What happened
On July 26, 1988, at approximately 18:10 local time, a Piper PA-18-125 (HB-PHP) was conducting a private visual flight rules (VFR) tour over the Swiss Alps. The aircraft departed from Lommis airfield with two pilots aboard. After circling the Säntis peak at roughly 12,000 feet for about two and a half hours, the crew began their descent back toward Lommis.
While flying near Gossau in the Höfrig/Degenau area, the second pilot, who possessed significantly more flight experience than the pilot in command, demonstrated an emergency landing approach at a site where another aircraft had previously made a forced landing. Following this demonstration, the pilot in command resumed control as they continued toward Lommis.
During the final approach phase, witnesses on the ground observed the engine sputtering and misfiring. Recognizing the imminent loss of power, the experienced second pilot took control of the aircraft. Due to the unsuitability of the surrounding terrain for a safe landing, he selected a nearby cornfield. The aircraft touched down approximately 17 meters into the field, rotating 110 degrees to the right before coming to a stop. Both occupants sustained minor injuries.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the mechanical condition of the Lycoming O-290-DTC engine and the maintenance history of HB-PHP. A post-accident inspection by Heliswiss in Bern revealed that the drive gear for the camshaft and magnetos had become loose on the crankshaft.
The investigation determined that a cylindrical pin (key) securing this gear had sheared off, causing the gear to detach from its intended position. The central screw holding the gear was found to be loose, although the locking washer remained correctly installed. The STSB concluded that the tightening torque during previous maintenance had likely not been applied according to specifications.
Crucially, the investigation traced the root cause of this mechanical failure to a prior incident on September 13, 1986. During that event, the aircraft had made an emergency landing where the propeller struck the ground, causing a sudden engine stoppage. Although repairs were performed, including a propeller replacement and a 100-hour inspection, a thorough control disassembly of the engine was not conducted following this strike.
Findings
The primary cause of the accident was a forced landing in unfavorable terrain resulting from an in-flight engine failure. The engine failure itself was caused by the shearing of the cylindrical pin on the drive gear for the camshaft and magnetos. This mechanical failure was attributed to the loosening of the central screw, which allowed the gear to shift under vibration and power changes.
Contributing factors included:
- Inadequate Post-Incident Inspection: The absence of a thorough engine control disassembly after the propeller ground strike on September 13, 1986. The initial impact likely sheared the pin and slightly displaced the gear, but the locking mechanism temporarily held it in place. This latent defect persisted for 76 flight hours until catastrophic failure occurred.
- Maintenance Torque Specification: The loosening of the central screw suggests that the specified tightening torque was not observed during previous maintenance work.
Both pilots were found to be properly licensed and medically fit. The aircraft was airworthy at the time of departure, with mass and balance within limits. The weather conditions were clear with good visibility, ruling out environmental factors as a cause for the initial engine malfunction.
Safety action
The STSB report highlighted that while Swiss civil aviation authorities had issued guidelines in November 1987 regarding procedures after propeller ground strikes or sudden engine stoppages, these measures did not prevent the oversight in this specific case. The guidelines emphasized that only a complete control disassembly or major overhaul could guarantee the detection of hidden defects following such incidents. The accident underscored the critical importance of adhering to strict maintenance protocols after any event involving potential internal engine stress.