What happened
On 29 August 2005, a Boeing 747-400, registered PH-BFO, was operating a scheduled passenger flight from Amsterdam Schiphol to Mexico City when the crew encountered a series of hydraulic failures. Approximately four hours into the flight, the crew noticed a drop in fluid levels within hydraulic system #4. Following consultation with Boeing's technical department, the pilots depressurized the system by deactivating both the engine-driven and demand pumps, which successfully stabilized the fluid level.
Thirty minutes later, the crew observed a similar decline in hydraulic system # 1. To prevent further loss, they also deactivated the pumps for this system. While the crew initially planned to divert to Toronto, a subsequent drop in hydraulic system #2 quantity necessitated an immediate diversion to the nearest suitable airport, Goose Bay, Canada. The crew declared a PAN-PAN urgency signal and performed a fuel dump to reduce the aircraft's weight for landing. Despite the progressive loss of fluid across three systems, the crew repressurized the systems prior to arrival to ensure the functionality of the landing gear and flaps. The aircraft landed without further incident, and no injuries or fatalities occurred among the 205 passengers and 12 crew members.
The investigation
Post-flight inspections of the aircraft revealed a visible leak near the left wing landing gear strut. Investigators identified the source as a damaged flexible brake hose at the second wheel of the left main gear. Laboratory analysis of the component determined that the failure was caused by incorrect installation of the flexible hoses during a recent landing gear maintenance event. This error reduced the necessary slack in the hoses, preventing them to move freely during gear extension and retraction.
Further inspection of the aircraft revealed that three additional hydraulic hoses passing through the same bracket were also damaged, though they were not yet leaking. These were subsequently replaced. The investigation also examined the mechanics of the fluid loss, noting that the leak rate was low enough to avoid triggering hydraulic fuses, which allowed the fluid to deplete slowly over time.
Findings
- The primary cause of the hose failure was the improper installation of flexible hoses during the last wing landing gear change, which restricted necessary slack.
- The sequential loss of fluid across systems #4, #1, and #2 was attributed to a slow leak downstream of the antiskid shuttle valves.
- A latent failure, likely a brake metering valve failing to return to its null position, allowed hydraulic pressure to be supplied to the leak site during flight.
Safety action
Following the incident, Boeing issued an Operations Manual Bulletin (OMB) providing specific instructions on how to manage hydraulic pump shutdowns during low quantity indications and updated diversion recommendations. Additionally, a software update was implemented for the Engine Indicating and Crew Alerting System (EICAS) to ensure the crew is explicitly prompted to take required actions when low quantity indications occur in systems #1 or #4.