What happened
On June 1, 2002, a pilot was conducting a local flight in an RSchneider LS 4, registration PH-1027, near the Terlet glider airfield. After approximately 45 minutes of flight, the pilot returned to the airfield circuit. While at an altitude of roughly 600 meters, the pilot attempted to retract the airbrakes. Upon doing so, the aircraft began behaving erratically, as if encountering intense thermal activity.
The pilot discovered that while the right airbrake had retracted, the left airbrake remained extended. Attempts to retract the left brake were unsuccessful. To regain control, the pilot increased airspeed to between 15 and 16 km/h and applied full right rudder and aileron inputs, which eventually induced a slow right-hand turn. Due to the significant loss of altitude during this struggle, a landing at the Terlet airfield was no longer possible. The pilot managed to execute a left-hand turn to land into the wind, eventually bringing the aircraft to a stop in a heathland area outside the airfield. There were no injuries and no damage to the aircraft.
The investigation
The investigation focused on the mechanical connection between the fuselage and the wing control surfaces. The airbrakes and ailerons are operated via rods that extend from the fuselage to the wings. These connections utilize l’Hotellier-type quick-release mechanisms, which allow the wings to be easily detached. Because these mechanisms have a history of potential failure or improper installation, an Airworthiness Directive (AD 1993-001) was implemented, requiring a specific securing method (such as a pin, sleeve, or clip).
The PH-1027 was equipped with an 'LS-Safety Sleeve' designed to comply with these safety requirements. This sleeve serves two purposes: it can only be rotated into place if the ball is correctly seated, and it prevents the connection from disconnecting if the spring becomes worn. The investigation examined how the pilot's pre-flight check failed to detect the malfunction.
Findings
- The primary cause of the incident was that the safety sleeves for the quick-release couplings were not correctly installed during the assembly of the airbrake control mechanism, allowing both couplings to become disconnected.
- The pilot's pre-flight inspection, which involved pulling and pushing on the airbrakes, was insufficient to verify the integrity of the connection.
- A more effective verification method involves applying approximately 5 kg of force while attempting to twist and slide the coupling to ensure the ball is properly seated and locked.