What happened
On 5 August 2008, a CESSNA 172 RG, registration EC-HHX, was conducting a dual flight training mission departing from Madrid-Cuatro Vientos Airport. The flight included stops at Marugán and Robledillo de Mohernando. During the downwind leg of the circuit at Madrid-Cuatro Vientos, the crew attempted to extend the landing gear, but visual and aural alarms immediately indicated that the gear was not down and locked. Upon inspection, the crew observed that the right main landing gear leg was stuck halfway down, while the nose and left legs were properly positioned.
The instructor pilot took control and communicated with the tower, maintaining the aircraft at 4,000 ft. The crew performed several cycles of the landing gear using both normal and emergency hydraulic procedures, but the right leg remained partially extended. After approximately 40 minutes of flight to burn off fuel, the crew proceeded to land on the runway. During the landing roll, the right main gear leg collapsed completely. This caused the aircraft to veer and come to a stop on its right side, resting on the right wing tip, the right horizontal stabilizer, and the aft fuselage. The three occupants (one instructor and two students) sustained no injuries and were able to exit the aircraft under their own power.
The investigation
The investigation focused on the mechanical failure of the right main landing gear actuating mechanism. Physical examination of the aircraft revealed that the rotational axis of the right gear leg had broken approximately 3 mm from the actuator housing. Laboratory analysis of the fractured component, an Al-Zn aluminum alloy, identified macrofractographic features consistent with progressive fatigue failure.
Detailed inspection of the actuator's internal surfaces revealed friction marks and indentations. These marks indicated that the pivot axis had moved both parallel to the actuator's longitudinal axis and circumferentially. The investigation established that these movements were likely caused by a misalignment of the pivot axis during operation.
Findings
- The primary cause of the landing gear failure was a fatigue fracture of the right main landing gear's rotational axis.
- The fracture was driven by torsional loads acting on the axis.
- The fatigue was triggered by a mechanical misalignment of the pivot axis relative to its axis of rotation, which may have resulted from improper assembly or excessive clearances.
- The failure of the axis prevented the gear from being lowered even when using the manual emergency hydraulic procedure.
- The aircraft's landing gear components had been subject to recent inspections, and the specific service bulletin for inspecting actuators for cracks was current at the time of the accident.