What happened
United Flight 97 was operating an 11-hour transpacific route when the crew began experiencing indications of uneven fuel flow from the main No. 2 and No. 3 tanks at flight level 360, approximately four hours after departure. The fuel system had been configured to supply all engines from these two tanks. Although the No. 2 crossfeed valve later failed in the closed position, the first officer initially reported to the captain that the in-transit light illuminated when the selector was moved to closed, suggesting normal operation.
The crew misdiagnosed the fuel system anomaly as faulty fuel gauge indications. Monitoring continued to show insufficient fuel flow from No. 2 tank during crossfeed operations. Consequently, engines 1, 3, and 4 flamed out as fuel was depleted from all tanks except No. 2. The crew initiated an emergency descent to Tokyo-Narita Airport. While the crew reported using flaps-20 for landing, the digital flight data recorder indicated that only flaps-1 were deployed. Three tires blew out upon touchdown.
The investigation
The investigation revealed that the Boeing 747 suffered from a mechanical failure of the No. 2 crossfeed valve, which failed in the closed position. This mechanical state contradicted the crew's interpretation of the system status based on the in-transit light. The discrepancy between the crew's reported flap setting and the DFDR data highlighted a significant deviation from standard landing procedures.
Findings
The primary contributing factor was fuel exhaustion resulting from the inability to properly manage fuel distribution due to the valve failure and subsequent misdiagnosis. Additionally, the use of insufficient flap settings for the emergency landing contributed to the tire blowouts. Notably, all three flight crew members had qualified in the B-747 within the 13 months preceding the incident.