Narrative:

I flew [this] aircraft for two legs. The first leg was uneventful. Preflight was normal; no circuit breakers were tripped. On the second leg we were fueled to 17;400. Both main tanks were indicating equal and full. I checked the takeoff fuel before we got on the runway and it showed 16;800. Five hundred pounds above our takeoff fuel. The tanks were equal within about 100-200 pounds; but the APU had been used and that would cause this minor difference. So this was normal. Approximately six minutes after takeoff; we noticed a 900 pound imbalance: the number one tank low; the number two tank appeared to be depleting normally. I had not selected the crossfeed. The first officer and I began calculating fuel to our next point and destination and compared it to the release. So far the numbers were on par.the fuel burn was normal on both engines at 2;400 pounds/hour. I elected not to start crossfeeding until we were sure there was not a fuel leak. About a minute later we had a 1;000 pound imbalance. Our fuel calculations to destination and next point started to decrease as well. The left tank number one started to decrease at a much faster rate. [Departure airport] was the nearest suitable airport and we started to turn around. I called for the imbal checklist. It told us not to crossfeed and to determine if a fuel leak was suspected. I called the flight attendants to have them look out the captain's side for vaper coming from the engine or wing. They saw no signs. The QRH also said to suspect a possible fuel leak if the total fuel was less than planned arrival fuel.our calculation before we turned around had shown a 300 pound decrease at destination from the previous calculation. We now had a 1;700 pound imbalance; number one tank low at just 15 minutes after takeoff. With this information and our number one tank decreasing rapidly; we suspected a possible fuel leak that the flight attendants could not see. We got vectors toward [departure airport]; called dispatch; [advised ATC of the situation]; and informed the flight attendants. I told the passengers that we were going back for a malfunction; but did not go into detail. The QRH then led us to the fuel leak checklist. The first officer had the left throttle pulled back and two units of rudder trim to help control the adverse yaw created by the unbalanced weight distribution.I manually took the controls; disconnected the autopilot; centered the rudder trim; and matched the throttles to feel if there was in fact a fuel imbalance and we weren't looking at a bad fuel gauge. This maneuver proved that there was in fact a significant weight imbalance and the gauge was accurate. The right wing was very heavy. This was quickly becoming an aircraft controllability issue. The imbalance was now at 3;000 pounds in 25 minutes. The fuel leak QRH said if there is a fuel imbalance of 500 pounds or more in 30 minutes that a fuel leak is confirmed. Aircraft operating manual (aom) states the exact same thing. We had six times that. We continued the QRH which guided us to shut down the number one engine. This helped the controllability of the aircraft.the QRH then said to open the crossfeed selector and verify all main fuel pumps on. Later in the checklist it said to balance the fuel as needed; and at that point I turned off the two left main pumps. We then ran the one engine inoperative landing checklist and landed flaps 15 uneventfully. The landing was made underweight and with a 2;800 pound imbalance. The firemen stated that they did not see signs of a fuel leak so we taxied to the gate. The mechanics confirmed the imbalance and said it was probably a valve malfunction and appeared that both engines were taking primarily from the left tank.come to find out there was no fuel leak; but a valve malfunction. Hypothetically speaking if we had run the crossfeed valve selector inoperative QRH it would have asked us to check a circuit breaker. Maintenance later said the circuit breakerhad tripped. The checklist would have then led us to verify sufficient fuel for both engines. This could not be accurately verified since the number one tank was the primary tank being used for dual engine operation. With limited information; short time; rapid increasing problem; and QRH guidance; I feel we did what was needed to get the aircraft on the ground safely. My concern is some parts of the QRH could have been better.

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Original NASA ASRS Text

Title: The Captain of a B737-700 reported that a fuel imbalance was increasing during fight at a very rapid rate.

Narrative: I flew [this] aircraft for two legs. The first leg was uneventful. Preflight was normal; no circuit breakers were tripped. On the second leg we were fueled to 17;400. Both main tanks were indicating equal and full. I checked the takeoff fuel before we got on the runway and it showed 16;800. Five hundred pounds above our takeoff fuel. The tanks were equal within about 100-200 pounds; but the APU had been used and that would cause this minor difference. So this was normal. Approximately six minutes after takeoff; we noticed a 900 pound imbalance: the number one tank low; the number two tank appeared to be depleting normally. I had not selected the crossfeed. The First Officer and I began calculating fuel to our next point and destination and compared it to the release. So far the numbers were on par.The fuel burn was normal on both engines at 2;400 pounds/hour. I elected not to start crossfeeding until we were sure there was not a fuel leak. About a minute later we had a 1;000 pound imbalance. Our fuel calculations to destination and next point started to decrease as well. The left tank number one started to decrease at a much faster rate. [Departure airport] was the nearest suitable airport and we started to turn around. I called for the IMBAL Checklist. It told us not to crossfeed and to determine if a fuel leak was suspected. I called the flight attendants to have them look out the Captain's side for vaper coming from the engine or wing. They saw no signs. The QRH also said to suspect a possible fuel leak if the total fuel was less than planned arrival fuel.Our calculation before we turned around had shown a 300 pound decrease at destination from the previous calculation. We now had a 1;700 pound imbalance; number one tank low at just 15 minutes after takeoff. With this information and our number one tank decreasing rapidly; we suspected a possible fuel leak that the flight attendants could not see. We got vectors toward [departure airport]; called Dispatch; [advised ATC of the situation]; and informed the flight attendants. I told the passengers that we were going back for a malfunction; but did not go into detail. The QRH then led us to the fuel leak checklist. The First Officer had the left throttle pulled back and two units of rudder trim to help control the adverse yaw created by the unbalanced weight distribution.I manually took the controls; disconnected the autopilot; centered the rudder trim; and matched the throttles to feel if there was in fact a fuel imbalance and we weren't looking at a bad fuel gauge. This maneuver proved that there was in fact a significant weight imbalance and the gauge was accurate. The right wing was very heavy. This was quickly becoming an aircraft controllability issue. The imbalance was now at 3;000 pounds in 25 minutes. The fuel leak QRH said if there is a fuel imbalance of 500 pounds or more in 30 minutes that a fuel leak is confirmed. Aircraft Operating Manual (AOM) states the exact same thing. We had six times that. We continued the QRH which guided us to shut down the number one engine. This helped the controllability of the aircraft.The QRH then said to open the crossfeed selector and verify all main fuel pumps on. Later in the checklist it said to balance the fuel as needed; and at that point I turned off the two left main pumps. We then ran the one engine inoperative landing checklist and landed flaps 15 uneventfully. The landing was made underweight and with a 2;800 pound imbalance. The firemen stated that they did not see signs of a fuel leak so we taxied to the gate. The Mechanics confirmed the imbalance and said it was probably a valve malfunction and appeared that both engines were taking primarily from the left tank.Come to find out there was no fuel leak; but a valve malfunction. Hypothetically speaking if we had run the crossfeed valve selector Inoperative QRH it would have asked us to check a circuit breaker. Maintenance later said the circuit breakerhad tripped. The checklist would have then led us to verify sufficient fuel for both engines. This could not be accurately verified since the number one tank was the primary tank being used for dual engine operation. With limited information; short time; rapid increasing problem; and QRH guidance; I feel we did what was needed to get the aircraft on the ground safely. My concern is some parts of the QRH could have been better.

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.