What happened
On 3 June 2010, a Diamond Aircraft Industries DA42 Twin Star, registration G-SLCT, was conducting a training flight when the pilot experienced a landing gear malfunction. During the approach to Cranfield, the landing gear unsafe light illuminated, and the position indicator for the right main gear remained extinguished.
In an attempt to rectify the issue, the pilot cycled the landing gear several times and performed high-g manoeuvres to dislodge the mechanism. After these efforts failed, the pilot returned to the home airfield at Stapleford. Following several unsuccessful touch-and-go landings, the pilot attempted a landing on Runway 22R. During the landing roll, the pilot attempted to keep the right wing elevated for as long as possible while maintaining control with the rudder and brakes. Eventually, the wing settled, causing the aircraft to yaw to the right and come to rest in a field of crops. The pilot and passenger escaped without injury, but the aircraft was extensively damaged.
The investigation
An investigation by the AAIB focused on the right landing gear assembly. Metallurgical examination of the trunnion, which connects the landing gear damper to the wheel trailing arm, revealed a fracture running through the 14 mm bore. This failure caused the trailing arm to hang lower than normal, which subsequently jammed the gear within the wheel well.
Investigators found evidence of secondary cracking and exfoliation corrosion on the trunnion. The analysis determined that the failure originated from a corrosion pit within the 14 mm bore, likely caused by galvanic corrosion between the attachment bolt and the bore walls where the protective anodised layer had been compromised. The material used for the trunnion was noted to be particularly vulnerable to environmental degradation.
Findings
- The right main landing gear failed to extend because the trunnion was jammed in the wheel well.
- The failure was caused by stress corrosion cracking (SCC) within the trunnion.
- The cracking likely initiated at a corrosion pit in the 14 mm bore where the protective anodising had been damaged, possibly by the movement of the attachment bolt.
- The aluminium alloy used in the trunnion is susceptible to SCC, especially when exposed to moisture and dirt thrown up by the wheels.
Safety action
Following the accident, the AAIB issued three safety recommendations to the aircraft manufacturer regarding the inspection, lubrication, and design of the trunnion components to reduce susceptibility to corrosion and cracking.