What happened
On July 8, 2021, an Aquila AT01 departed from the Nordholz-Spieka airfield for a local VFR flight. During the pre-flight inspection, the pilot had been interrupted by the arrival of another aircraft and had been attending to the engine's oil level. Shortly after takeoff, the pilot notified air traffic control that an engine cowling service flap had opened and expressed the intention to perform a short circuit to land immediately.
As the aircraft entered a left-hand turn at an altitude of approximately 150 to 180 feet AGL, it experienced a significant increase in bank angle. The aircraft subsequently stalled, wing-dropped to the left, and impacted the ground in a cornfield, where it immediately caught fire. A nearby helicopter crew attempted a search for survivors but found the aircraft heavily engulfed in flames. The pilot and the passenger, the pilot's wife, both sustained fatal injuries.
The investigation
The BFU investigation examined the aircraft's maintenance and pre-flight procedures, noting that the oil filler cap was found on the apron where the pre-flight check had been conducted. While the missing cap would not typically cause an engine failure, its absence indicated an incomplete pre-flight sequence. The investigation also analyzed radio communications, which revealed an active stall warning siren in the background of the pilot's transmission.
Flight testing of an identical aircraft model was conducted to understand the aerodynamic behavior. The tests demonstrated that at speeds below 50 KIAS, the aircraft was prone to stalling and wing-dropping, especially during uncoordinated turns. Specifically, at approximately 36 KIAS, the aircraft would drop a wing to the left, and at 45 KIAS, even a slight reduction in right rudder pressure during a climb could trigger a similar loss of control.
Findings
- The primary cause of the accident was an aerodynamic stall and loss of control during an uncoordinated turn.
- The pilot was distracted by the open service flap, which likely led to an abrupt maneuvering decision.
- The aircraft was flying at a speed significantly below the recommended climb speed of 65 KIAS, as evidenced by the audible stall warning.
- The pre-flight inspection was interrupted and left incomplete, resulting in the service flap not being properly secured and the oil filler cap being left behind.