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.
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
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
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
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.
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.
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
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
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
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
ensure that the nose does not continue yawing to the right. If
a go-around is required, increasing the collective too much or
too rapidly with airspeeds below translational lift may cause
a rapid spinning to the right.
Once the helicopter has landed and is sliding/rolling to a
stop, the heading can be controlled with a combination of
collective, cyclic and throttle. To turn the nose to the right,
raise the collective or apply aft cyclic. The throttle may be
increased as well if it is not in the full open position. To turn
the nose to the left, lower the collective or apply forward
cyclic. The throttle may be decreased as well if it is not
already at flight idle.
Loss of Tail Rotor Effectiveness (LTE)
Loss of tail rotor effectiveness (LTE) or an unanticipated
yaw is defined as an uncommanded, rapid yaw towards the
advancing blade which does not subside of its own accord.
It can result in the loss of the aircraft if left unchecked. It is
very important for pilots to understand that LTE is caused
by an aerodynamic interaction between the main rotor and
tail rotor and not caused from a mechanical failure. Some
helicopter types are more likely to encounter LTE due to the
normal certification thrust produced by having a tail rotor
that, although meeting certification standards, is not always
able to produce the additional thrust demanded by the pilot.
A helicopter is a collection of compromises. Compare the
size of an airplane propeller to that of a tail rotor. Then,
consider the horsepower required to run the propeller. For
example, a Cessna 172P is equipped with a 160-horsepower
(HP) engine. A Robinson R-44 with a comparably sized tail
rotor is rated for a maximum of 245 HP. If you assume the
tail rotor consumes 50 HP, only 195 HP remains to drive
the main rotor. If the pilot were to apply enough collective
to require 215 HP from the engine, and enough left pedal to
require 50 HP for the tail rotor, the resulting engine overload
would lead to one of two outcomes: slow down (reduction
in rpm) or premature failure. In either outcome, antitorque
would be insufficient and total lift might be less than needed
to remain airborne.
Every helicopter design requires some type of antitorque
system to counteract main rotor torque and prevent spinning
once the helicopter lifts off the ground. A helicopter is heavy,
and the powerplant places a high demand on fuel. Weight
penalizes performance, but all helicopters must have an
antitorque system, which adds weight. Therefore, the tail
rotor is certified for normal flight conditions. Environmental
forces can overwhelm any aircraft, rendering the inherently
unstable helicopter especially vulnerable.
As with any aerodynamic condition, it is very important for
pilots to not only to understand the definition of LTE, but
more importantly, how and why it happens, how to avoid
it, and lastly, how to correct it once it is encountered. We
must first understand the capabilities of the aircraft or even
better what it is not capable of doing. For example, if you
were flying a helicopter with a maximum gross weight of
5,200 lb, would you knowingly try to take on fuel, baggage
and passengers causing the weight to be 5,500 lb? A wise
professional pilot should not ever exceed the certificated
maximum gross weight or performance flight weight for any
aircraft. The manuals are written for safety and reliability.
The limitations and emergency procedures are stressed
because lapses in procedures or exceeding limitations can
result in aircraft damage or human fatalities. At the very least,
exceeding limitations will increase the costs of maintenance
and ownership of any aircraft and especially helicopters.
Overloaded parts may fail before their designed lifetime. There
are no extra parts in helicopters. The respect and discipline
pilots exercise in following flight manuals should also be
applied to understanding aerodynamic conditions. If flight
envelopes are exceeded, the end results can be catastrophic.
LTE is an aerodynamic condition and is the result of a control
margin deficiency in the tail rotor. It can affect all single-rotor
helicopters that utilize a tail rotor. The design of main and
tail rotor blades and the tail boom assembly can affect the
characteristics and susceptibility of LTE but will not nullify
the phenomenon entirely. Translational lift is obtained by
any amount of clean air through the main rotor disk. Chapter
2, Aerodynamics of Flight, discusses translational lift with
respect to the main rotor blade, explaining that the more
clean air there is going through the rotor disk, the more
efficient it becomes. The same holds true for the tail rotor.
As the tail rotor works in less turbulent air, it reaches a point
of translational thrust. At this point, the tail rotor becomes
aerodynamically efficient and the improved efficiency
produces more antitorque thrust. The pilot can determine
when the tail rotor has reached translational thrust. As more
antitorque thrust is produced, the nose of the helicopter
yaws to the left (opposite direction of the tail rotor thrust),
forcing the pilot to correct with right pedal application
(actually decreasing the left pedal). This, in turn, decreases
the AOA in the tail rotor blades. Pilots should be aware of the
characteristics of the helicopter they fly and be particularly
aware of the amount of tail rotor pedal typically required for
different flight conditions.
LTE is a condition that occurs when the flow of air through
a tail rotor is altered in some way, by altering the angle or
speed at which the air passes through the rotating blades of
the tail rotor disk. As discussed in the previous paragraph, an
effective tail rotor relies on a stable and relatively undisturbed