What happened
On Thursday, December 12, 1987, a private pilot departed from Birrfeld aerodrome at 14:40 local time for a short local flight. The aircraft involved was a Cessna Skylane 182 R, registered as HB-CFK. Weather conditions were characterized by low temperatures (-4°C) and high humidity, with overcast skies.
The pilot experienced difficulty starting the engine due to the cold ambient temperature. A witness observed the engine "coughing" at the beginning of the roll but noted it smoothed out before takeoff. Shortly after liftoff, reaching an altitude of approximately 10-15 meters, the same witness heard a loud bang and saw smoke from the exhaust. The aircraft crossed the nearby National Highway N1 at low altitude and turned left without gaining height. As it flew over the Reuss valley, which was lower than the airfield, the plane lost altitude. The pilot attempted to reach lower terrain but collided with a wooded slope rising from the ground 1,100 meters north of the runway end at 14:43.
The aircraft was destroyed, and minor forest damage occurred. The pilot sustained severe injuries and died eleven days later in the hospital from complications related to the crash.
The investigation
SUST examined the wreckage, the engine components, and the flight conditions. Post-accident analysis of the carburetor float chamber revealed only fuel, with no water or contamination, but small water droplets were found in the air channel. Examination of the spark plugs showed normal deposits covered in soot, indicating the engine had been running rich. Engine tests confirmed no mechanical defects.
The investigation focused on whether technical failure or pilot error contributed to the incident. The weather report indicated conditions conducive to carburetor icing: low temperature and high humidity. The aircraft flight manual explicitly recommended using carburetor heat during warm-up, takeoff, climb, and cruise in cold weather. However, post-crash checks showed the carburetor heat was off.
Findings
The primary cause of the accident was a loss of engine power during the initial climb, most likely due to carburetor icing. Contributing factors included:
- Insufficient use of carburetor heat given the low temperature and high humidity.
- The pilot's decision-making after recognizing power loss; instead of landing straight ahead on the extended runway centerline where terrain was flat, the pilot turned toward lower ground which featured an upward slope, leading to collision with rising terrain.
The investigation found no evidence of medical issues affecting the pilot or mechanical failure of the engine prior to the icing event. The aircraft was airworthy and within weight and balance limits.
Safety action
SUST highlighted the importance of adhering to manufacturer guidelines for cold weather operations. The report noted that similar carburetor icing had been observed during ground runs at Birrfeld under comparable weather conditions, emphasizing the need for strict compliance with carburetor heat procedures during critical phases of flight in such environments.