Narrative:

On climbout and upon reaching cruise altitude FL380; we experienced moderate turbulence. Immediately; in search of a smoother altitude; we requested lower and found FL360 to be a better ride. As I was doing my cruise flow; I noticed we had a fuel imbalance of around 500 pounds. This alerted us to examine the possibility of a fuel leak. As per the QRH; verifying fuel load and calculated as per the flight plan's anticipated burn; we established the correct frob and both engine's ff to be correct at around 2;500 pounds. We continued to closely monitor the burn and found the imbalance increasing rapidly with no logical explanation. To prevent the imbalance exceeding any limitations; we tried to opening the crossfeed; which turned out not to be working because of a tripped crossfeed valve circuit breaker. At this point; we agreed to get maintenance and dispatch in the loop and my first officer tried to establish communication. Unfortunately this took about 10 to 15 minutes and by this point the imbalance had reached over 1;000 pounds. The left tank was by now below 2;800 pounds. We were still well over 220 miles from [destination] with [another suitable airport] on the other hand in clear sight only about 120 miles away.during the lengthy attempt to contact maintenance; we tried to reduce the fuel burn by starting a slow descent. We reduced the left engine to idle and made small thrust adjustments with the right engine to maintain a descent rate of about 1;000 FPM to slow down the increase of the imbalance. According to our calculations; if we were to continue to [destination]; we were going to burn the left tank well below the 2;000 pounds or we were considering an idle descent into [the nearby airport] keeping the fuel level in the left tank above emergency fuel level.additionally; we had to consider that [destination] was landing north with gusty conditions; which would have required an additional fuel burn. After finally getting everyone in the loop; we had reached an altitude of about FL240 and were just about 80 miles out of [diversion airport]. The mutual decision was made to divert. An emergency was declared and we pulled up to the gate with 2;400 pounds in the left tank and 3;700 pounds in the right tank.

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Original NASA ASRS Text

Title: B737-700 Captain reported apparent fuel leak in the left tank that led to a decision to declare an emergency and divert to nearest suitable airport.

Narrative: On climbout and upon reaching cruise altitude FL380; we experienced moderate turbulence. Immediately; in search of a smoother altitude; we requested lower and found FL360 to be a better ride. As I was doing my cruise flow; I noticed we had a fuel imbalance of around 500 LBS. This alerted us to examine the possibility of a fuel leak. As per the QRH; verifying fuel load and calculated as per the flight plan's anticipated burn; we established the correct FROB and both engine's FF to be correct at around 2;500 LBS. We continued to closely monitor the burn and found the imbalance increasing rapidly with no logical explanation. To prevent the imbalance exceeding any limitations; we tried to opening the crossfeed; which turned out not to be working because of a tripped crossfeed valve circuit breaker. At this point; we agreed to get Maintenance and Dispatch in the loop and my First Officer tried to establish communication. Unfortunately this took about 10 to 15 minutes and by this point the imbalance had reached over 1;000 LBS. The left tank was by now below 2;800 pounds. We were still well over 220 miles from [destination] with [another suitable airport] on the other hand in clear sight only about 120 miles away.During the lengthy attempt to contact Maintenance; we tried to reduce the fuel burn by starting a slow descent. We reduced the left engine to idle and made small thrust adjustments with the right engine to maintain a descent rate of about 1;000 FPM to slow down the increase of the imbalance. According to our calculations; if we were to continue to [destination]; we were going to burn the left tank well below the 2;000 LBS or we were considering an idle descent into [the nearby airport] keeping the fuel level in the left tank above emergency fuel level.Additionally; we had to consider that [destination] was landing north with gusty conditions; which would have required an additional fuel burn. After finally getting everyone in the loop; we had reached an altitude of about FL240 and were just about 80 miles out of [diversion airport]. The mutual decision was made to divert. An emergency was declared and we pulled up to the gate with 2;400 LBS in the left tank and 3;700 LBS in the right tank.

Data retrieved from NASA's ASRS site as of July 2013 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.