What happened
Following takeoff, the left engine cowl of a DHC-1 Chipmunk began flapping during flight. The pilot, likely facing a high cognitive workload and distraction due to this in-flight emergency, initiated a left turn. During this maneuver, the aircraft's bank angle increased to approximately 55 degrees. This steep bank caused a rapid increase in stall speed, leading the aircraft to enter an aerodynamic stall at an altitude that precluded recovery. The aircraft subsequently collided with the terrain.
The investigation
The investigation focused on why the engine cowl was unfastened and the structural integrity of the cockpit. Investigators found that the left engine cowl latches had not been secured prior to taxi, possibly due to the pilot opening the cowl for fuel line priming or carburetor flooding.
Furthermore, the investigation identified non-conforming rivets used in the mountings between the front and rear cockpits. These rivets, which did not meet design specifications, significantly reduced the strength of the structure holding the front cockpit shoulder harness. While it could not be determined if the impact deceleration exceeded human tolerance or if standard rivets would have held, the presence of these rivets represented a latent threat to the aircraft's crashworthiness.
Findings
- The pilot failed to notice that the left engine cowl latches were unfastened before commencing taxi.
- The aircraft entered an aerodynamic stall during a high-angle bank turn while the pilot was managing the distraction of the flapping cowl.
- Non-conforming rivets were present in the cockpit mounting structure, which reduced the strength of the shoulder harness attachment point.
- An Australian airworthiness directive regarding these inspections had been cancelled in 2008, which may have left such structural discrepancies undetected in older aircraft.