What happened
On 2 May 2008, an Agusta A109A II helicopter, registration G-ELTE, was conducting a private passenger flight from Manchester to London. During the cruise phase of the flight, the commander noticed that the landing gear lever felt unusual and began to rotate in his hand when he attempted to select the gear-up position.
Upon further investigation by the co-pilot, it was discovered that the handle and spindle had become detached from the selector mechanism. The crew found that the threaded end of the spindle could not be reattached to the lever, and the landing gear could not be lowered using the manual lever. As fuel levels became critically low, the commander decided to divert to Redhill Aerodrome, where maintenance personnel were available.
To mitigate the risk of a rollover due to the gear-up configuration, the commander opted to disembark the four passengers while the helicopter was in a low hover. Following the disembarkation, the commander executed a landing on two parallel lines of pre-positioned car tyres. The landing resulted in minor damage to the aircraft, and there were no injuries to the crew or passengers.
The investigation
An AAIB field investigation examined the landing gear selector assembly. The investigation established that the spring circlip, which is responsible for retaining the locking collar, had moved out of its locating groove. This displacement allowed the locking collar to move longitudinally, which in turn permitted the handle and spindle to rotate freely and eventually unthread from the lever.
Testing by the component manufacturer demonstrated that the failure could occur after a certain number of operating cycles if the circlip was not correctly seated. The investigation also found that the specialized tooling used by the manufacturer to install the circlip was heavily damaged, which likely hindered the correct alignment of the component during the previous overhaul.
Findings
- The landing gear selector failure was caused by the dislodgement of the spring circlip from its retaining groove.
- The displacement of the circlip allowed the handle and spindle to rotate, leading the spindle to unthread from the lever.
- It is probable that the circlip was incorrectly positioned during the last assembly of the selector mechanism.
- The installation of the circlip was made difficult by the degraded condition of the manufacturer's assembly tools.