What happened
On 11 January 2003, a Druine D31 Turbulent, registration G-ARGZ, was performing a private flight at Headcorn Aerodrome, Kent. Following a routine landing on the grass runway, the pilot taxied the aircraft across the grass area. During this maneuver, an audible noise was heard coming from beneath the inboard starboard wing. Immediately following this sound, the starboard undercarriage leg folded forward, causing the wing to drop and the aircraft to rotate approximately 90 degrees. The pilot managed to cut the magnetos before the propeller made contact with the ground. There were no injuries to the pilot, and the aircraft sustained minor damage to the mounting brackets and undercarriage.
The investigation
Investigators reviewed the accident report provided by the pilot, interviewed the maintenance engineer, and inspected the failed components. The investigation focused on the attachment point of the aft drag brace to the wing structure. This assembly utilized a bolt passing through two light-gauge brackets attached to wing ribs. The inspection revealed that the lugs on both brackets had failed.
Analysis of the metal indicated the brackets were approximately 0.064 inches thick but appeared to be made of non-aircraft grade material. The failure showed signs of significant elastic deformation and long-term wear. Discoloration at the failure site suggested frettage had occurred. Furthermore, the maintenance engineer noted that other aircraft in the same fleet utilized a superior design involving 4130 steel brackets with brazed washers to better distribute loads and prevent wear.
Findings
- The primary cause of the undercarriage collapse was the failure of the attachment lugs on the drag brace brackets.
- The brackets used were of a thickness of 0.064 inches and did not appear to be constructed from aircraft-grade material.
- Evidence of long-standing wear and frettage suggested that the bolt holes had enlarged over time, reducing the structural integrity of the lugs.
- It is probable that previous landing or taxiing stresses caused hole elongation, which was then exploited during the taxiing phase following the final landing.
- The aircraft, which was 42 years old at the time, had a history of a landing gear collapse in 1988, though the specifics of that repair were unknown.