1967 Stinson 108-3 Accident at Ecuvillens (HB-TRR)

Overview and Investigation Context On 27 August 1967, a Stinson 108-3 (Voyager) registered HB-TRR was involved in an accident at the Ecuvillens airfield. The Swiss Federal Aircraft Accident Investigation Commission issued its final report on 12 December 1968. The aircraft was being used for parachute training by the Paraclub romand, and three parachutists were aboard for practice jumps. ## Aircraft and Operational Details The aircraft was a Stinson 108-3, serial number 3920, built in 1947 by the Stinson Division in Michigan, USA. It was a braced, single-engine, four-seat high-wing aircraft with fixed tailwheel landing gear and disc brakes. The engine was a Franklin 6A-4-165-B3 producing 165 hp, and the propeller was a fixed-pitch metal Sensenich M 74 DE. The aircraft was owned and operated by Coop-Aviation, Posieux. Its airworthiness certificate and operating permit were valid at the time of the accident. The total flight time was 2,012 hours, with 614 hours since major overhaul. The last 100-hour inspection was on 19 May 1967, and 33-hour inspections were performed on 7 July 1967 and 29 July 1967. The pilot, born in 1940, held a private pilot license and a restricted commercial pilot license, both valid at the time, with type ratings for the Stinson Voyager. He had approximately 198 hours of powered flight experience and about 24 hours on HB-TRR since his type rating, including 16.75 hours and 60 takeoffs for parachute flights at Ecuvillens. ## Accident Sequence Earlier that morning, another pilot had completed four training flights with three parachutists each, using the same aircraft. The accident pilot then took over for further training flights. For the accident flight, three parachutists were aboard, including one weighing 123 kg with equipment, seated behind the pilot. The right cabin door was removed as usual. During taxi, the pilot checked the wheel brakes and found both satisfactory, though the right brake worked better than the left. He performed a magnet check, set the flaps to position 1, and set the elevator trim to full nose-up. He then applied full power and the aircraft began to roll. The pilot felt the aircraft was accelerating less than usual and had not lifted the tailwheel after about 400 m of the approximately 630 m runway. With about 200 m remaining and a 2.5% uphill gradient at the end, he decided to abort the takeoff, reduced power, and applied the wheel brakes. The braking effect was poor on the damp grass, and the aircraft passed over a small rise at the runway end into the rear slope. An attempt to steer the aircraft into a ditch failed, and it crossed a road cut into the slope, coming to rest about 80 m outside the airfield, damaged. ## Damage and Findings The occupants were uninjured. The aircraft sustained significant damage to the landing gear, propeller, and right wing. No significant ground damage was reported. Post-accident examination of the brake system found that the left wheel could only be turned with difficulty. A mechanic found a swollen rubber O-ring in the left brake system and later determined that the pressure piston was slightly bent and impaired in function. It could not be determined whether the piston was damaged during the accident or was already in that condition. The right wheel had broken off at a weld between the strut and wheel hub; an expert found the weld was of good quality and had failed only under excessive stress. ## Contributing Factors The density altitude at the time was estimated at about 900 m, based on a pressure of 1020 mb QNH and a temperature of 20–22°C at the 700 m elevation airfield. According to the manufacturer's handbook, a fully loaded Stinson 108-3 required a takeoff distance of about 1,030 m at 900 m density altitude, with about 60% of that distance for ground roll, or approximately 620 m. The runway was 637 m long, made of grass, and had a 2.5% uphill gradient near the end. The investigation concluded that the takeoff in this direction was risky, and conditions were further worsened by incomplete use of the runway length, unfavorable loading, and crosswind. The aircraft's takeoff weight was about 1,050 kg, just below the maximum allowable weight of 1,090 kg. The 123 kg parachutist in the rear seat increased tail heaviness, though the center of gravity remained within limits. ## Probable Cause The commission concluded unanimously that the accident was most likely caused by the decision to abort the takeoff, which was conducted under tight and unfavorable conditions, being made too late.