Accident Sequence
On the afternoon of Saturday, 4 August 1979, a pilot was conducting glider towing flights from the Bex airfield. During the fourteenth departure at 18:38 local time, the pilot was at the controls of the Piper PA-18-150 aircraft registered HB-OQB, towing a glider of type Kranich III (HB-538) carrying an instructor and a pilot on an introductory flight. Shortly after takeoff, the pilot observed a rapid decrease in engine rpm and felt strong vibrations. The power loss prevented maintaining speed and altitude. The pilot immediately signaled the glider pilot by rocking the wings, who released the tow and landed the glider without damage in a field.
The Super Cub pilot decided to return to the airfield. After a 180° right turn, he saw a high-tension line blocking the approach, while continuing to lose height and experiencing persistent vibrations. He then attempted a forced landing. He switched off the magnetos and master switch. The proximity of the high-tension line forced another 180° left turn, bringing the aircraft onto final approach over the chosen field. He immediately lowered full flaps and shortened the approach with a slip. Excessive speed prevented stopping within the intended area: the aircraft crossed an agricultural path and entered a beet field, causing it to overturn. The accident occurred around 18:40 local time.
Aircraft and Pilot Information
The pilot held a valid private pilot license and had logged 324 total flight hours, of which 91:06 were on type. The aircraft was approved for operation. The last periodic inspection (50-hour check) was performed on 28 July 1979 at 1847.88 flight hours; at the time of the accident, it had accumulated 1859.42 hours. Technical records indicated a complete overhaul and replacement of the valves on cylinder no. 4 on 12 December 1977 at 1410 service hours.
Investigation and Cause
Examination of the Lycoming O-320-A2B engine revealed a breakage of the exhaust valve seat on cylinder no. 3. Inspection showed the breakage was due to fatigue, and the underside of the seat was covered with a layer of carbon deposit. Analysis of engine oil and fuel samples (100 LL) from the aircraft's tanks revealed nothing significant.
The engine was designed for use with low-lead aviation fuel (80/87 octane). Because of rationalization, fuel suppliers discontinued that grade in favor of 100 LL, which contains about four times more lead. Experience had shown that engines designed for low-lead fuel could suffer disturbances or damage when using 100 LL, including conductive deposits on spark plug electrodes and poor heat transfer on valves. This risk was highlighted in technical communication CT-C FM 70.110-20 published on 15 March 1975 by the Federal Office of Civil Aviation. Engine manufacturers, including Avco-Lycoming, issued recommendations to replace intake and exhaust valves with parts better resistant to heat. The valves on HB-OQB had not been replaced; the investigation could not determine to what extent recommended operating and maintenance procedures had been followed.
The engine was frequently used for mountain landings and glider towing, subjecting it to large thermal variations. Cylinders no. 3 and 4 are generally the least cooled on this engine type. The failure origin was attributed with high probability to excessive thermal stress on the exhaust valve of cylinder no. 3, leading to the valve seat breakage.