What happened
On 15 February 2005, a DH8LS Tiger Moth, registration G-ALIW, was conducting a private flight at a private airstrip in Littlebredy, Dorchester, Dorset. The flight was being conducted by an instructor to revalidate the pilot's Private Pilot's Licence, with a planned itinerary including circuit practice, upper air work, and a simulated forced landing.
During the landing phase, the instructor demonstrated a three-point landing technique. The pilot subsequently attempted a 'wheeler' landing, intending to let the tail settle before applying power for a potential go-around. Before the tailskid made contact with the ground, the right-hand undercarriage collapsed. This caused the aircraft to tip onto its nose and right wingtip, though it eventually settled back into an upright position. There were no injuries to the two crew members on board, though the aircraft sustained damage to the propeller, the underside of the lower wing, and the right-side undercarriage strut.
The investigation
Investigators examined the wreckage to determine the cause of the structural failure. The instructor, who possessed a background in metallurgy, identified that the right undercarriage drag strut had detached from its fuselage fitting. This was caused by the failure of the swivel bolt (part number H.22186).
An examination of the fractured bolt by the AAIB confirmed the presence of a fatigue crack. The analysis suggested that low-cycle fatigue had developed over a relatively short duration. Furthermore, the investigation noted that the bolt passing through the strut fork and the swivel bolt appeared to have been excessively tightened at some point in the past. While the specific maintenance history of this component was unavailable, the investigators noted a 1960 De Havilland report describing a nearly identical failure mechanism.
Findings
- The right undercarriage drag strut detached due to the failure of the swivel bolt.
- The fracture was caused by low-cycle fatigue.
- Evidence suggested the bolt had been subjected to excessive tightening.
- The lack of recorded history for such components and the variable loads experienced by the undercarriage make these parts difficult to monitor.