What happened
On July 16, 2009, an Aquila AT01, registration HB-SFV, was conducting a private VFR flight near Thun, Switzerland. The flight, which began as an Alpine tour, proceeded normally through the Kandertal and over the Gemmi Pass. However, during the final approach to runway 32 at Thun airfield, the engine abruptly ceased operation while the aircraft was in a turning maneuver.
The pilot, realizing that the remaining altitude was insufficient to reach the runway, opted for an emergency landing in a nearby cornfield. During the descent, the pilot retracted the flaps and maintained a controlled airspeed. As the aircraft contacted the tall corn stalks, a heavy impact occurred, causing the aircraft to flip onto its back. While the pilot escaped without injury, the passenger sustained a light head injury. The aircraft was heavably damaged and subsequently written off as a total loss.
The investigation
SUST examined the mechanical condition of the engine and the maintenance history of the aircraft. The investigation focused on the sudden loss of power and the physical evidence found within the engine assembly. Investigators also reviewed the pilot's training records and the operator's maintenance procedures, noting that the aircraft's flight manual was not at the most recent revision level.
Findings
Technical analysis revealed that the engine failure was caused by a mechanical failure of the camshaft drive. Specifically, the keyway of the camshaft drive had failed, leading to the rotation of the control wheel on the camshaft, which rendered the engine inoperable. This failure was the result of a chain of mechanical issues:
- Significant wear on the mating surfaces of the two engine crankcase halves.
- Reduced clearance in the camshaft and crankshaft bearings.
- Increased resistance to rotation (stiffness) within the engine.
Furthermore, the investigation found that the developing engine stiffness was not detected during routine operations. While the engine manufacturer requires a daily check for such stiffness, the investigation concluded that this check is difficult to perform effectively in daily practice. Additionally, the operator's maintenance program lacked specific instructions for addressing or inspecting such developing issues.
Safety action
While no specific safety recommendations were detailed in the provided text, the investigation highlighted that the pilot's decision to prioritize flight attitude and minimize fire risk by switching off electrical components contributed to the survivability of the accident.