Date: 202001
Local time of day: 0601-1200
Location: ZZZ.TRACON, US
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Troubleshooting
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During climb around 10;000 ft. to 15;000 ft. the PIC first noticed the aircraft required an unusually high amount of left rudder trim. We then noticed the left wing tank had an amber caution imbalance and was about 1;500 lbs. lighter than the right wing tank. I asked the PIC if we were in balance before departure; and he reminded me that we both had verified the quantity and balance before taking the runway. We then passed our first fix on the fuel log and noticed we were 1;000 lbs. down. The PIC then continued to trim the aircraft to engage the autopilot; while asking me to retrieve the Fuel Imbalance Checklist from the QRH; which lead us to the Fuel Leak Engine checklist. Once I retrieved the checklist the PIC transferred control and radios of the aircraft to me for the duration of the flight. We then weighed our options. Because we were by now 2;000 lbs. light in our left tank and knowing there are not many airports between ZZZ and ZZZ1; we decided to return to ZZZ. As the Captain proceeded on with the checklist; he asked me to [advise ATC] and to start heading back to ZZZ. I asked Center for direct for the field; center then handed us off to approach. On initial check in I asked approach for straight in Runway XX; knowing that it was the longest runway with a headwind. Approach asked if we wanted the visual; I then asked for the RNAV Z Runway XX. Approach cleared us direct to TACOH to cross TACOH at 10;000 ft. and cleared for the approach. Somewhere in the process approach asked if we wanted ARFF standing by and I replied; 'affirmative'. Approach then asked for number of souls on board and fuel remaining. Meanwhile the Captain busy completing checklists; notifying flight attendants; and notifying company; informed me that the flight attendants had smelled fuel on departure. The flight attendants smelling of fuel helped us confirm that our decision to return to ZZZ was correct . Then on about a 5-10 mile left base for Runway XX; the Captain came to the part of the Fuel Leak Engine checklist that required us to shut down the left engine. We weighed our options. There was 5;000 lbs. of fuel in the left tank; the fuel leak had subsided at the low thrust setting; engine indications were normal; there was a lot of traffic in the terminal area (a missed approach wasn't out of the question); we were chose to landing; and we didn't want to do a single engine missed at a high altitude/high terrain airport. We decided to to leave the engine running and complete the decent and before landing checklist. We landed without event and I stopped the aircraft and transferred control of the aircraft to the Captain. The Captain told the passengers to remain seated as we retracted the spoilers and set flaps to 40. ARFF then advised us that there were no leaks or fire and followed us off the runway to the gate; where they boarded and checked on the status of the passengers. All of the passengers were fine. The entire flight from takeoff until touchdown lasted approximately 15 minutes. The Captain did an amazing job of delegating responsibilities among the crew while still keeping everyone in the loop. Although there was a lot to do in a short amount of time; I never once felt rushed or in the 'red'. I attribute this to the Captain setting a great tone on day one of our trip that promoted great team work and communication.


Second narrative: On departure; passing 10;000 ft. we noticed a rapidly growing fuel imbalance between wing tanks. Left was 2;000 pounds less than right tank. Sight right roll tendency was noticed. I passed control and comms to the First Officer. I contacted Tower to query about fuel venting from left wing. None noted. [Advised ATC] and initially leveled off. First Officer flew a return to ZZZ. I ran the fuel leak checklist; emergency landing checklist; briefed the Flight Attendants and made PA. notified Dispatch via ACARS. I had #1 (Flight Attendant) confirm no visible fuel leak on left engine/pylon and left wing areas. First Officer flew a flawless approach and landing to Runway XX. We stopped straight ahead for ARFF inspection; then taxied to gate with ARFF chase.I elected to not shut down the left engine and land based on the following- No visible indications of a fuel leak. All engine indications were normal throughout. Landing was to be conducted in mountainous terrain with high traffic volume. We agreed that with the above indications; the best choice was to land ASAP and not expose ourselves to higher risk of a single engine approach and landing.


Synopsis: B737 flight crew reported that a fuel imbalance resulted in a return to the departure airport.