What happened
During flight, a hydraulic line (part number 137311D1443) within the main hydraulic system failed. Following the failure, the captain and first officer performed numerous attempts to extend the landing gear by following the emergency checklist and the aircraft flight manual. While two of the three landing gear legs were successfully extended, the crew was unable to extend the left main landing gear. As a result of the gear configuration, the aircraft sustained damage during the landing process.
The investigation
Investigators examined the failed hydraulic line, which was a curved component measuring approximately 15 feet from end to end. The failure occurred at the base of a flared fitting, an event that allowed hydraulic fluid from the main system reservoir to be pumped overboard.
The investigation also reviewed the emergency hydraulic system, which utilizes a three-position selector: Normal, Flaps Down, and Gear Down. Testing of the gear extension system confirmed it functioned according to the instruments specified in the emergency checklist. However, when the selector was set to the Normal position, the line failure allowed fluid to be pumped from the emergency reservoir into the main system.
A metallurgical examination of the hydraulic line revealed that the component had failed due to multiple fatigue origins located on the outside diameter of the tube and on diametrically opposite sides.