American Champion Aircraft 7GCAA N519MA accident near Longmont, Colorado

Probable cause

The failure of the landing gear through bolt primarily due to fatigue during a hard landing, which resulted in a landing gear collapse and runway excursion. Contributing to the accident was noncompliance with the manufacturer service letter.

NTSB analysis

The flight instructor and private pilot were conducting an instructional flight when the private pilot made a hard landing and the airplane bounced. The flight instructor heard a "snapping" sound during the landing and subsequently took over the flight controls. He increased engine power to compensate for the bounced landing then landed the airplane on the runway. Upon touchdown, the left landing gear collapsed up and aft. The airplane continued forward on its nose and left wing, then departed the left side of the runway. The left landing gear through bolt was found fractured.

A metallurgical examination of the landing gear through bolt fracture revealed two opposing regions of fatigue comprising 80-90% of the cross-sectional area of the bolt. These were separated by a narrow region of overstress, consistent with reverse bending fatigue crack propagation, which likely grew under low stress as evidenced by the length of the fatigue cracks.

The airplane was subject to a manufacturer service letter for inspection of the landing gear through bolts due to previous instances of cracking and failure. The service letter recommended that the inspections be performed on or before the next 100-hour inspection and at 100-hour intervals thereafter, and at more frequent intervals if the aircraft is used in soft or rough runway operations. The owner of the airplane was unaware of the service letter and stated that the airplane had not received the recommended inspection. Although not required, it is likely that had the service letter been complied with and the through bolt been regularly inspected, the fatigued bolt would have been replaced and the accident would have been prevented.

Text from the NTSB. Open the NTSB docket for CEN18LA059 for the full record.