Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Troubleshooting; Situational Awareness; Time Pressure
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed in Emergency Condition
Aircraft
While at cruise FL360 received a 'tank 3 pumps off' alert. Consulted QRH which stated that the alert could be considered normal if fuel was less than 10;500 pounds in tanks 1 or 3 because of tip fuel transfer balancing. Crew then received alert 'TNK 3 tip trapped'; followed by a 'Tank 3 QTY low' alert. Tank 3 fuel quantity showed 7;000 lbs. Consulted QRH which said that tip fuel (5800 lbs.) was trapped and unusable. This put us in a possible emergency fuel state. Our destination was 160 miles away and closest suitable was ZZZ which was 50 miles away. [Operations Control] and [Maintenance Control] were consulted and decision was made to divert to ZZZ. [ATC was notified about] the low fuel situation. An uneventful approach and landing was accomplished. Landed with 14;000 on fuel gauges and estimated 8;500 with the trapped fuel deducted.The Number 3 tip tank that contains roughly 5;800 lbs of fuel did not drain into main tank as it normally should. Thus becoming fuel that is unusable. There is no crew action or QRH procedure for crew to perform to correct this. Usually caused by a stuck float/valve switch in tank.
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Air carrier flight crew reported a fuel system tank anomaly causing fuel to become trapped and unusable and necessitating an immediate diversion.