What happened
On 30 December 2007, a Dyn’A_Aero MCR-01 ULC, registration G-BZXG, was performing a short flight between two airfields near Felton, Northumberland. The aircraft, operated as a private flight, was flying from a farm strip at Longframlington to Eshott Airfield. The flight was uneventful until the aircraft was approaching the crosswind leg of the landing circuit at approximately 800 ft.
During this phase of flight, a loud noise was heard by the occupants and people on the ground as the tailplane suddenly separated from the aircraft. The loss of the tailplane caused the aircraft to become uncontrollable, leading to a tumble and descent into a wooded area. The aircraft struck trees before coming to rest near a stream. The pilot and passenger both sustained serious injuries but survived the impact.
The investigation
The AAIB examined the wreckage and the detached tailplane, which was found 150 metres from the main crash site. Investigators focused on the attachment lugs that connect the tailplane to the fin. While the aircraft was equipped with newer Type 3 lugs, the investigation revealed that the left lug had been compromised by a stress corrosion mechanism.
Metallurgical analysis showed that the separation of the lugs occurred due to tensile overload. The investigation found that the left lug had been weakened by stress corrosion, which was likely facilitated by the orientation of the material grain relative to the applied loads. Furthermore, the investigation noted that the aluminium lugs lacked adequate corrosion protection, as no primer had been used and the paint was poorly adhered. The investigation also examined the seat harnesses, which had failed in the impact due to variability in the stitching strength.
Findings
- The tailplane separation was caused by the failure of the attachment lugs in an upload condition.
- Stress corrosion was present in the left lug, which weakened the component.
- The orientation of the material grain in the lug was such that applied loads acted in a direction that increased susceptibility to corrosion.
- The lack of surface conversion processes, such as anodising, and the absence of primer left the aluminium vulnerable to the corrosive UK atmosphere.
- Stress concentrations may have been introduced by the deep staking marks used to retain the bearings in the lugs.