What happened
On July 22, 1980, at approximately 11:30 local time, a Robin DR-400-180 aircraft with registration HB-EXN was involved in an accident during the initial climb phase of flight. The pilot, holding a valid private pilot license, intended to conduct a private cross-country flight to Sion with three passengers on board. The group arrived at Ecuvillens airfield at 10:30. Prior to departure, the aircraft underwent brake adjustments in a workshop.
The pilot taxied to runway 10, selecting the left fuel tank for the flight. Fuel quantities were recorded as 40 liters in the right tank, 30 liters in the left, and 80 liters in the rear tank. After completing engine checks, which appeared normal, the pilot aligned with the runway and applied full throttle. The aircraft initially accelerated normally but experienced two instances of deceleration due to soft spots on the damp grass surface.
After traveling approximately 180 meters, the airspeed reached roughly 60 mph (approx. 97 km/h). The pilot began pulling back on the control column to rotate. The aircraft lifted off momentarily but settled back onto the runway. Subsequent attempts to lift the nose failed. The aircraft continued past the end of the paved/grass strip, flew over a perpendicular road, passed under a power line, and impacted a wheat field at a high angle of attack.
The impact resulted in the destruction of HB-EXN. The pilot sustained minor injuries, while one passenger suffered serious injuries. No third-party injuries or significant property damage occurred outside the aircraft.
The investigation
The Swiss Transportation Safety Investigation Board (STSB/SUST) examined the operational and mechanical factors surrounding the event. Key findings included:
- Pilot Qualifications: The pilot held a valid license with 167 hours of total flight time, including 20 hours in the last 90 days. However, he had only 9 hours of experience specifically on the Robin DR-400 type.
- Aircraft Condition: Post-accident inspection confirmed the aircraft was airworthy prior to the incident. Weight and center of gravity were within limits, approximately 50 kg below the maximum takeoff weight of 1,100 kg. Flaps were set at 15 degrees and locked; trim was neutral.
- Runway Conditions: Runway 10 at Ecuvillens is 637 meters long with a 2.5% upward slope in its second half. The surface was damp grass, with the first half being softened/mushy due to moisture.
- Weather: Wind was from the northeast at approximately 5 knots, visibility was good (20 km), and temperature was +17°C.
- Performance Analysis: Calculations indicated that the available runway length was sufficient for takeoff under these conditions. The aircraft's propeller had a low pitch (64'), which generally aids acceleration. Engine compression tests post-crash were inconclusive due to potential damage from impact or debris ingress.
Findings
The investigation determined that the primary cause was improper takeoff technique by the pilot. Contributing factors included:
- The pilot failed to account for the increased ground roll required due to the soft, damp runway surface.
- Excessive nose-up attitude during the rollout created unnecessary drag, impeding acceleration.
- Lack of experience on the specific aircraft type (Robin DR-400).
- The decision not to abort the takeoff despite deceleration events was likely influenced by the airspeed already reaching 50-60 mph and the loss of braking effectiveness due to the soft ground.
Safety action
The report highlights the critical importance of adjusting takeoff techniques for runway surface conditions. Pilots must anticipate longer ground rolls on damp or soft grass runways and avoid excessive nose-up attitudes during the acceleration phase to maintain optimal airspeed growth. No specific regulatory changes were mandated in this summary, but the findings serve as a cautionary example for general aviation operations on unimproved surfaces.