Accident
On 9 August 1985, at approximately 1620 local time, a Piper PA-28-140 aircraft, registration HB-PAP, departed from Biel-Kappelen airfield for a private pleasure flight. The pilot had two passengers on board and about two hours of fuel. After overflying Chasseral at an altitude of 6500-7000 feet QNH, the pilot headed toward Schwarzenburg, intending to descend gradually back to the departure point. During the descent, the pilot reduced engine power to idle. After passing Lake Wohlen, the pilot attempted to level off at 2500-3000 feet QNH over Säriswil and increase power. However, the throttle lever became stuck and could not be moved forward or backward despite considerable effort. Without engine power, the pilot initiated a forced landing. After a 180-degree right turn, the aircraft touched down in a potato field. After a ground roll of about 45 meters, the aircraft struck a field embankment and nosed over. The pilot and the front-seat passenger sustained minor head injuries.
Investigation
The investigation revealed that the throttle cable had been routed to the right side of the vertical exhaust pipe, contrary to the aircraft manufacturer's specifications, which required routing to the left. After the accident, the carburetor was inspected but not disassembled; no cause for the blockage was found. Repairs were made, and the aircraft returned to service. On 6 March 1986, during a final approach to Runway 14 at Bern-Belp Airport, a flight instructor and student pilot in the same aircraft experienced an identical throttle lever blockage. The instructor managed to land safely without damage. Subsequent disassembly and examination of the carburetor revealed several factors contributing to the blockage:
- An unfavorable material pairing (steel-on-steel) for the throttle shaft and bushings.
- Significant wear marks on the throttle shaft bearings, including partial rotation and binding at bushing edges.
- Imprints on the housing, indicating damaged bearing seats.
- The installed throttle shaft was made of ordinary structural steel, whereas an earlier shaft was made of stainless steel.
- Additional adhesive was used to secure the throttle plate screws.
- The thermal expansion coefficient ratio between shaft and bushing materials was less favorable than in the earlier stainless steel design, leading to a stronger bimetallic effect.
- The use of the manufacturer's prescribed reamer resulted in an unusually large clearance, which mechanics might consider excessive.
Probable Cause
The accident was caused by a forced landing from low altitude after the throttle lever blocked. Contributing technical factors: insufficient clearance during assembly of the carburetor throttle shaft and non-compliance with the engine manufacturer's assembly instructions.
Incident
On 6 March 1986, the same aircraft, after repair, experienced a similar throttle lever blockage during final approach at Bern-Belp Airport. The flight instructor successfully landed the aircraft without damage.