What happened
On April 6, 2009, a Cessna 172 RG, registration EC-DNG, was performing a local instructional flight at Madrid-Cuatro Vientos Airport. Following a normal landing on runway 10, the aircraft was taxiing right to exit the runway via the first available exit (E2). During this maneuver, the nose gear collapsed, causing the propeller to strike the ground and resulting in significant damage to the propeller. The two crew members on board were uninjured and evacuated the aircraft on their own.
The investigation
Investigators examined the fractured component, identified in the Illustrated Parts Catalog (IPC) as the nose gear actuator fitting. Laboratory analysis revealed that the part, which should have been constructed from 201ered aluminum alloy 2014-T6, contained a chemical defect. Specifically, the copper (Cu) content was measured at 5.8%, exceeding the maximum allowable limit of 5.0% established by the ASTM B247 standard.
This excess copper resulted in higher hardness levels and lower electrical conductivity than specified. The metallurgical study determined that the presence of intermetallic phases caused by the high copper content reduced the material's toughness, facilitating the nucleation and propagation of cracks. Fractographic analysis indicated that the failure was the result of low-cycle fatigue, where cracks developed and grew through successive heavy landings during the aircraft's service life as a training vehicle.
Findings
- The primary cause of the incident was the collapse of a nose gear component due to material fatigue.
- A contributing factor was a defect in the alloy composition, specifically an excess of copper, which decreased the material's toughness and accelerated crack propagation.
- The aircraft's history of heavy landings during flight training provided the high-load cycles necessary to drive the fatigue process.