What happened
On January 13, 2022, an Aquila AT01, registration HB-SFU, was conducting a VFR training flight from Hausen am Albis (LSZN). After performing a touch-and-go maneuver on runway 09, the aircraft entered a climb phase. At an altitude of approximately 200 meters, the crew experienced sudden, intense vibrations accompanied by a significant loss of engine power.
The flight instructor immediately took control, reduced power, and initiated a descent. After attempting to increase power briefly—which only intensified the vibrations—the instructor performed a turn back toward the airfield to land on runway 27. The aircraft landed safely without further incident, and no injuries were reported.
The investigation
SUST investigators examined the engine and found that cylinder #2 could not maintain compression. A borescope inspection revealed an exhaust valve with a significant area of damage characterized by a crater-like appearance. Metallurgical analysis of the cylinder head showed heavy deposits of combustion byproducts and lead compounds. Specifically, the investigators found evidence of melting (burn-through) on the valve disc.
The investigation also reviewed the aircraft's fuel history. While the aircraft primarily uses unleaded MOGAS, it had been operated with AVGAS 100LL for approximately 10 hours during a period two months prior to the incident. The investigation noted that the engine's maintenance intervals for oil changes and compression tests had not been shortened despite the use of leaded fuel.
Findings
- The primary cause of the engine power loss and vibrations was a defect in an exhaust valve.
- Heavy lead deposits on the valve and within the combustion chamber caused a leak between the valve disc and the seat.
- This leakage allowed hot combustion gases to overheat the valve disc, leading to the observed melting and structural failure.
- The accumulation of these deposits was attributed to the recent use of AVGAS 100LL, which contains tetraethyl lead.
- The aircraft's PowerFlarm collision warning system was non-functional due to expired firmware, though this did not contribute to the engine failure.