What happened
While operating in cruise flight at flight level 330, the crew of the aircraft received a master caution light indicating hydraulic system 1 low pressure. This was accompanied by warning lights for the outboard spoilers, outboard spoilerons, and outboard ground spoilers. Although hydraulic fluid quantity and temperature readings initially remained stable, the system pressure began to drop.
The crew executed the emergency checklist and proceeded to Denver, utilizing the backup (1B) hydraulic system. During the final approach and landing phase, the captain reported a complete loss of both pressure and quantity in the number 1 system. In response, the crew shut off the 1B system to facilitate the landing. Following the arrival, the crew shut down the number one engine and taxied the aircraft to the gate.
The investigation
A post-incident examination of the aircraft's number 1 hydraulic system revealed that the hydraulic pump had experienced an internal failure and seized. Investigators discovered metal fragments within the hydraulic filters and lines. Further inspection of the pump identified that several piston shoes in the pump's hanger and piston subassembly had detached. These metal fragments caused significant damage to the hold-down plate, the hanger, and other internal components, which ultimately led to the pump seizure.