What happened
On April 14, 1962, a Piper Apache PA-23, registration D-GIRL, was performing a repositioning flight from Malaga to Hannover. After several intermediate stops in Barcelona and Lyon, the pilot proceeded under instrument flight rules (IFR) toward Zurich. While flying at 8,000 ft near Bern, the aircraft entered a thick cloud layer characterized by heavy icing conditions.
Seeking to avoid worsening conditions, the pilot requested a descent to 6,000 ft. During this descent, near Willisau, the right engine failed. The pilot was unable to maintain altitude with only one engine operating and attempted an emergency landing. While attempting to find a suitable spot in the dark, the aircraft struck treetops and crashed at low altitude near Ebersecken. The pilot sustained severe injuries, and the aircraft was destroyed.
The investigation
Investigators examined the aircraft's fuel system and the environmental conditions at the time of the accident. The investigation noted that the pilot had observed irregularities with the right engine on previous legs, including lower operating temperatures and higher fuel consumption.
Upon inspecting the wreckage, investigators found that the fuel priming pump for the right engine was defective. The knurled nut holding the pump mechanism had loosened, allowing the pump piston to be partially extended. This defect caused excess fuel to be drawn into the cylinders, leading to an overly rich mixture. Additionally, the investigation found that the propeller de-icing fluid was depleted and that the right engine's propeller was at a significantly lower RPM than the left at the moment of impact.
Findings
- The primary cause was a combination of heavy icing and a pre-existing fuel system defect that led to the failure of the right engine.
- The loose priming pump mechanism caused an over-rich fuel mixture, which lowered the engine's operating temperature and increased its susceptibility to carburetor icing.
- The pilot's decision to fly into a zone of known icing hazards, despite having limited de-icing capabilities and observing engine irregularities, contributed to the accident.
- The loss of the right engine rendered the aircraft unable to maintain altitude in the icing conditions.