What happened
On June 26, 2009, a DG-500/20 Elan glider, registration PH-1108, was performing a routine winch launch at Malden Airfield. The flight, which included a pilot and one passenger, proceeded normally until the aircraft reached an altitude of approximately 50 meters. At that moment, the pilot experienced a loud impact and a physical shock, accompanied by a tearing sound.
As the aircraft climbed to 250 meters and disconnected from the winch cable, the pilot discovered significant flight control issues. While the rudder and ailerons remained functional, the elevator control was restricted. Furthermore, the airbrake lever had moved out of its secured position and became jammed, leaving the airbrakes partially open. The flaps were also immobilized. Despite these complications, the pilot managed to fly a wide circuit, using S-turns and slips to manage altitude, and eventually completed a safe landing.
Upon inspection, the aircraft sustained heavy damage. The fuselage featured a hole, the junction between the fuselage and the wing was broken, and the left wing's under-laminate had separated from the spar and ribs. Internal fuselage skinning was also torn, and the control mechanisms for the flaps and elevator had been displaced.
The investigation
The investigation focused on the wing attachment mechanism. The DG-500/20 Elan features detachable wings for transport, where wing stubs from both sides meet within the fuselage. These are secured using two main bolts, which are further secured by locking bolts and clips to prevent them from sliding forward.
Investigators found that the left main bolt had moved forward significantly, causing the left wing stubs to lose their connection. This allowed the right wing stub to pivot upward. This upward movement of the right wing was eventually halted when the wing stub struck the aircraft's control mechanisms, causing the observed jams in the elevator and airbrake systems.
Findings
- The primary cause of the incident was that the main wing bolts were not secured during the most recent assembly of the wings.
- The person responsible for assembling the wings 13 days prior failed to engage the locking mechanism, citing the unusual design of the locking system on this specific model compared to other gliders.
- While a daily pre-flight inspection (A-inspection) is required to check these bolts, the error went undetected during three separate inspections performed by different individuals since the assembly.
- A lack of familiarity with this specific locking mechanism contributed to the oversight.