What happened
On Saturday, October 22, 1983, at approximately 13:47 local time, a Robin DR400/180 R aircraft with registration HB-EXS departed from Sion Airport. The pilot was accompanied by three passengers, including his mother and a couple of friends, for a scenic flight over the Valais Alps. After traversing the Val d'Hérens, the Monte Rosa region, and the Rhône Valley, the crew turned back toward the Ginal valley south of Raron.
During the return leg, the pilot flew over the Aletsch Glacier before heading toward Eischoll. Intending to show his passengers their hometown, he performed several low passes at altitudes between 150 and 200 meters above ground level. While exiting a left turn toward Sion at an airspeed of roughly 60 mph (approximately 97 km/h), the pilot reported hearing a noise described as "like a stone falling into water" but did not perceive the stall warning. The aircraft stalled and entered a left spin.
The pilot attempted to recover by increasing engine power and pulling back on the control stick during the first rotation, which is contrary to standard recovery procedures. After approximately two and a half rotations, the aircraft exited the spin at an altitude of 200–250 meters but struck a hillside at an angle of roughly 18 degrees. The impact occurred around 15:40. The aircraft was destroyed, damaging two fruit trees on the slope.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the accident to determine causes and contributing factors, focusing on prevention rather than liability. Technical inspections of the wreckage revealed no mechanical defects; the engine was producing power at impact, and the airframe was within weight and balance limits. The stall warning device was found to be functional but weak.
The investigation focused heavily on pilot experience and flight tactics. The pilot held valid licenses for both fixed-wing aircraft and helicopters. However, his total flight time of 160 hours consisted of approximately 120 hours in helicopters and only about 40 hours in fixed-wing aircraft, with recent training ending in late 1982. Weather conditions were clear with good visibility, ruling out environmental factors as a cause.
Findings
The accident was primarily caused by the aircraft entering a stall due to airspeed falling below the minimum safe speed for level flight, followed by an inappropriate spin recovery technique. Key contributing factors included:
- Inadequate Flight Tactics: The pilot reduced power to near-idle while flying low over populated areas, likely distracted by the scenery. This led to an unmonitored loss of airspeed.
- Improper Recovery: Instead of reducing power and pushing the stick forward to break the stall, the pilot pulled back on the controls and increased power, exacerbating the spin.
- Experience Mismatch: The pilot had significantly more experience in helicopters than fixed-wing aircraft. Helicopter flight dynamics differ vastly from fixed-wing aerodynamics; habits formed in rotary-wing flight (such as maintaining altitude through power adjustments rather than airspeed management) likely contributed to the error.
- Weak Stall Warning: The acoustic stall warning was audible but too faint to capture the pilot's attention during a high-workload situation.
Safety action
The report highlighted the need for stall warnings to be sufficiently loud to alert pilots under stress. It also underscored the risks associated with pilots transitioning between different aircraft categories without adequate cross-training, particularly regarding the distinct handling characteristics of fixed-wing versus rotary-wing aircraft.