What happened
On September 12, 1991, at approximately 08:20 local time, a Piper PA-38-112 Tomahawk, registration HB-PGT, was involved in an incident during a flight training exercise near Sierre, Valais. The aircraft was being operated by a certified flight instructor and a student pilot for instructional purposes in the airspace between Sierre and St-Léonard.
The pair had taken over the aircraft from another instructor who had just completed five circuit flights and confirmed the airframe was in normal condition. After completing pre-flight checks, which indicated less than 10 US gallons of fuel remained in each tank, the student pilot took the controls for a climb to 3,000 feet. During the maneuvering phase, the student performed 360-degree left turns and pitch attitude changes (climb, level, descent) while the instructor monitored.
The instructor conducted a carburetor heat check, which showed no anomalies. However, during the third exercise at an altitude of 3,200 feet MSL, the student advanced the throttle to full power as part of a climb maneuver. Immediately, the engine RPM dropped to 600 and oscillated between 600 and 1,300 RPM. The instructor intervened instantly, activating the auxiliary fuel pump, switching fuel tanks, enriching the mixture, and applying carburetor heat. These actions failed to restore power.
Recognizing the inability to regain engine performance, the instructor initiated a forced landing procedure. After identifying a nearby field, he noticed an extensive irrigation system that made it unsuitable for landing. He selected an adjacent cornfield instead, extended the glide path slightly, deployed full flaps, and touched down in the crop. The aircraft skidded approximately 30 meters before coming to a stop. Both occupants, wearing shoulder harnesses, exited the aircraft unharmed and notified Sion air traffic control.
The investigation
The preliminary investigation was conducted by Mr. Remy Henzelin and concluded with a report submitted on January 9, 1992. The inquiry established that both pilots were properly licensed and medically fit, and the aircraft held valid navigation and circulation certificates. The mass and center of gravity were within prescribed limits.
The engine, a Lycoming O-235-L2C with a Marvel-Schebler carburetor, had undergone a complete overhaul by Scanaviation in Copenhagen in February 1988 after 1,981 hours of service. It had accumulated an additional 1,292.45 hours since the overhaul. Post-accident examination revealed that engine controls and linkages functioned correctly. The upper spark plugs were clean, while lower plug electrodes showed some carbon buildup but remained functional.
A bench test of the recovered engine confirmed it could not increase RPM above 1,200 during startup. However, when the original carburetor was replaced with a known-good unit, the engine operated normally across all RPM ranges. Examination of the faulty carburetor revealed aluminum shavings in the bowl and a partial loosening of the mixture lever. This defect caused a misalignment of the control linkage when set to "rich," preventing the engine from achieving maximum RPM due to insufficient fuel flow.
Findings
The accident was caused by a forced landing resulting from a significant loss of engine power. Contributing factors included:
- Carburetor malfunction: A defective assembly led to the loosening of the mixture lever, which obstructed proper fuel delivery during high-power settings.
- Environmental debris: Aluminum shavings found in the carburetor bowl likely originated from improper installation procedures during previous maintenance, potentially using an electric screwdriver on a misaligned assembly screw. These particles may have contributed to the wear and eventual failure of the mixture control mechanism.
- Terrain constraints: The area overflown was not conducive to emergency landings, though the instructor successfully identified an alternative site despite the presence of irrigation infrastructure.
The investigation could not definitively rule out that the aluminum debris resulted from improper maintenance practices during the carburetor's assembly or repair. However, the primary cause remained the mechanical failure of the mixture control due to defective fitting.
Safety action
No specific safety recommendations were issued in this final report beyond the standard procedural conclusions regarding the maintenance and inspection of carburetor components.