What happened
On August 1, 2005, a S.T.R.A CB 15 "Crystal" glider, registered F-CBCK, was performing a local flight following a winch launch from the Chambéry Challes-les-Evers aerodrome. The pilot was flying in company with another glider. During the return leg of the flight, while descending at approximately 160 km/h, the pilot deployed the airbrakes to maintain separation from the preceding aircraft, then subsequently retracted them.
While flying straight at a speed of approximately 220 km/h, the pilot experienced an aerodynamic imbalance in the wings and discovered that the left airbrake had deployed. Attempts to retract the left airbrake via the cockpit controls were unsuccessful, as only the right airbrake remained functional. The pilot attempted to use the rudder to maintain symmetrical flight but was forced to perform an emergency landing in a field. During the landing maneuver, the glider struck trees before hitting the ground, resulting in heavy damage to the aircraft.
The investigation
The investigation focused on the failure of the left airbrake control linkage. Investigators recovered the left airbrake control rod for examination at the BEA laboratory. The analysis revealed that the rod had detached from the ball end at the level of the four rivets. Two of the rivets remaining in the ball end had failed due to shear stress caused by excessive tensile forces.
Technical examinations of the airbrake components showed that the upper blade's spacer fittings were deformed in a direction consistent with aerodynamic forces. Testing on the wreckage demonstrated that if the airbrake becomes partially jammed—specifically if the lower blade and the rear of the upper blade make contact—the force required to operate the control becomes four times higher than normal. While this high force is sufficient to lock the system, it also places extreme stress on the control linkage.
Findings
- The primary cause of the accident was the deployment of airbrakes at speeds exceeding the manufacturer's limits.
- The investigation established that the airbrake components and fittings had been subjected to abnormally high aerodynamic loads, likely due to operating at speeds above the maximum permitted velocity (VNE) of 270 km/h.
- When the pilot attempted to retract the airbrakes, the interaction between the upper and lower blades caused a mechanical jam. The resulting high tension in the control linkage caused the rivets in the control rod to shear, leading to the uncontrolled deployment of the left airbrake.