What happened
On August 4, 1979, a Piper PA-18-150, registration HB-OQB, was performing a glider towing operation departing from the Bex airfield. The aircraft was towing a Kranich III glider, HB-538, which carried an instructor and a student pilot.
Shortly after takeoff, the pilot of the towing aircraft noticed a significant drop in engine power accompanied by intense vibrations. Due to the inability to maintain necessary airspeed and altitude, the pilot signaled the glider pilot to release the towline. The glider pilot successfully released and landed the glider safely in a nearby field.
Attempting to return to the airfield, the pilot of the Piper PA-18-150 performed a 180-degree turn but encountered high-voltage power lines obstructing the approach path. As the engine continued to lose power, the pilot was forced to execute an emergency landing. After another maneuver to avoid the power lines, the aircraft attempted a landing in a beet field. The high approach speed caused the aircraft to overshoot a farm path and enter the crop, resulting in the aircraft flipping over (capotage). The pilot escaped the crash uninjured, though the aircraft sustained damage to the wings, propeller, engine, and fuselage.
The investigation
The investigation focused on the mechanical failure of the Lycoming 0-320-A2B engine. Examination of the engine revealed that the seat of an exhaust valve in cylinder number 3 had fractured due to fatigue. Carbon deposits were also noted on the underside of the valve seat.
Investigators reviewed the engine's maintenance history, noting that while a valve change had been performed on cylinder number 4 in 1977, the valves in cylinder number 3 had not been replaced. The investigation also considered the impact of using 100 LL aviation gasoline, which contains higher lead content than the 80/87 grade for which the engine was originally designed.
Findings
- The primary cause of the accident was an engine failure caused by the fracture of an exhaust valve.
- The use of valves that were not properly adapted for operation with 100 LL fuel may have contributed to the valve failure.
- The engine was subjected to significant thermal stresses due to its frequent use in glider towing and mountain operations.
- The rear cylinders (numbers 3 and 4) are naturally prone to less efficient cooling, increasing the risk of excessive thermal loading.