Schempp-Hirth Nimbus-3 glider accident at Sion Airport (HB-KAW)

Accident Overview and Investigation Scope The Swiss Transportation Safety Investigation Board (STSB) issued final report number 2356 concerning the accident involving a Schempp-Hirth Nimbus-3 glider, registration HB-1638, which occurred on 26 June 2016 at Sion Airport (LSGS) in the canton of Valais. The investigation was conducted under the mandate of the Swiss Federal Act on Air Navigation and ICAO Annex 13, with the sole objective of accident prevention and not the determination of legal or civil liability. The reference version of the report is in German, and all times are given in Coordinated Universal Time (UTC), with local time corresponding to Central European Summer Time (CEST), which is UTC + 2 hours. ## Aircraft, Operator, and Pilot Information The aircraft was a single-seat Schempp-Hirth Nimbus-3 glider with a wingspan of 25.5 meters, operated and owned privately. It was equipped with a center-of-gravity (C.G.) tow hook at the bottom of the fuselage but did not have a nose hook. At the time of the accident, the wing water ballast tanks, with a total capacity of 338 liters, contained approximately 120 liters of water. Both the mass and center of gravity were within the limits prescribed in the aircraft's flight manual. The pilot was a Swiss citizen born in 1952, holding a Sailplane Pilot Licence (SPL) issued by the Federal Office of Civil Aviation (OFAC) under the European Aviation Safety Agency (EASA). The pilot had a total of 1,963 hours and 32 minutes of flight experience, including 1,208 hours and 43 minutes on the type. In the 90 days preceding the accident, the pilot had logged 10 hours and 17 minutes, with 5 takeoffs and landings. ## Flight History and Sequence of Events The pilot had begun the 2016 gliding season in March with a supervised check flight. Because the landing was not satisfactory, a second supervised check flight was conducted. Subsequently, the pilot made three flights in the HB-1638 before the accident. On 26 June 2016, the pilot arrived at the Sion gliding club at approximately 09:00 local time, prepared the glider in front of the club hangar, and filled the wing tanks with about 120 liters of water. The pilot performed the pre-flight inspection and control check. Although the weather was fair, morning gliding conditions were mediocre, so the pilot planned a 13:00 takeoff. For the takeoff, the HB-1638 was positioned to the right of grass runway 25 to maintain the minimum 75-meter distance between the center of the paved runway and the left edge of the glider. The tow aircraft, a Robin DR400 with registration HB-KAW, aligned in front of the glider on the same axis. The towed formation appeared slightly offset to the left, pointing toward grass runway 25. At 13:04 UTC, the towed formation received takeoff clearance and began to move. After the ground crew member held the glider level for more than 5 seconds and the formation had moved a few meters, the right wing, once released, dropped and touched the ground. The glider then turned markedly to the right. The pilot corrected to the left with sustained pressure on the left rudder pedal, and after about 3 seconds, the right wingtip lifted. The glider then moved to the left and crossed the runway centerline behind the tow aircraft. The pilot then corrected sharply to the right, crossing the centerline again, and once more corrected harshly to the left. During this phase, the left wingtip touched the ground, and the glider rotated rapidly around its vertical axis and suddenly rose at an angle of approximately 45 degrees. It then rolled to the left around the left wingtip and came to rest upside down on the paved runway. Shortly before the glider fell, the tow cable broke. The tow pilot, upon noticing the absence of the glider in the rearview mirror and feeling the tow cable break, aborted the takeoff shortly before reaching takeoff speed. Only while taxiing back up the runway did the tow pilot realize that the glider had crashed on the paved runway. ## Injuries and Damage The glider pilot sustained various abrasions, a deep cut to the forehead, and anterograde amnesia between the time of the accident and arrival at the hospital emergency department. The pilot was properly restrained, which very likely saved his life. The glider was destroyed. Two runway edge lights were damaged. ## Meteorological and Aerodrome Conditions At the time of the accident, a valley breeze was blowing in gusts, with dry weather and good visibility. The sky had 3/8 to 4/8 cloud cover at 7,000 feet above aerodrome elevation and 3/8 to 4/8 at 10,000 feet. Visibility was 25 kilometers. The wind was from 245 degrees at 15 knots, gusting to 25 knots. The temperature was 23 degrees Celsius, with a dew point of 10 degrees Celsius. The QNH was 1016 hPa. Sion Airport, which recorded approximately 40,000 movements in 2014, is equipped with a 2,000-meter paved runway and a 660-meter grass runway. It is classified as an aerodrome reference code number 3 under ICAO Annex 14 due to the length of its paved runway. The gliding season runs from March to September, and takeoffs and landings are performed almost exclusively on the grass runway. According to ICAO Annex 14, no object may be located less than 75 meters from the axis of a code 3 paved runway during flight operations. Because the two runway axes are only 50 meters apart, no aircraft may be present on the grass runway when the paved runway is in use. To comply with the 75-meter distance, the glider is normally positioned north of the grass runway for departure, while the tow aircraft is oriented toward the runway axis. During the taxi phase, the tow aircraft makes a slight turn to join the grass runway axis. This procedure was introduced in March 2011 and did not meet with particular opposition from glider and tow pilots.