What happened
On June 25, 1989, at approximately 14:57 local time, a Blanik L-13 glider (HB-942) was involved in a fatal accident near Cortebert, Switzerland. The aircraft was undergoing an initiation flight under the auspices of the Biel Gliding Group, based at Courtelary aerodrome. The pilot, a licensed instructor with extensive experience, was training a young passenger.
During the towed takeoff by a Robin aircraft, the glider failed to climb as expected. The tug pilot immediately observed that the glider's air brakes were fully extended and alerted the glider pilot via radio. Ground personnel also attempted to warn the glider pilot on the local frequency, but due to high traffic congestion on the channel, these calls went unheard by the glider pilot, who was focused on the lack of altitude gain.
As the tug approached obstacles near Cortebert, it released the glider. The glider pilot attempted a right-hand reversal turn at low altitude to avoid terrain. During this maneuver, the aircraft lost airspeed rapidly and entered a stall, crashing into a field. The passenger sustained serious injuries and later died from them, while the pilot survived uninjured. The glider was destroyed.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the circumstances surrounding the accident. Key findings included:
- The glider's weight and balance were within limits, though the front seat load was approximately 8 kg below the prescribed minimum.
- The pilot held valid licenses and had flown the Blanik L-13 regularly since 1968.
- The aircraft was airworthy, with its last state control performed in August 1988.
- Weather conditions were favorable, with light winds and moderate turbulence at cloud level.
- Witnesses confirmed that the air brakes were deployed shortly after takeoff.
Findings
The investigation concluded that the primary cause of the accident was a stall occurring at low altitude during a reversal turn. Contributing factors included:
- Inadvertent deployment of air brakes due to an incomplete pre-flight check of the locking mechanism.
- The pilot's decision to execute a low-altitude reversal turn after release from the tug.
- The front seat load being slightly below the minimum required weight, which exacerbated the loss of speed during the turn.
The radio warnings from the tug and ground crew were ineffective due to frequency congestion and the pilot's preoccupation with the climb performance issue.
Safety action
This report highlights the critical importance of thorough pre-flight checks, particularly regarding air brake locking mechanisms. It also underscores the risks associated with low-altitude maneuvers following an emergency release and the necessity of maintaining proper weight and balance configurations.