Narrative:

I was the first officer and pm. This was my first leg of the day and first leg ever with the crew. Preflight and ground ops were uneventful. I recall looking at the fuel totalizer gauges as we were number two for takeoff and mentally noting we still had approximately 300 pounds before we hit our min takeoff number of 16.3. Approximately five minutes after takeoff; the captain and I noticed the left main fuel tank was approximately 850 pounds less than the right main tank. The center tank was empty from the beginning and a nonfactor. The captain continued to perform PF duties as we began analyzing the problem. We wanted to ensure we didn't have a fuel leak before we started cross feeding. As the pm; I began to document the fuel imbalance to determine if the situation was worsening. My first recorded reading was 940 pounds imbalance approximately six to seven minutes after takeoff. Both tanks were losing ten pounds every few seconds; but eventually the left tank decreased at a substantially higher rate. Fuel flow and all engine instrument indications appeared normal. The captain was referencing the dispatch paperwork to compare expected fuel versus FMC predicted fuel over the next departure point; as well as expected fuel at our destination. Initially; FMC calculations had us over the next point within 100 to 200 pounds of dispatch. I also noted we were 1000 pounds above dispatch destination fuel. Eight minutes after takeoff; my recorded fuel imbalance was 980 pounds. Shortly thereafter; the fuel imbalance exceeded 1000 pounds and we began the fuel imbalance QRH checklist. While running the imbalance checklist; the imbalance increased to 1700 pounds 15 minutes after takeoff. Destination fuel decreased 300 pounds; as well as the predicted fuel over the next point. We concluded; per the QRH imbal checklist; we might have a suspected fuel leak based on the worsening conditions. The captain handed PF duties over to me as she talked with the flight attendant and dispatch. The F/a did not see any fuel leak indications. We coordinated for a return to [departure airport]; our closest suitable runway; 100 miles away. Initially; the decision was made to keep the number 1 engine at idle as we continued to analyze the situation; run checklist; and execute the return. I input two units of right rudder trim to help center the slip indicator. Step six in the fuel leak engine checklist confirmed we might have a fuel leak since our imbalance had increased more than 500 pounds in less than 30 minutes (700 pounds in seven minutes). We proceeded to run the checklist; shutdown the number 1 engine; and [advised ATC]. The maximum fuel imbalance was 3000 pounds just prior to shutting down the number 1 engine. The captain also hand flew the aircraft prior to shutting down the number 1 engine to help confirm our right wing was in fact 'heavy'. We removed the two units of right rudder trim while the captain was hand flying. We confirmed the right wing felt heavy and proceeded to shut down the number 1 engine in accordance with QRH checklist. Per the single engine inoperative landing checklist; the imbalance was reduced after crossfeeding to 2500 pounds on final. We landed uneventfully; taxied clear of the runway; and stopped on the taxiway. Fire rescue confirmed no fuel leaking from the aircraft; so we taxied to the gate and shutdown.I look forward to providing a recommendation as soon as maintenance determines what happened. Maybe the QRH needs a new checklist to handle this malfunction. Maybe we could have kept both engines running and stopped the worsening imbalance conditions some other way. But given the conditions we faced; we could not rule out a possible fuel leak which led us to that checklist.

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Original NASA ASRS Text

Title: B737-700 First Officer reported returning to the departure airport after shutting down the #1 engine in response to a possible fuel leak.

Narrative: I was the FO and PM. This was my first leg of the day and first leg ever with the Crew. Preflight and Ground Ops were uneventful. I recall looking at the fuel totalizer gauges as we were number two for takeoff and mentally noting we still had approximately 300 pounds before we hit our min takeoff number of 16.3. Approximately five minutes after takeoff; the Captain and I noticed the left main fuel tank was approximately 850 pounds less than the right main tank. The center tank was empty from the beginning and a nonfactor. The Captain continued to perform PF duties as we began analyzing the problem. We wanted to ensure we didn't have a fuel leak before we started cross feeding. As the PM; I began to document the fuel imbalance to determine if the situation was worsening. My first recorded reading was 940 pounds imbalance approximately six to seven minutes after takeoff. Both tanks were losing ten pounds every few seconds; but eventually the left tank decreased at a substantially higher rate. Fuel flow and all engine instrument indications appeared normal. The Captain was referencing the Dispatch paperwork to compare expected fuel versus FMC predicted fuel over the next departure point; as well as expected fuel at our destination. Initially; FMC calculations had us over the next point within 100 to 200 pounds of Dispatch. I also noted we were 1000 pounds above Dispatch destination fuel. Eight minutes after takeoff; my recorded fuel imbalance was 980 pounds. Shortly thereafter; the fuel imbalance exceeded 1000 pounds and we began the fuel imbalance QRH Checklist. While running the Imbalance Checklist; the imbalance increased to 1700 pounds 15 minutes after takeoff. Destination fuel decreased 300 pounds; as well as the predicted fuel over the next point. We concluded; per the QRH IMBAL Checklist; we might have a suspected fuel leak based on the worsening conditions. The Captain handed PF duties over to me as she talked with the Flight Attendant and Dispatch. The F/A did not see any fuel leak indications. We coordinated for a return to [departure airport]; our closest suitable runway; 100 miles away. Initially; the decision was made to keep the number 1 engine at idle as we continued to analyze the situation; run checklist; and execute the return. I input two units of right rudder trim to help center the slip indicator. Step six in the Fuel Leak Engine Checklist confirmed we might have a fuel leak since our imbalance had increased more than 500 pounds in less than 30 minutes (700 pounds in seven minutes). We proceeded to run the checklist; shutdown the number 1 engine; and [advised ATC]. The maximum fuel imbalance was 3000 pounds just prior to shutting down the number 1 engine. The Captain also hand flew the aircraft prior to shutting down the number 1 engine to help confirm our right wing was in fact 'heavy'. We removed the two units of right rudder trim while the Captain was hand flying. We confirmed the right wing felt heavy and proceeded to shut down the number 1 engine in accordance with QRH Checklist. Per the Single Engine Inoperative Landing Checklist; the imbalance was reduced after crossfeeding to 2500 pounds on final. We landed uneventfully; taxied clear of the runway; and stopped on the taxiway. Fire Rescue confirmed no fuel leaking from the aircraft; so we taxied to the gate and shutdown.I look forward to providing a recommendation as soon as Maintenance determines what happened. Maybe the QRH needs a new checklist to handle this malfunction. Maybe we could have kept both engines running and stopped the worsening imbalance conditions some other way. But given the conditions we faced; we could NOT rule out a possible fuel leak which led us to that checklist.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.