What happened
On March 27, 1982, a private flight was scheduled from the airfield in Triengen, Switzerland, to Lugano and back. The aircraft involved was a Cessna F 172 N with registration HB-CNV. The pilot, a Swiss citizen born in 1941, had not flown for several months prior to this incident. He intended to fly with three passengers: a fellow pilot friend seated in the front right seat and two acquaintances in the rear.
The aircraft was equipped with optional long-range fuel tanks, increasing its total capacity to 204 liters compared to the standard 163 liters. The owner and operator of the aircraft, who also served as the flight instructor for the pilot, had refueled the plane the previous day. Knowing the pilot's intention to carry three passengers, the instructor should have limited the fuel load to approximately 100 liters (half a tank in each wing) to ensure the aircraft did not exceed its maximum takeoff weight.
On the morning of the accident, the pilot arrived at Triengen around 07:45 local time. He performed an external check and discussed the route with his instructor on the ground. The instructor informed him that the plane had already been fueled. No specific weight and balance calculation was performed by the pilot before departure. The pilot checked the fuel gauges, which indicated "almost full" or "full," but did not verify the actual weight.
The takeoff was conducted from Runway 33 at Triengen, which has a length of 395 meters. The aircraft had flaps extended to 10 degrees. The pilot applied full power and began rolling. According to witness accounts, including that of the instructor observing from the ground, the aircraft accelerated normally but lifted off prematurely, approximately 300 meters into the takeoff roll, which was earlier than expected for the conditions.
Shortly after becoming airborne, at an altitude of roughly 30 meters, the pilot retracted the flaps too early. This action caused the aircraft to lose lift and sink. The pilot attempted to correct the descent by pulling back on the controls, but the aircraft had already lost sufficient airspeed. It stalled, rolled to the right, and crashed into a meadow about 400 meters past the end of Runway 33. The impact with a small mound caused the aircraft to flip over.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) conducted an inquiry into the accident. Key findings included:
- Pilot Experience: The pilot had only 99 total flight hours, with none in the preceding 90 days. He had flown the HB-CNV only three times previously.
- Aircraft Configuration: The Cessna 172 N was equipped with long-range fuel tanks. The exact fuel quantity at takeoff could not be precisely determined due to discrepancies in records and gauge readings, but estimates suggested between 110 and 130 liters.
- Weight and Balance: Calculations indicated the aircraft was overloaded by approximately 21 kg (up to 43 kg depending on fuel estimates), exceeding the maximum takeoff weight of 1,043 kg. The center of gravity was within limits but towards the rear.
- Weather Conditions: Weather was clear, with no wind and good visibility, which did not contribute to the accident.
- Witness Accounts: The passenger in the front right seat confirmed the fuel gauges appeared full and noted the aircraft lifted off earlier than expected. He also stated he felt the airspeed was insufficient before the stall.
Findings
The primary cause of the accident was improper takeoff technique by the pilot. Specifically:
1. Premature Rotation: The pilot rotated (pulled back on the controls) too early during the takeoff roll, causing the aircraft to lift off at a speed lower than optimal for the overloaded condition. 2. Early Flap Retraction: The flaps were retracted while the aircraft was still at a low altitude and airspeed, leading to a loss of lift and a subsequent stall. 3. Overloading: The aircraft was overloaded due to excessive fuel load. The pilot failed to perform a weight and balance calculation, relying instead on the instructor's prior refueling without verifying the total weight against the maximum takeoff limit. 4. Lack of Training: The pilot's recent lack of flying experience contributed to poor judgment regarding performance limits.
Contributing factors included the pilot's failure to use shoulder harnesses (resulting in a minor head injury for the front passenger) and the absence of a pre-takeoff weight calculation.
Safety action
The investigation highlighted the critical importance of pilots performing their own weight and balance calculations, regardless of who refueled the aircraft. It also emphasized the need for adequate training and recent experience before flying with passengers in overloaded configurations.