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and altitude are critical.
c. Partial Power Condition. Under partial power
conditions, the engine may operate smoothly at
reduced power or it may operate erratically with
intermittent surges of power. In instances where a
power loss is experienced without accompanying power
surging, the helicopter may be flown at reduced power
to a favorable landing area. Under this condition, the
pilot should always be prepared for a complete power
9-10
loss. In the event a partial power condition is
accompanied by erratic engine operation or power
surging, and flight is to be continued, the throttle may
be adjusted in an attempt to correct the surging
condition. If flight is not possible, close the throttle
completely and complete an autorotational landing.
d. Complete Power Loss. When the crew
experiences an engine failure, immediate action is
required to make a safe autorotative descent and
landing. Flight conducted within the caution area of the
height velocity diagram (fig. 9-2) exposes the helicopter
to a high probability of damage despite the best effort
of the pilot.
The altitude and airspeed at which the engine failure
occurs will dictate the action to be taken. After the
failure, main rotor RPM will decay rapidly and the
aircraft will yaw to the left and begin to descend. At
gross weights above 5000 Ibs, serious consideration
should be given to jettisoning the armament stores.
During cruise at airspeeds to VNE, reduce collective
immediately to regain NR and then adjust as necessary
to maintain rotor RPM. The collective should be
adjusted as necessary to maintain rotor RPM. The
cyclic should be adjusted as necessary to attain and
maintain airspeed in the optimum range (fig. 9-3).
normally between 55 and 80 KIAS. Sixty-five knots
provides an optimum balance between minimum rate of
descent airspeed and that required for maximum glide.
A landing area must be selected immediately after an
engine failure and control inputs made to fly to the
intended site. Every effort must be made to land in an
area that will afford a safe touchdown into the wind, as
wind direction will have a pronounced effect on
autorotational success.
Throughout the descent, adjust collective as necessary
to maintain NR within normal range. At high gross
weights, the rotor will rapidly overspeed and the
collective must be used to maintain the desired rotor
RPM (fig. 5-1). NR should be maintained at or slightly
above 100% to allow ample rpm before touchdown, and
desired trim maintained by pedals. Main rotor rpm will
increase momentarily when the cyclic is moved aft with
no change in collective pitch setting. An autorotative
rpm of approximately 100% provides for a good rate of
descent. NR significantly above or below 100% will
result in a higher than desired rate of descent.
At approximately 100 feet AGL, use cyclic to
decelerate. This reduces airspeed and rate of descent
and causes an increase in NR. The degree of increase
depends upon the amount and rate of deceleration. An
increase in NR can be beneficial at this point. More
inertial energy in the rotor system will be available to
be used to cushion the landing.
Ground contact should be made with minimum forward
speed. Some ground run is permissible when landing to
a prepared surface (runway or hard-surfaced road), but
if the only available landing area is a rough sod field, a
more pronounced deceleration is necessary and
touchdown speed should approach zero. In that case, it
is necessary to decelerate at a higher altitude and
assume a landing attitude from a vertical descent.
Initial collective pitch-application may vary from 15-25
feet AGL, depending upon rate of descent and landing
attitude. All available collective pitch should be used to
effect the softest landing possible. A hard landing
should be expected when operating at high gross
weights or high density altitude.
e. Low Airspeed and Low Altitude. Under low
airspeed and low altitude conditions, the deceleration
capability is limited. Initial collective reduction may vary
after an engine malfunction, dependent upon the
TM 1-1520-248-10
altitude and airspeed at the time of occurrence. For
example, collective pitch must not be decreased when
an engine failure occurs at a hover in ground effect;
whereas, during cruise flight conditions, altitude and
airspeed are sufficient for a significant reduction in
collective pitch, thereby allowing rotor rpm to be
maintained in the safe operating range during
autorotational descent.
Pilots should avoid using the collective pitch to extend
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OPERATOR’S MANUAL FOR ARMY OH-58D HELICOPTER 2(87)
