What happened
On April 17, 2002, at 17:45 hours, an Aerocommander 680F, registration EC-CTO, was performing a dual-instruction flight at Cuatro Vientos Airport in Madrid. The flight was part of a training session for a student pilot seeking multi-engine piston instructor certification.
Following a normal landing on runway 28, the crew exited via taxiway G. While taxiing parallel to the runway toward hangar 9, the pilot executed a right turn to transition from the paved surface to a gravel area. During this maneuver, the right main landing gear unexpectedly retracted. This caused the aircraft to strike the ground, resulting in damage to the right propeller, the right main landing gear, and the lower portion of the fuselage. The crew successfully shut down the engines and all aircraft systems before evacuating the aircraft without injury.
The investigation
Investigators performed a visual inspection of the right main landing gear strut and identified that the bracket connecting the locking actuator to the strut had failed. The investigation focused on the mechanical properties of this bracket and the functionality of the landing gear actuators.
Technical analysis conducted by the INTA (Instituto Nacional de Técnica Aeroespacial) determined that the bracket, made of high-strength steel, was free of material defects or pre-existing cracks. The analysis revealed that the failure was a result of a static bending overload. The fracture surface showed plastic deformation and a 45-degree fracture plane, characteristic of a sudden high-load event rather than fatigue.
Two hypotheses were considered: that the gear was not fully locked during the initial landing, or that the bracket had been incorrectly adjusted. The investigation examined the possibility that the gear was improperly secured during landing, which would have required a simultaneous failure of both the hydraulic and pneumatic systems to provide a false 'locked' indication in the cockpit. However, the aircraft successfully completed 20 flight cycles following repairs without further issues.
Findings
- The primary cause of the accident was the failure of the bracket connecting the locking actuator to the right main landing gear strut due to a static bending overload.
- The investigation concluded that the most likely cause of this overload was inadequate adjustment of the bracket.
- The failure occurred when the aircraft transitioned from the paved taxiway to a gravel surface, which subjected the gear to vertical and horizontal forces that the compromised bracket could no longer withstand.
- While a Service Letter (No. 376) recommended inspections of this bracket every 500 hours due to reported cracking in other aircraft, no pre-existing cracks were found in this specific component.