Rotorcraft Flying Handbook, FAA-H-8083-21

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 autorotations,
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 helicopters
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 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. Leave the collective
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 throttle
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 collective
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 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 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 movement
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 airspeeds
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 cushion
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.
11-4