What happened
On 11 May 2018, a Windlass Aquilla, registration ZU-DLY, departed Brits Airfield (FABS) for a private scenic flight. The aircraft was carrying a pilot and one passenger. While flying the circuit, the pilot transitioned from a descent into a fly-past, applying full throttle to initiate a climb. Shortly after this power application, the engine suffered a sudden loss of power.
In an attempt to execute an emergency forced landing, the aircraft descended toward a nearby private farm. During this maneuver, the left main landing gear struck overhead railway power lines. The aircraft subsequently impacted the ground a few meters from the railway tracks. The impact resulted in serious injuries to the pilot and minor injuries to the passenger. The aircraft sustained significant damage to the engine mount, instrument panel, propeller, and landing gear.
The investigation
SACAA AIID investigators examined the wreckage and the engine components to determine the cause of the power loss. A compression test performed on the Rotax 5/2 engine revealed a significant discrepancy between cylinders; while cylinder no. 2 showed 625kPa, cylinder no. 1 measured only 100kPa. Further inspection confirmed that the piston in the first cylinder had seized.
Technical analysis focused on the thermal dynamics of the engine during the flight sequence. The investigation noted that the engine, which lacks a thermostat, had been running at idle during the descent. This caused the cylinder walls to cool rapidly. When the pilot applied full throttle for the fly-past, the piston expanded at a different rate than the cylinder sleeve, leading to a mechanical lock.
Findings
- The primary cause of the engine failure was a cold seizure of the number one piston.
- The seizure was triggered by a thermal imbalance occurring when full power was applied following a period of engine idling during descent.
- The aircraft's left rear wheel collided with railway overhead power lines during the forced landing attempt.
- The pilot was operating with a valid national license and medical certificate.
- The aircraft had undergone a periodic inspection approximately 20 flight hours prior to the accident.