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under a very specific set of circumstances, and has
nothing to do with climb performance, nor is it the
optimum airplane attitude or configuration for climb
performance.
During dynamic VMC determination in aircraft certification,
cuts of the critical engine using the mixture
control are performed by flight test pilots while
gradually reducing the speed with each attempt. VMC
is the minimum speed at which directional control
could be maintained within 20° of the original entry
heading when a cut of the critical engine was made.
During such tests, the climb angle with both engines
operating was high, and the pitch attitude following
the engine cut had to be quickly lowered to regain
the initial speed. Pilots should never attempt to
demonstrate VMC with an engine cut from high
power, and never intentionally fail an engine at
speeds less than VSSE.
The actual demonstration of VMC and recovery in flight
training more closely resembles static VMC determination
in aircraft certification. For a demonstration,
the pilot should select an altitude that will allow
completion of the maneuver at least 3,000 feet AGL.
The following description assumes a twin with
noncounter-rotating engines, where the left engine
is critical.
With the landing gear retracted and the flaps set to the
takeoff position, the airplane should be slowed to
approximately 10 knots above VSSE or VYSE
(whichever is higher) and trimmed for takeoff. For the
remainder of the maneuver, the trim setting should not
be altered. An entry heading should be selected and
high r.p.m. set on both propeller controls. Power on the
left engine should be throttled back to idle as the right
engine power is advanced to the takeoff setting. The
landing gear warning horn will sound as long as a
Figure 12-20. Effect of CG location on yaw.
A
B
Inoperative
Engine
Operative
Engine
B x R = A x T
Inoperative
Engine
Operative
Engine
A
R
B
R
T T
Ch 12.qxd 5/7/04 9:55 AM Page 12-29
12-30
throttle is retarded. The pilots should continue to carefully
listen, however, for the stall warning horn, if so
equipped, or watch for the stall warning light. The left
yawing and rolling moment of the asymmetrical thrust
is counteracted primarily with right rudder. A bank
angle of 5° (a right bank, in this case) should also be
established.
While maintaining entry heading, the pitch attitude is
slowly increased to decelerate at a rate of 1 knot per
second (no faster). As the airplane slows and control
effectivity decays, the increasing yawing tendency
should be counteracted with additional rudder pressure.
Aileron displacement will also increase in order
to maintain 5° of bank. An airspeed is soon reached
where full right rudder travel and a 5° right bank can
no longer counteract the asymmetrical thrust, and the
airplane will begin to yaw uncontrollably to the left.
The moment the pilot first recognizes the uncontrollable
yaw, or experiences any symptom associated
with a stall, the operating engine throttle should be
sufficiently retarded to stop the yaw as the pitch
attitude is decreased. Recovery is made with a minimum
loss of altitude to straight flight on the entry heading at
VSSE or VYSE, before setting symmetrical power. The
recovery should not be attempted by increasing power
on the windmilling engine alone.
To keep the foregoing description simple, there were
several important background details that were not
covered. The rudder pressure during the demonstration
can be quite high. In certification, 150 pounds of force
is permitted before the limiting factor becomes rudder
pressure, not rudder travel. Most twins will run out of
rudder travel long before 150 pounds of pressure is
required. Still, it will seem considerable.
Maintaining altitude is not a criterion in accomplishing
this maneuver. This is a demonstration of
controllability, not performance. Many airplanes will
lose (or gain) altitude during the demonstration. Begin
the maneuver at an altitude sufficient to allow completion
by 3,000 feet AGL.
As discussed earlier, with normally aspirated engines,
VMC decreases with altitude. Stalling speed (VS),
however, remains the same. Except for a few models,
published VMC is almost always higher than VS. At
sea level, there is usually a margin of several knots
between VMC and VS, but the margin decreases with
altitude, and at some altitude, VMC and VS are the
same. [Figure 12-21]
Should a stall occur while the airplane is under asymmetrical
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