What happened
The aircraft was established on final approach to Chicago O'Hare International Airport when a sudden structural failure of the number three flap track occurred. This mechanical event resulted in substantial damage to several critical components, including the left wing, the left inboard flap assembly, and the left inboard spoilers along with their support beam.
Despite the significant structural compromise, the pilot experienced significant uncommanded roll and yaw but was able to regain control of the aircraft. The pilot subsequently executed an uneventful landing without further incident.
The investigation
A thorough examination revealed a long-standing history of flap track cracking problems associated with stress corrosion and design inadequacy. Although the manufacturer accomplished a design change of the member in 1971, flap tracks of the earlier design were allowed to remain in service on 239 aircraft.
Findings
The investigation determined that the airworthiness directives and service bulletins applicable to this particular structural member were not adequate in methodology and periodicity to ensure crack detection. Furthermore, these regulations did not require inspection for the cause of the crack-corrosion pits.