What happened
On August 8, 1995, at approximately 16:45 local time, a Reims-Cessna F-152 (registration HB-CFA) experienced a sudden loss of engine power during the climb-out phase following a planned go-around. The aircraft was operating out of Speck-Fehraltorf airfield in Switzerland. The flight involved an instructor and a student pilot preparing for a private pilot license checkride.
Prior to departure, the pilot conducted the exterior inspection and refueling. Relying on cockpit fuel quantity indicators and visual checks that suggested the tanks were roughly half-full, the pilot added 15 liters of fuel. This resulted in a total indicated quantity of approximately 60 liters. Based on standard consumption tables, the pilot estimated a maximum flight time of 2.5 hours.
The flight consisted of several short segments totaling 1 hour and 15 minutes. During the final segment, after returning to the airfield for the go-around maneuver, the engine suffered a significant power drop. The instructor immediately took control, checked the fuel selector, mixture, and carburetor heat, but only observed a brief, insufficient improvement in performance. With the aircraft at an altitude of only 30-50 meters above ground level and surrounded by obstacles including hills, houses, and high-voltage lines, the instructor initiated an immediate emergency landing on a nearby harvested field. The aircraft sustained significant damage to the nose gear, propeller, and engine but remained structurally intact enough to stop approximately 30 meters from a main road. Both occupants exited without injury.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the technical condition of the aircraft and the operational context. Post-accident inspection revealed that the fuel system had drained 10 liters of usable fuel, leaving virtually none in the lines or carburetor. Both fuel filters were clean, and no water or contaminants were found in the drained fuel. The spark plugs were in good condition, and a new carburetor allowed the engine to run perfectly during ground tests, ruling out mechanical failure.
The investigation focused on the fuel quantity indicators. Reconstructing the tank contents showed that the left indicator read empty (0) while the right read nearly 1/4 full, despite the actual distribution being asymmetric and critically low. The aircraft's design merges the fuel lines before the selector valve, meaning the pilot could not isolate individual tanks; the system relied on a balanced supply from both wings.
The SUST found that the fuel quantity indicators were grossly inaccurate, particularly in the lower range. This was not due to a stuck float but an internal system error. The maintenance manual for the Reims-Cessna F-152 did not require periodic verification of these gauges by draining and measuring fuel. Although the operator had an internal policy to check gauges annually, the last such check on HB-CFA had been performed in July 1990.
Findings
The primary cause of the accident was fuel starvation resulting from a disruption in fuel supply when the remaining quantity was slightly above the "unusable" limit. This was directly caused by the great inaccuracy of the fuel quantity indicators, which misled the pilot into believing there was sufficient fuel for the planned training program.
Contributing factors included:
- Asymmetric fuel distribution: The remaining 10 liters were unevenly distributed between the tanks, a condition the aircraft's design could not correct during flight.
- Optimistic fuel planning: The pilot underestimated consumption by using standard cruise figures rather than accounting for higher fuel burn rates during steep turns and go-arounds typical of training maneuvers.
- Low altitude maneuvering: The go-around was performed at a low altitude in a curved path, leaving minimal margin for error or recovery.
- Lack of verification: The absence of mandatory periodic calibration of fuel gauges allowed the discrepancy to persist unchecked for five years.
Safety action
The SUST issued three safety recommendations: 1. Use precise measuring devices (pipettes, graduated rulers) during pre-flight refueling whenever technically possible. 2. Aircraft fuel quantity indicators should be verified annually by draining a measured quantity of fuel; inaccuracies should not exceed 5%. 3. Flight manuals should be redesigned to be more user-friendly and clear.