North American TB-25N N9079Z accident near Georgetown, Delaware

Probable cause

Maintenance personnel's inadequate examination of the main landing gear during the airplane’s most recent conditional inspection, which resulted in the fatigue failure of the right main landing gear shock strut drag arm during the landing roll.

NTSB analysis

According to both pilots, they landed the airplane and felt the right wing drop during the landing roll. They attempted to maintain directional control, but the right wing dropped further and the airplane veered off of the right side of the runway. During a subsequent examination of the wreckage, the right main landing gear shock strut drag arm was found broken. An examination of the broken drag arm revealed that it fractured in the I-beam at the blend radius of an orthogonal arm due to bending fatigue. Given the extent of fatigue crack propagation in the flange of the I-beam, it is likely that the fracture occurred due to high-cycle, low-stress fatigue. Since the depth of the fatigue crack was significant relative to the I-beam flange thickness, the fatigue crack probably developed and propagated over a long time under numerous low-stress cycles. The most recent inspection of the main landing gear was on July 7, 2011, which was in accordance with a conditional inspection. The inspection required a mechanic to verify the condition and security of the main landing gear. It is most likely that the fatigue crack would have been detectable at the time of that inspection.

Text from the NTSB. Open the NTSB docket for ERA11LA484 for the full record.