades, worn bearings, dampers out
of adjustment, or worn parts.
MEDIUM AND HIGH FREQUENCY VIBRATIONS
Medium frequency vibrations (1,000 - 2,000 cycles per
minute) and high frequency vibrations (2,000 cycles
per minute or higher) are normally associated with out-
of-balance components that rotate at a high r.p.m., such
as the tail rotor, engine, cooling fans, and components
of the drive train, including transmissions, drive shafts,
bearings, pulleys, and belts. Most tail rotor vibrations
can be felt through the tail rotor pedals as long as there
are no hydraulic actuators, which usually dampen out
the vibration. Any imbalance in the tail rotor system is
very harmful, as it can cause cracks to develop and
rivets to work loose. Piston engines usually produce a
normal amount of high frequency vibration, which is
aggravated by engine malfunctions such as spark plug
fouling, incorrect magneto timing, carburetor icing
and/or incorrect fuel/air mixture. Vibrations in turbine
engines are often difficult to detect as these engines
operate at a very high r.p.m.
TRACKING AND BALANCE
Modern equipment used for tracking and balancing the
main and tail rotor blades can also be used to detect
other vibrations in the helicopter. These systems use
accelerometers mounted around the helicopter to detect
the direction, frequency, and intensity of the vibration.
The built-in software can then analyze the information,
pinpoint the origin of the vibration, and suggest the
corrective action.
FLIGHT DIVERSION
There will probably come a time in your flight career
when you will not be able to make it to your destination.
This can be the result of unpredictable weather conditions,
a system malfunction, or poor preflight planning. In any
case, you will need to be able to safely and efficiently
divert to an alternate destination. Before any cross-
country flight, check the charts for airports or suitable
landing areas along or near your route of flight. Also,
check for navaids that can be used during a diversion.
Computing course, time, speed, and distance informa-
tion in flight requires the same computations used
during preflight planning. However, because of the
limited cockpit space, and because you must divide
your attention between flying the helicopter, making
calculations, and scanning for other aircraft, you should
take advantage of all possible shortcuts and rule-of-
thumb computations.
When in flight, it is rarely practical to actually plot a
course on a sectional chart and mark checkpoints and
distances. Furthermore, because an alternate airport is
usually not very far from your original course, actual
plotting is seldom necessary.
A course to an alternate can be measured accurately
with a protractor or plotter, but can also be measured
with reasonable accuracy using a straightedge and the
compass rose depicted around VOR stations. This
approximation can be made on the basis of a radial
from a nearby VOR or an airway that closely parallels
the course to your alternate. However, you must
remember that the magnetic heading associated with
a VOR radial or printed airway is outbound from
the station. To find the course TO the station, it may
be necessary to determine the reciprocal of the
indicated heading.
Distances can be determined by using a plotter, or by
placing a finger or piece of paper between the two and
then measuring the approximate distance on the
mileage scale at the bottom of the chart.
Before changing course to proceed to an alternate, you
should first consider the relative distance and route of
flight to all suitable alternates. In addition, you should
consider the type of terrain along the route. If circum-
stances warrant, and your helicopter is equipped with
navigational equipment, it is typically easier to navi-
gate to an alternate airport that has a VOR or NDB
facility on the field.
After you select the most appropriate alternate, approx-
imate the magnetic course to the alternate using
a compass rose or airway on the sectional chart. If time
permits, try to start the diversion over a prominent
ground feature. However, in an emergency, divert
promptly toward your alternate. To complete all
plotting, measuring, and computations involved before
diverting to the alternate may only aggravate an
actual emergency.
Once established on course, note the time, and then
use the winds aloft nearest to your diversion point to
calculate a heading and groundspeed. Once you have
calculated your groundspeed, determine a new arrival
time and fuel consumption.