What happened
On 4 July 2022, a Windlass Aquila 912 trike, registered ZU-FWM, was conducting a routine circuit training flight at Oudtshoorn Aerodrome in the Western Cape. The flight, operated by Centurion Flight Academy, included a flight instructor and a student pilot. Following an uneventful landing on Runway 2/2, the aircraft began taxiing toward the apron on taxiway Alpha.
During the taxi, the engine mount pylon—the component supporting both the engine and the seat frame—suffered a structural failure. This caused the seat to drop, which simultaneously pulled the throttle cable into an open position. The throttle became stuck at approximately 80% power, causing the aircraft to accelerate rapidly and uncontrollably. The instructor managed to steer the aircraft away from obstacles, eventually coming to a stop in a nearby field after the engine died when the carburettors were dislodged. There were no injuries to the two occupants, though the aircraft sustained minor damage, including a bent rear axle and radiator bracket.
The investigation
An investigation by the SACAA AIID focused on the mechanical failure of the engine mount. Investigators examined the aircraft's maintenance history and the physical condition of the failed component. While the aircraft's total airframe hours were well within the 1500-hour service life limit for the engine mounting bracket, a post-incident inspection of the parts revealed significant manufacturing and maintenance discrepancies. Specifically, the spar tube used in the assembly had a wall thickness of only 1.6mm, which was significantly thinner than the 3mm thickness required by the official engineering drawings.
Findings
- The primary cause of the incident was a fatigue fracture of the engine mount pylon.
- The failure was exacerbated by burn-through welding, where excessive heat during the welding process created holes in the metal and reduced its structural integrity.
- The fracture occurred near the vertical square tube/cross member welded joint.
- The use of a spar tube with a 1.6mm wall thickness, rather than the required 3mm, contributed to the structural weakness.