What happened
On August 15, 1989, at approximately 16:04 local time, a Robin DR 400/180 R aircraft with registration HB-KAE impacted the flat roof of an industrial building in Sion, Switzerland. The accident resulted in one fatality and serious injuries to another occupant.
The incident occurred during a private VFR flight operated by the motor gliding group of the A&CS Valais section. Earlier that day, the aircraft had been used for two other flights totaling 126 minutes of flight time before this third leg. The pilot in command, holding a private pilot license, departed Sion at 14:45 with three passengers. During the descent toward Sion, the low-fuel warning light began flashing. The pilot attempted to switch to the auxiliary fuel tank but failed because the control knob was missing. Unable to restore engine power despite multiple throttle attempts, the pilot declared a distress situation and executed an emergency landing on the building roof.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the technical condition of the aircraft, the pilot's qualifications, and the fuel management procedures. Technical inspection revealed no mechanical defects that could have caused engine failure; the engine stopped solely due to fuel exhaustion. Post-accident checks confirmed the main tank contained only 1 deciliter of residual fuel, while the auxiliary tank was completely empty.
The investigation highlighted significant discrepancies in the pilot's pre-flight planning. The pilot estimated the aircraft's fuel consumption at 25 liters per hour, whereas the manufacturer's data indicated an average consumption of approximately 32 liters per hour under normal operating conditions. Furthermore, the flight plan listed an endurance of three hours, while the actual available fuel allowed for only one hour of flight time.
Findings
The primary cause of the accident was a forced landing following total fuel exhaustion. Contributing factors identified by the investigation included:
- Inadequate Fuel Planning: The pilot failed to account for the cumulative fuel burn of previous flights earlier that day and underestimated the aircraft's actual consumption rate.
- Lack of Familiarity with Aircraft Documentation: The pilot demonstrated insufficient knowledge of the Aircraft Flight Manual (AFM) regarding fuel system limitations and capacity.
- Improper Decision Making: Upon seeing the low-fuel warning, the pilot did not divert to a suitable airfield or declare an emergency early enough. Additionally, the pilot attempted to use the auxiliary tank despite it being empty, partly because the control knob was missing, which may have been lost during previous operations.
- Injury Severity: The pilot and front passenger sustained serious injuries due to the high-energy impact on a rigid structure. Rear passengers were less severely injured as they were properly restrained.
Safety action
The investigation noted that the missing control knob for the auxiliary fuel tank was not reported in the aircraft logbook after previous flights, highlighting a gap in maintenance and pre-flight inspection protocols. The case underscores the critical importance of accurate fuel consumption data and strict adherence to reserve fuel requirements for local flights.