What happened
During the landing roll-out, the nose landing gear collapsed. The incident occurred as the aircraft was completing its landing sequence on the runway.
The investigation
An examination of the nose landing gear identified that the nose gear upper lock link (PN 3914464-503) had failed and separated into two pieces. This specific component was subject to recurring non-destructive testing (NDT) every 5,000 cycles in accordance with AD 97-02-10. The operator had performed an eddy current inspection on the link 1,075 cycles prior to the accident.
Investigators determined that the link was part of a series manufactured from plate stock instead of being forged. This manufacturing change meant the machined parts lacked draft angle allowances, which reduced the load-carrying cross-sectional area compared to forged links, thereby decreasing overall strength.
Metallurgical analysis showed approximately 10,000 major fatigue progression cycles within about 0.6 inches of the crack progression. Given that there are two major stress cycles per gear retraction and extension cycle, calculations indicated the crack length would have been greater than 0.25 inches at the time of the inspection 1,075 cycles before the accident.
Findings
Investigators found that the inspection procedure provided by the airframe manufacturer did not require the removal of the upper lock link from the aircraft for recurring NDT. Furthermore, NTSB and FAA inspectors noted that access to the upper lock link for NDT was limited when the part remained installed on the airplane.