Nose Gear Actuator Fitting Failure During Taxi at Madrid-Cuatro Vientos (EC-DNG)

Incident Overview and Aircraft Details On 6 April 2009 at 16:52 local time, a Cessna F-172-RG with registration EC-DNG, operated by Aerofan, was conducting a dual-command instructional flight at Madrid-Cuatro Vientos Airport. The aircraft, manufactured in 1980 and registered on 1 June 1981, had been dedicated to training operations since its acquisition. It was powered by a single Lycoming O-360-F1A6 engine. The pilot in command was a 42-year-old commercial pilot with 2,768 total flight hours and 231 hours on type. ## Sequence of Events Following a normal landing on runway 10, the aircraft was taxiing to the right to exit via the first taxiway (E2) when the nose gear leg folded. The propeller subsequently struck the ground, resulting in significant damage. Both crew members exited the aircraft unassisted and were uninjured. The aircraft sustained minor damage, with no other damage reported. ## Technical Investigation Findings Post-incident inspection revealed that the fitting to which the nose gear actuator was attached had fractured into three pieces: a central section remaining attached to the actuator and two lateral sections remaining attached to the structure. The fracture also sheared one of the brake system hoses. Metallurgical analysis of the fitting, identified in the Illustrated Parts Catalog as the nose gear actuator fitting, indicated it was specified to be manufactured from 2014 aluminum alloy with T6 heat treatment per ASTM B247. Chemical analysis detected a copper content of 5.8%, exceeding the standard maximum of 5.0% (range 3.9% to 5.0%). Hardness values were higher than specified for the 2014-T6 alloy, and electrical conductivity was lower than typical, both consistent with the excess copper. The excess copper had generated a significant quantity of intermetallic phases, which reduced the material's toughness and likely promoted the nucleation and propagation of fatigue cracks. A corrosion process associated with fatigue crack propagation may also have contributed, though this could not be confirmed with absolute certainty during laboratory testing. Fractographic analysis showed that the fracture processes on both sides of the central portion of the fitting developed in a similar and simultaneous manner. The initial stage of failure was attributed to low-cycle fatigue, indicating the cracks grew progressively during successive landings until reaching a critical size that caused the component to collapse. The manufacturer reported that no special inspections beyond routine procedures existed to detect this type of fracture. ## Contributing Factors and Prior History The aircraft's extensive training use involved numerous hard landings, which imposed high loads transmitted through the nose gear hydraulic actuator to the fitting. The low-cycle fatigue failure was consistent with this operational history. The prior accident on 30 August 2008, in which the nose wheel collapsed during landing at the same airport due to an uncommanded nose gear leg unlock, was determined not to have made a significant contribution to the component failure.