mast bumping

mast bumping

A phenomenon to which helicopters fitted with semirigid rotor systems, teetering types, and underslung types are susceptible. In this phenomenon, the fuselage begins to right roll in a low-g or zero-g condition. If a lateral cyclic is applied to recover from the right roll, the mast bumps against the hub and may break in the process. The main rotor also can hit the tail cone. Mast bumping is caused by an incorrect pilot response to an abrupt, unexpected change in the helicopter's pitch-and-roll attitude. These changes may be because of a low-g maneuver, engine failure, synchronized elevator failure, or tail rotor failure. Mast bumping can be avoided by making smooth, gradual control movements by the cyclic aft to recover low-g conditions. Thereafter, the lateral cyclic should be applied to recover from the right roll.
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Dynamic rollover, mast bumping, damage to the rotors or landing gear, and injuries to the crew are just some of the consequences.
Bell has long known that its teeter-rotor system is susceptible to mast bumping.
Despite Bell's claim that pilot shenanigans were causing most mast bumping, an Army study revealed that more than half of the mast-bumping accidents occurred when the helicopters were in "normal cruise flight.
Agreeing that mast bumping just might be the result of pilots pushing their helicopters beyond their designed capabilities, Bell and the Army toyed for the next few years with several fixes that would protect Army pilots and their passengers from mast bumping--for which Bell accepted a string of lucrative Pentagon contracts.
In the summer of 1983, 18 years after the first recorded Bell mast bumping accident, a respected pilot, Major Larry B.
In July 1984, the investigators recommended nearly two dozen changes to help curtail mast bumping.
Following the decision, a juror said the panel had searched in vain for a clear and unmistakable warning of the mast bumping problem from Bell to the Army.