collective bounce

A phenomenon in which the collective is in resonance with the vertical motion of the helicopter, making the motion worse. It is a divergent, vertical helicopter oscillation caused by interaction between the vertical motion of the helicopter and the pilot’s body, with the left arm moving the collective pitch stick control. The amplitude of bouncing oscillation increases with each up-and-down movement. This rapid motion, occurring at about 3 Hz, if not immediately corrected, might cause the helicopter to crash into the ground on a down bounce, might cause the pilot to lose control of the helicopter, or cause severe structural damage. Collective bounce usually occurs during transition to or from a hover when the helicopter is at a high gross weight with either an internal or an external load. Some of the recovery methods include: letting go of the collective pitch stick; transfering flight controls to the other pilot; freezing the position of the collective; bending the elbow so that the pilot’s arm does not provide a rigid connection between the fuselage motion and the collective; jettisoning any external stores or loads; or tightening the friction nut on the collective pitch stick.