Which flight regime would create conditions that are more susceptible to mast bumping in a helicopter equipped with a teetering rotor head?
Mast bumping is a dangerous situation that mainly occurs on helicopters with teetering rotor heads that may come into contact with the main rotor mast under certain conditions.
It is characterised by an unusual airframe vibration with the sound of sharp metal-to-metal contact.
Mast bumping can occur due to various factors, including:
- Low G (nose down) manoeuvres such as an immediate dive after a steep climb or during a pushover from moderate or high airspeed, as the helicopter noses over.
- Turbulence, especially severe downdrafts.
- Abrupt corrective actions, opposite to the fuselage angle, left or right cyclic inputs.
Pilots should prevent mast bumping accidents by:
- Avoiding abrupt cyclic inputs.
- Recognizing the weightless feeling associated with the onset of low G and quickly take corrective action before the situation becomes critical.
- Recovering from a low-G situation by first gently applying aft cyclic to restore normal G condition before attempting to correct any roll.
- Reducing power and using a slower than normal cruise speed if turbulence is expected or encountered.
Therefore, level flight in turbulence conditions would create conditions that are more susceptible to mast bumping.
NOTES:
- During a flight in turbulence, momentary excursions in airspeed, altitude, and attitude are to be expected. Pilots should respond with smooth, gentle control inputs and avoid over controlling.
- Mast bumping is less likely at lower airspeeds.
- Multi-bladed rotors may experience a phenomenon similar to mast bumping known as “droop stop pounding”
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