11
Reduction in directional stability at high power settings
The conditions where the reduction in directional stability are most likely to occur are:
Low speed (aerodynamic controls are less effective, control feel is a function of dynamic
pressure)
Flaps 50% (high rudder boost, allows generation of larger side slip angles)
Gear up (Landing gear down is stabilizing)
High power settings (more momentum change across the propeller disk, larger
destabilizing normal force)
Left rudder pedal inputs, right side slip
The primary indication to the pilot of reduced directional stability is the reduction in rudder
pedal force gradient. In some cases only 25 pounds of rudder force are needed to command 25
degrees of side slip. The cockpit side forces that might provide warning to the pilot of high side
slip are too low to be noticeable at the low airspeeds where rudder force lightening is most likely
to happen.
To avoid over control, excessive side slip angles and rudder overbalance (reversal in rudder
pedal forces, aka “rudder lock”), the pilot must anticipate and recognize the low rudder force
gradient. The pilot will experience the rudder force lightening when the rudder pedals begin to
move easily and side slip continues to increase. If the pilot continues with rudder pedal input,
the rudder pedal force will continue to decrease until the rudder floats towards full deflection by
itself. The pilot experiences this as a reversal in rudder pedal force and it is called rudder
overbalance. If rudder overbalance occurs, neutralizing the rudder pedals will not recover the
aircraft. The pilot must actively push on the opposite rudder to bring the rudder back towards
center. If the rudder is not promptly centered, the airplane can reach an extreme side slip, roll to
a high bank angle, and depart controlled flight.
