PILOT INDUCED OSCILLATIONS

The pilot may purposely induce various motions to the airplane by the action of the controls. In addition, certain undesirable motions may occur due to inadvertent action on the controls. The most important con­dition exists with the short period longitu­dinal motion of the airplane where pilot – control system response lag can produce an unstable oscillation. The coupling possible in the pilot-control system-airplane combi­nation is most certainly capable of producing damaging flight loads and loss of control of the airplane.

When the normal human response lag and control system lag are coupled with the air­plane motion, inadvertent control reactions by the pilot may furnish a negative damping to the oscillatory motion and dynamic in­stability exists. Since the short period motion is of relatively high frequency, the amplitude of the pitching oscillation can reach dangerous proportions in an unbelievably short time. When the pilot induced oscillation is en­countered, the most effective solution is an immediate release of the controls. Any at­tempt to forcibly damp the oscillation simply continues the excitation and amplifies the oscillation. Freeing the controls removes the unstable (but inadvertent) excitation and allows the airplane to recover by virtue of its inherent dynamic stability.

The pilot induced oscillation is most likely under certain conditions. Most obvious is the case of the pilot unfamiliar with the “feel’’ of the airplane and likely to overcontrol or have excessive response lag. High speed flight at low altitude (high q) is most likely to provide low stick-force gradients and periods of oscillation which coincide with the pilot – control system response lag. Also, the high ^ flight condition provides the aerodynamic capability for failing flight loads during the oscillation.

If a pilot induced oscillation is encountered the pilot must rely on the inherent dynamic stability of the airplane and immediately release the controls. If the unstable excitation is continued, dangerous oscillation amplitudes will develop in a very short time.