Introduction
On 30 August 1991, at 1519 local time, a Cessna F-152 (registration HB-CFU) operated by Fluggruppe Pro Altenrhein was on a cross-country flight from Sion (Sitten) to Samedan when the engine lost power over the Splügenpass area. The aircraft was severely damaged during the subsequent emergency landing. Both occupants, a flight instructor and a student pilot, were uninjured.
Flight History
The flight was the final leg of a "Tour de Suisse" arranged by the instructor and the student, who had decided to end the student's flight training due to insufficient progress. The aircraft departed Zurich, made stops at Neuchâtel, Gruyères, and Sion, where it was refueled with 65 liters of aviation gasoline. The intended next destination was Locarno, but due to a solid cloud cover over Domodossola, the flight was redirected toward Samedan or Altenrhein.
Engine Failure and Emergency Landing
Approximately 1 km north of Piz Tamborello, at a cruising altitude of 9,500 feet QNH, the engine RPM suddenly dropped from 2350 to 1300-1400. The instructor took over the controls and found that the throttle could only vary RPM by 200-300. He set the throttle to idle and initiated a glide toward the relatively flat valley floor of Andeer, which was not yet visible. Soon after, realizing Andeer was unreachable, he identified a suitable landing field on the southwestern edge of Splügen.
During the approach, the aircraft crossed a high-tension line. After a textbook approach, the first touchdown was smooth, but the aircraft bounced due to a ground swell. The second contact was harder, and a subsequent ground undulation caused the nose wheel to collapse, the propeller to strike the ground, and both wingtips to contact the terrain due to a lateral oscillation. The aircraft came to rest parallel to the old cantonal road, about 150 meters before the end of the field, between the Splügen village sign and the local tourist office.
Investigation Findings
The investigation revealed that the engine failure was caused by a broken pushrod (ventil-stösselrohr) on the exhaust valve of cylinder No. 3. The fracture was attributed to a combination of thermal and mechanical factors. Metallurgical examination of the pushrod showed multiple cracks in the press-fit area between the steel ball stud and the aluminum tube, with evidence of prior loosening and a progressive fracture. Additionally, corrosion products and cracks were found on several other pushrods. The cylinder head and valve components exhibited varying degrees of wear.
The aircraft, a Cessna F-152 built in 1980 with a Lycoming O-235-L2C engine, had accumulated 2,428 hours on the airframe and 1,634 hours on the engine since overhaul. The last 100-hour inspection was performed on 2 August 1991. Weight and balance were within limits at the time of the accident. Weather conditions were reported as 1/8 high cloud, visibility over 10 km, wind from 120° at 10-15 knots, temperature 12°C, and QNH 1020 hPa.
Conclusion
The accident was attributed to a failed emergency landing in difficult terrain following an engine failure, which was caused by a broken exhaust valve pushrod on cylinder No. 3, induced by thermal and mechanical factors.