What happened
On August 27, 1967, a Stinson 108-3 (registration HB-TRR) was performing training flights for skydivers at the Ecuvillens airfield. During the final flight of the morning, the aircraft was carrying a pilot and three passengers. As the aircraft accelerated down the grass runway, the pilot noted that the aircraft was not accelerating as expected and failed to lift the tailwheel after approximately 400 meters of the roll.
Faced with a remaining runway length of only about 200 meters and an uphill gradient of 2.5% at the end of the strip, the pilot attempted to abort the takeoff by applying the brakes. However, due to the low friction of the damp grass, the aircraft could not be stopped within the remaining distance. The aircraft overran the end of the runway, crossed a small embankment, and eventually came to rest approximately 80 meters beyond the airfield boundary after descending a roadside slope.
The investigation
The investigation examined the aircraft's weight, the runway conditions, and the mechanical state of the braking system. The aircraft was operating at a weight of approximately 1050 kg, which was near its maximum allowable limit of 1090 kg. One passenger, weighing 123 kg, was seated in the rear, contributing to a tail-heavy loading configuration.
Investigators analyzed the runway at Ecuvillende, noting it was a grass strip with a 2.5% incline at the end. While the aircraft's engine and controls were functional, the investigation into the braking system revealed a swollen O-ring and a slightly bent piston in the left brake mechanism, though it could not be determined if this was a pre-existing condition. The investigation also noted that the required takeoff roll for such a load at the prevailing density altitude would have been approximately 620 meters, leaving very little margin for error.
Findings
- The takeoff was conducted under tight and unfavorable conditions, including a near-maximum takeoff weight and a grass runway with an uphill gradient.
- The decision to abort the takeoff was made too late, leaving insufficient distance to stop the aircraft.
- The effectiveness of the brakes was significantly reduced by the dampness of the grass surface.
- The aircraft's loading, specifically the heavy passenger in the rear, influenced the takeoff performance.