What happened
On 15 November 2004, a Robinson R22 Beta, registration G-DERB, was conducting a training flight from Manston to Biggin Hill. During the flight, the pilot noted an unusual vibration. After landing safely at Biggin Hill, the crew decided to hover taxi to a nearby maintenance facility. During this taxiing maneuver, the vibration became intense, prompting the pilot to land and shut down the engine.
Upon inspection, investigators found that one of the main rotor blades had developed a crack extending from the trailing edge through approximately 75% of its chord, reaching as far as the spar at the one-third span position. There were no injuries to the two crew members on board.
The investigation
The AAIB examined the rotor system and discovered that the teeter hinge was extremely stiff. The investigation revealed that the main rotor head, blades, and spindles had been reassembled approximately 20 hours prior to the incident. During this maintenance, a mechanic working under supervision had assembled the head without the necessary shim washers because none were in stock.
Although the initial assembly was rejected due to stiffness, a second assembly was performed using the same components. The investigation found that the teeter bolt's cadmium plating had been damaged and the thrust washers and bushes were deformed. This caused the teeter motion to occur between the bolt and the bushes rather than between the bushes and the head, significantly increasing friction.
Findings
- The stiff teeter hinge was the primary cause of the blade failure.
- The assembly of the rotor head without required shims caused the thrust washers to apply excessive loads directly to the teeter head.
- The lack of shims resulted in the teeter motion occurring between the teeter bolt and its bushes, leading to high friction and the displacement of soft cadmium plating.
- The resulting mechanical constraint forced the blades to absorb stresses via the coning hinges, inducing high-stress loads through the Coriolis effect.
- The fatigue crack in the blade originated from these higher than normal stresses rather than from corrosion or material defects.