What happened
On February 6, 2012, a Piper PA-28-181 Archer II, registration HB-PPV, was conducting a VFR training flight near Grünenmatt, Switzerland. The flight was part of a training program for Swiss Aviation Training (SAT), utilizing aircraft and infrastructure from the Grenchen Flight School. During the flight, the crew was performing simulated emergency landing exercises.
While executing the second simulated engine failure maneuver, the aircraft was in a downwind leg at approximately 3,500 ft QNH. About 30 to 40 seconds after initiating the procedure, the engine experienced a significant drop in RPM and a loss of remaining power. The pilot took control and attempted to increase power, but the engine failed to respond. The pilot subsequently performed an emergency landing on a frozen field. The aircraft remained undamaged, and both the pilot and student remained uninjured.
The investigation
SUST examined the technical condition of the aircraft, the maintenance practices, and the operational procedures of the involved flight schools. The investigation focused on the engine's performance in extreme cold, as ambient temperatures had dropped to approximately -17 °C during the preceding night, with temperatures remaining well below freezing during the flight.
Investigators reviewed the engine starting and preheating procedures, noting conflicting accounts regarding whether the engine had been properly preheated according to manufacturer specifications. The investigation also scrutinized the communication and coordination between the two flight schools regarding safety guidelines for training in low temperatures.
Findings
Technical analysis revealed that the engine power loss was primarily caused by a combination of two factors: the aircraft was operated without necessary winterization equipment—such as a winterization plate for the oil cooler or an engine breather tube kit—due to economic reasons, and the carburetor was set to an excessively lean mixture.
Additional technical issues identified included improper spark plug electrode gaps, excessive mechanical play in the carburetor heat valve, and missing baffles within the exhaust system. Furthermore, the investigation found that the engine was started using procedures that contradicted manufacturer specifications, and the preheating method used was also inconsistent with manufacturer guidelines.
Safety action
The investigation identified several risk factors, including poor coordination and conflicting instructions between the participating flight schools regarding the execution of emergency landing exercises. The investigation also noted that the checklists for engine starting were unclear and that the training exercises themselves were inherently high-risk when performed in extreme cold temperatures.