• Wing flaps in the takeoff position. For most
twins, this will be 0° of flaps.• Cowl flaps in the takeoff position.
• Airplane trimmed for takeoff.
• Airplane airborne and the ground effect negligible.
• Maximum of 5° angle of bank. Vmc is highly
sensitive to bank angle. To prevent claims of
an unrealistically low Vmc speed in aircraft
certification, the manufacturer is permitted to
use a maximum of a 5° bank angle toward the
operative engine. The horizontal component of
lift generated by the bank assists the rudder in
counteracting the asymmetrical thrust of the
operative engine. The bank angle works in the
manufacturer's favor in lowering Vmc.
Vmc is reduced significantly with increases in bank
angle. Conversely, Vmc increases significantly with
decreases in bank angle. Tests have shown that Vmc
may increase more than 3 knots for each degree of
bank angle less than 5°. Loss of directional control
may be experienced at speeds almost 20 knots above
published Vmc when the wings are held level.
The 5° bank angle maximum is a regulatory limit
imposed upon manufacturers in aircraft certification.
The 5° bank does not inherently establish zero sideslip
or best single-engine climb performance. Zero sideslip,
and therefore best single-engine climb performance,
occurs at bank angles significantly less than 5°. The
determination of Vmc in certification is solely concerned
with the minimum speed for directional control
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
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.
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