Pilatus B4-PC 11AF accident during aerobatic exam at Zweisimmen
On July 19, 1986, a Pilatus B4-PC 11AF was involved in an accident at Zweisimmen, Switzerland, during an aerobatic exam. After releasing the tow cable at approximately 700 meters, the pilot performed a series of aerobatic maneuvers. The maneuvers were initially executed correctly, but the spirals were too tight, leading to a significant loss of altitude. During the final approach, the glider stalled and crashed into the Simme River. The pilot sustained serious injuries and hypothermia but survived. The aircraft was destroyed.
- Aerotow aerobatics flight: The pilot aerotowed from Zweisimmen runway 35 to about 700 m AGL and began the planned aerobatic exam sequence overhead the airfield.
- Overly steep spirals: During the three right spirals, the bank angle was about 80°, causing an abnormally high sink rate and greater than expected loss of altitude.
- Low exit altitude: Despite the examiner’s relayed request to stop the program, which the pilot did not acknowledge, he completed the third spiral and exited into downwind at only about 80 m AGL and 85 km/h, well below the recommended spiral exit height.
- Low-speed turn to final: From this low downwind, the pilot initiated a 180° left turn toward final for runway 35 at a speed near minimum flying speed and with possible sideslip and tailwind gust influence.
- Stall and wing drop: After turning about 150° and as the nose was raised toward the runway heading, the pilot felt a jolt and tail lift, and the glider abruptly rolled steeply left into a stall from which recovery was not possible at the low height.
- Crash - serious injury: The glider impacted nose-down in the Simme river near the airfield, was destroyed, and the pilot sustained serious injuries and severe hypothermia while trapped for about 90 minutes in cold water.