Sunday, February 15, 2009

Cross-Control Stall

The objective of a cross-control stall demonstration
maneuver is to show the effect of improper control
technique and to emphasize the importance of using
coordinated control pressures whenever making turns.
This type of stall occurs with the controls crossed—
aileron pressure applied in one direction and rudder
pressure in the opposite direction.

In addition, when excessive back-elevator pressure is
applied, a cross-control stall may result. This is a stall
that is most apt to occur during a poorly planned and
executed base-to-final approach turn, and often is the
result of overshooting the centerline of the runway
during that turn. Normally, the proper action to correct
for overshooting the runway is to increase the rate of
turn by using coordinated aileron and rudder. At the
relatively low altitude of a base-to-final approach turn,
improperly trained pilots may be apprehensive of
steepening the bank to increase the rate of turn, and
rather than steepening the bank, they hold the bank
constant and attempt to increase the rate of turn by
adding more rudder pressure in an effort to align it
with the runway.

The addition of inside rudder pressure will cause the
speed of the outer wing to increase, therefore, creating
greater lift on that wing. To keep that wing from rising
and to maintain a constant angle of bank, opposite
aileron pressure needs to be applied. The added inside
rudder pressure will also cause the nose to lower in
relation to the horizon. Consequently, additional
back-elevator pressure would be required to maintain a
constant-pitch attitude. The resulting condition is a
turn with rudder applied in one direction, aileron in the
opposite direction, and excessive back-elevator
pressure—a pronounced cross-control condition.

Since the airplane is in a skidding turn during the
cross-control condition, the wing on the outside of the
turn speeds up and produces more lift than the inside
wing; thus, the airplane starts to increase its bank. The
down aileron on the inside of the turn helps drag that
wing back, slowing it up and decreasing its lift, which
requires more aileron application. This further causes
the airplane to roll. The roll may be so fast that it is
possible the bank will be vertical or past vertical before
it can be stopped.

For the d

emonstration of the maneuver, it is important
that it be entered at a safe altitude because of the
possible extreme nosedown attitude and loss of
altitude that may result.

Before demonstrating this stall, the pilot should clear
the area for other air traffic while slowly retarding the
throttle. Then the landing gear (if retractable gear)
should be lowered, the throttle closed, and the altitude
maintained until the airspeed approaches the normal
glide speed. Because of the possibility of exceeding
the airplane's limitations, flaps should not be extended.
While the gliding attitude and airspeed are being
established, the airplane should be retrimmed. When
the glide is stabilized, the airplane should be rolled into
a medium-banked turn to simulate a final approach
turn that would overshoot the centerline of the runway.

During the turn, excessive rudder pressure should be
applied in the direction of the turn but the bank held
constant by applying opposite aileron pressure. At the
same time, increased back-elevator pressure is
required to keep the nose from lowering.

All of these control pressures should be increased until
the airplane stalls. When the stall occurs, recovery is
made by releasing the control pressures and increasing
power as necessary to recover.

In a cross-control stall, the airplane often stalls with
little warning. The nose may pitch down, the inside
wing may suddenly drop, and the airplane may
continue to roll to an inverted position. This is usually
the beginning of a spin. It is obvious that close to the
ground is no place to allow this to happen.

Recovery must be made before the airplane enters an
abnormal attitude (vertical spiral or spin); it is a simple
matter to return to straight-and-level flight by
coordinated use of the controls. The pilot must be able
to recognize when this stall is imminent and must take
immediate action to prevent a completely stalled
condition. It is imperative that this type of stall not
occur during an actual approach to a landing, since
recovery may be impossible prior to ground contact
due to the low altitude.

The flight instructor should be aware that during traffic
pattern operations, any conditions that result in
overshooting the turn from base leg to final approach,
dramatically increases the possibility of an
unintentional accelerated stall while the airplane is in a
cross-control condition.

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