What happened
On 4 September 2021, a Boeing A75N1(PT17) Stearman, registration N707TJ, was participating in the Bournemouth Air Festival as part of a two-aircraft wing walking display. During the performance over the sea, the pilot noticed a drop in engine RPM and a lack of climb capability. Following established procedures, the pilot signaled the wing walker to dismount and return to the front cockpit.
As the aircraft flew west along the coast toward Bournemouth Airport, the engine began to misfire, and oil was observed streaming from the engine cowling. To avoid flying over populated areas, the pilot sought an alternative landing site. After determining that the nearby golf course was unsuitable, the pilot opted to ditch the aircraft in the entrance to Poole Harbour. The aircraft maintained a stable descent and struck the water with a nose-high attitude, but the impact caused the plane to flip onto its back. Both the pilot and the wing walker successfully exited the submerged cockpits and were rescued by a nearby leisure craft.
The investigation
The AAIB investigation focused on the mechanical failure of the engine's oil system. Examination of the wreckage revealed that the engine oil inlet pipe had broken at the point where it connected to the engine casing. The investigation also scrutinized the metal strap intended to support the weight of the rigid section of the pipe.
Metallurgical analysis of the support strap showed evidence of mechanical damage around a fastener hole, which was consistent with the over-tightening of a retaining nut. This damage served as the starting point for a fatigue crack that propagated through the strap. Furthermore, the investigation examined the threaded joint of the oil pipe, finding evidence of fatigue and potential stress caused by shrinkage porosity in the casting.
Findings
- The engine power loss was caused by a failure of the oil inlet pipe, which interrupted the supply of oil to the engine.
- The failure was initiated by the fatigue of a metal support strap, which had been weakened by mechanical damage caused by over-tightening a retaining nut.
- Once the support strap failed, the weight of the oil pipe was transferred to the threaded joint, leading to the subsequent fracture of the pipe itself.
- The pilot's decision to ditch in an area clear of boats and populated land prevented injury to third parties.