What happened
On June 12, 2021, a private VFR flight involving a Grob G103C glider, registration OY-XTG, was conducting local thermalling near Herning (EKHG). While established on the final approach to runway 27, the pilot attempted to deploy the airbrakes. Upon moving the cockpit lever, the pilot noted an unusual sound and resistance. Shortly thereafter, the aircraft experienced a sudden, heavy sink rate.
Upon inspection, the pilot discovered that both airbrakes had fully deployed. Attempts to retract the brakes were unsuccessful. Due to the aerodynamic drag caused by the deployed brakes, the pilot was unable to reach the runway and instead landed in a cornfield approximately 5 and 50 to 100 meters short of runway 27. The aircraft subsequently taxied onto the runway and came to a stop. There were no injuries to the pilot and no damage to the aircraft.
The investigation
The investigation focused on the mechanical failure of the airbrake linkage. Technical inspections of the airbrake system revealed a broken bolt that secured a pivot arm to the fuselage. This breakage severed the mechanical connection between the cockpit control lever and the airbrakes, preventing the pilot from retracting them once they had deployed.
Analysis of the broken bolt showed evidence of a double-sided fatigue failure. While the bolt was the correct M6 size, it was identified as a "NORM 8.8" type rather than the specific part number prescribed by the manufacturer and aviation authorities. Investigators found that although an Airworthiness Directive (AD) had been issued in 2008 to replace this specific bolt with a higher-strength version (part number LN9037), there was no documentation proving the correct part had been used during the 2008 maintenance task. The installed bolt possessed both lower static strength and lower fatigue strength than the required component.
Findings
- The use of an unprescribed and weaker bolt type in the airbrake system caused the bolt to fail under the loads required to overcome the system's over-centering mechanism.
- The failure of the bolt broke the connection between the cockpit lever and the airbrakes, leaving them stuck in the fully deployed position.
- The aerodynamic drag from the deployed airbrakes forced the pilot to land short of the runway in a field.