Date: 202102
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Human-Machine Interface; Other / Unknown; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight; Routine Inspection
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Captain was pilot monitoring. FO (First Officer) was pilot flying. Passing through 4;000 feet MSL on climbout from ZZZ we noticed a fuel imbalance developing - right wing main tank fuel indication already 400 pounds lower than left wing. Right wing main tank fuel appeared to be feeding at approximately twice the rate of the left and it appeared that the center tank fuel was not feeding at all. Engine fuel flow readings appeared normal and equal. By 10;000 feet MSL the imbalance had increased to the point that the IMBAL alert was activated. We coordinated with ATC to level off at 12;000 feet MSL in order to reduce fuel flow and begin taking appropriate action. Upon leveling at 12;000 feet MSL we observed the right fuel tank quantity feed rate appear to normalize and then observed a 50 pound increase in right main tank fuel quantity. Fuel imbalance at that point was 1;100 pounds. We initiated the IMBAL checklist in the QRH. Observing what then appeared to be normal tank quantity indications for a crossfeed operation; we coordinated with ATC to continue our climb and proceed on course to the destination knowing we would also coordinate with Dispatch and maintenance promptly to determine if proceeding to ZZZ1 was advisable. As soon as we established climb fuel flow settings and pitch attitude; the right main tank quantity began to decrease again at what appeared to be the same abnormal rate observed during the initial climb. We then requested priority handling initiated the return to ZZZ (nearest airport) and started the APU as a precaution. I advised Dispatch; the cabin crew; and the passengers of the situation using SOP. As soon as we stopped the climb to initiate the return to ZZZ the right main fuel tank indication again stopped decreasing at an abnormal rate. We concluded the IMBAL checklist and for the remainder of the flight we observed no further indications of abnormal rate of decrease in right main wing tank fuel. During the descent; I spoke with the flight attendants and they confirmed that there were no visible indications of fuel leaks on either wing or engine. We landed uneventfully at ZZZ with a total imbalance of 1;220 pounds with 8;370 pounds in the left wing; 7;150 pounds in the right wing and 13;470 pounds in the center tank - 29;000 points total. The aircraft was below max landing weight at touchdown. ARFF (Airport Rescue and Firefighting) personnel were standing by as requested. After landing and before further taxi; ARFF personnel inspected the aircraft closely and reported no leaks; no visible damage; and no abnormalities. We shut down the number 2 engine as an additional precaution and requested that the ARFF vehicles follow us to our gate. Maintenance personnel were debriefed in person at the gate upon arrival.Captain suspects suspect faulty fuel crossfeed valve.Reporter recommends company consider revisiting the IMBAL checklist in light of this particular malfunction.




Synopsis: B73-800 Captain reported an uneventful air return due to an airborne fuel system malfunction during initial climb.