What happened
On February 23, 1985, a Cessna F172P aircraft with registration HB-CGY departed from Buttwil airport. The pilot, accompanied by his father-in-law who had no flight experience, climbed to 10,000 feet. While cruising, the engine began to sputter before failing completely below 1,000 RPM. The pilot attempted to restart it unsuccessfully and initiated a forced landing on a snow-covered field near Schönenberg, Zurich.
As the aircraft entered the final approach, the passenger suddenly grabbed the control yoke and pulled it fully back. Despite the pilot's efforts to push forward and his verbal commands to release the controls, he could not overcome the passenger's grip. The aircraft exceeded its critical angle of attack, stalled at low altitude, and crashed into a hillside.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the wreckage, the engine components, and the fuel samples. Technical inspections revealed no evidence of pre-existing mechanical defects, fuel leaks, or improper maintenance. The carburetor heat indicator was outside the danger zone for icing, and humidity levels were too low to support carburetor ice formation. Although a fatty residue was found in the fuel sample, it was attributed to oil-contaminated snow during evidence collection rather than in-flight contamination.
The investigation concluded that the cause of the engine failure could not be determined definitively. However, the sequence of events suggested the pilot may have inadvertently pulled the mixture lever instead of applying carburetor heat, though this was denied by the pilot. The passenger's retrograde amnesia prevented him from recalling the event, but his account of an anxiety-driven reaction was considered plausible.
Findings
The accident resulted in severe injuries to both occupants and significant damage to the aircraft. The primary causal factors were:
- An unexplained engine failure during cruise flight.
- Passenger interference with the flight controls during the critical final approach phase.
- A stall caused by exceeding the minimum lift speed near the ground due to the control blockage.
Safety action
The report highlights the inherent risks of non-flying passengers operating aircraft controls, particularly in emergency situations. While no specific regulatory changes were mandated in this summary, the findings underscore the importance of crew resource management and passenger briefing procedures during abnormal operations.