Accident
On 3 June 2010 at 0900 UTC, a Diamond Aircraft Industries DA42 Twin Star (registration G-SLCT) was on a training flight from Stapleford Airfield, Essex. The pilot, aged 38 with 2,276 hours total flying experience (58 on type), reported that during the approach to Cranfield, the landing gear unsafe light illuminated and the position indicator for the right main landing gear remained extinguished. Despite cycling the landing gear several times, the indications persisted. The air traffic controller confirmed that the right main gear had not extended. The pilot returned to Stapleford and performed high-g manoeuvres and touch-and-go landings in an attempt to dislodge the gear, but without success. A landing was then made on Runway 22R. The pilot held the right wing up as long as possible during the landing roll while maintaining directional control with the rudder and brakes. The wing eventually settled on the runway, and when directional control was lost, the aircraft yawed to the right and came to rest in a field of crops on a heading of approximately 270°. The pilot and passenger were uninjured, but the aircraft was extensively damaged.
Investigation
The AAIB investigation established that the right landing gear had jammed in the wheel well due to the failure of a trunnion (Part No D60-3217-23-51) that connected the landing gear damper to the wheel trailing arm. The failure was caused by stress corrosion cracking (SCC).
Metallurgical Findings
Examination of the right trunnion, manufactured from aluminium alloy 3.4365.71 with an anodised finish, revealed a fracture along the 14 mm bore through which the attachment bolt passed. A section approximately 5 mm wide had broken away. There was evidence of secondary cracking, corrosion pits, and exfoliation corrosion on the inner and outer faces of both lugs to a depth of 4.8 mm. Cracking and exfoliation corrosion were also found along the inner radius at the base of both lugs to a depth of 1.6 mm. The metallurgist concluded that SCC had initiated in the 14 mm bore where the protective anodising had been damaged. Movement of the attachment bolt in the bore may have compromised the anodised layer, accelerating SCC. The aluminium alloy is known to be susceptible to SCC. Examination of the left trunnion revealed exfoliation corrosion but no cracking.
Previous Occurrences
Diamond Aircraft Industries were aware of corrosion on the landing gear assembly and had inspected 550 aircraft over the previous three years, finding 13 occurrences of trunnion damage. However, this was the first in-flight failure. The manufacturer had issued a temporary Aircraft Maintenance Manual revision on 14 July 2009 detailing anti-corrosion coatings, a Service Information letter on 11 December 2009 advising new anti-corrosion methods, and a Mandatory Service Bulletin on 23 June 2010 requiring inspection of trunnions within 20 flight hours and then at every 100-hour maintenance inspection. The MSB only required disassembly if there was doubt about the condition.
Safety Recommendations
The AAIB made three safety recommendations to Diamond Aircraft Industries:
- 2010-066: Issue a Mandatory Service Bulletin for the trunnions (Part No D60-3217-23-51) on the main landing gear fitted to DA42 and DA42M aircraft to be removed, disassembled, and inspected for corrosion and cracking.
- 2010-067: Review the instructions for inspection and lubrication of the trunnions with a view to reducing their susceptibility to corrosion and stress corrosion cracking.
- 2010-068: Review the design of the trunnions with a view to making the components less susceptible to stress corrosion cracking.