What happened
During the approach to landing, the crew of the Boeing 737 (implied by context/airline) observed an unsafe nose gear light. The crew performed a gear cycle, after which all cockpit indications suggested the landing gear was down and locked. However, upon touchdown on runway 4R, the nose gear collapsed, causing the aircraft to skid to a stop. There were no injuries reported.
The investigation
An examination of the aircraft identified that the nose landing gear upper lock link had fractured. This specific component had accumulated 28,978 service hours and 24,511 cycles since its original manufacture. Records indicated the lock link had been overhauled 13,515 hours and 6,317 cycles prior to the failure.
A metallurgical examination of the part revealed a fracture through the 'I' beam section near the lower end, approximately 3 inches from the overcenter pivot with the lower lock link. The characteristics of the fracture were consistent with fatigue that had progressed across a large portion of the fracture surface. Investigators found no evidence of manufacturing defects, corrosion, or mechanical discontinuities at the origin of the fatigue.
At the time of the accident, Airworthiness Directive AD-97-02-10 was in effect regarding the upper lock link. This directive required inspections of assemblies prior to 10,000 total cycles or within 90 days of the directive's issuance on February 11, 1997. Continental Airlines was actively performing fleet-wide inspections for the affected components when the failure occurred.