What happened
On 30 June 1999, a SA314G Gazelle 1, registration G-PYOB, was performing a private flight near Braintree. Following a routine landing and shutdown on a concrete helipad, the pilot applied the rotor brake. As the rotor blades were decelerating, a loud noise occurred, and the main rotor blades dropped significantly toward the ground. During this movement, one blade made contact with the tail boom and the engine exhaust. The incident resulted in no injuries to the pilot, but caused damage to the tail boom, the drive train, and the main rotor blades.
The investigation
Investigators examined the main rotor head assembly and the Low Stop Ring supports. The Low Stop Ring is held by three supports located beneath each blade mounting. The investigation found that two of these three supports had failed, which allowed the ring to shift downwards. This displacement meant the rotor blade mounted stops no longer functioned correctly, permitting the blades to droop excessively.
Detailed analysis of the recovered supports revealed that the upper and lower flanges of the 'Yellow' support had suffered static ductile bending fractures. The 'Red' support also experienced a failure in the lower tab via static ductile bending. While the material properties and hardness of the components met all required specifications, measurements showed that the thickness of the flanges had been reduced. Specifically, the 'Blue' support's lower flange had thinned from a nominal 2.0 mm to 1.5 mm.
Findings
- The primary cause of the incident was the failure of two Low Stop Ring supports, which allowed the ring to move out of position.
- The failure was characterized by static ductile bending of the support flanges.
- Evidence suggests that wear had reduced the thickness of the support flanges over time.
- A historical precedent indicated that operational practices, such as tilting the rotor disc to facilitate passenger boarding while rotors were turning, could accelerate wear on these components by causing the ring to move radially against the supports.