What happened
On August 13, 1987, an instructor and a student pilot conducted basic flight training maneuvers, including touch-and-go landings and go-arounds, in a Piper PA-38-112 bearing registration HB-PFM. The aircraft was operated by the Flugsportgruppe Zürcher Oberland from Fehraltorf airfield. Before departure, the student performed the exterior walk-around inspection. Due to the aircraft being parked on uneven ground, checking the fuel levels visually proved difficult. The student estimated approximately 5 US gallons in the left tank and 6-7 US gallons in the right tank, deeming this sufficient for the planned duration.
The crew decided against refueling, citing the aircraft's known poor climb performance and high ambient temperatures of 26°C (79°F). The takeoff was conducted with the fuel selector set to the left tank. Throughout the flight, the instructor reported that prior to the final touch-and-go, the left tank contained slightly less than 2 US gallons, while the right held 3-4 US gallons. During the pre-approach check, the student did not switch to the fuller right tank, as they had agreed to exhaust the left tank first.
Shortly before landing, the instructor switched the selector to the right tank. The aircraft landed normally on runway 30. The student applied full power for a go-around and climbed to pattern altitude. As the student executed a standard 15-degree left turn, the engine began to sputter. The instructor took control and directed the aircraft toward a nearby field for an emergency landing. After rolling approximately 50 meters, the nose gear collapsed, causing the aircraft to flip onto its back. Both occupants exited the wreckage uninjured.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the accident to determine causal factors. Post-accident inspection revealed that the right tank contained 7.8 liters of fuel, while the left tank held only 0.5 liters. The remaining fuel was clean with no water contamination. The entire fuel system was found to be in order, ruling out mechanical failure as the cause of power loss.
Measurements confirmed the unusable fuel limit for this aircraft model is 1.0 US gallon per wing in critical attitudes. The investigation calculated that for a flight duration of approximately 1 hour and 13 minutes, at an estimated consumption rate of 7.9 US gallons per hour, roughly 9.9 US gallons should have been consumed. The discrepancy between the estimated and actual remaining fuel indicated significant operational error.
Findings
The accident was caused by a failed emergency landing following fuel exhaustion. Contributing factors included:
- Depletion of the left tank below the unusable fuel limit: The crew continued flying on the left tank after it had effectively run dry, leading to air entering the fuel lines.
- Delayed fuel selector switch: The switch to the fuller right tank was made too late in the approach phase. Although switched before landing, there was insufficient time to purge the air from the system during the subsequent go-around.
- Inadequate pre-flight planning: The crew underestimated fuel requirements given the high temperature and poor climb performance, choosing not to refuel despite marginal reserves.
The investigation noted that switching tanks while below the unusable fuel limit can introduce air into the fuel lines. The Pilot's Operating Handbook requires running the engine for at least three minutes at 1000 RPM on each tank after replenishment or depletion to ensure no air remains. This procedure was not possible during the short go-around phase.
Safety action
This incident highlights the critical importance of accurate fuel management and adherence to pre-flight planning procedures, particularly in high-performance or high-temperature conditions. It underscores the need for pilots to verify fuel quantities accurately and to switch tanks well before potential power demands increase, allowing time for system purging.