Narrative:

Departed normal. At cruise; FL400; we noticed a fuel imbal of approximately 800 pounds. We thought it was a possible gauge problem because both of us just looked at the fuel less than 5 mins before and it was only 200 pounds. We started a fuel log and 10 mins later calculated we were burning over 1000 pounds more per hour from the left engine. We also looked at the fuel flow and saw a very small difference which would have little effect. We did the fuel leak checklist and asked the flight attendant to look to see if fuel was leaking from the left engine/wing they saw nothing. Since we were not sure if the fuel was leaking or if it was a gauge malfunction; we did not shut down the #1 engine. At this time we had no other indications -- the yoke was centered. As we continued toward ZZZ we continued to monitor the fuel burn and an estimated arrival fuel if indeed it was a fuel leak. Approximately 240 mi from las we were asked to descend to FL330. We then declared 'minimum fuel' to stay high to conserve fuel just in case the cause was correct. We explained our situation to center and they gave us FL360. Another carrier aircraft asked if we were the airplane that just overflew them (we were) and they reported we had only 1 contrail. At this point we now knew we were leaking fuel. Also; the yoke showed a slight left deflection (less than 1 ft); at this point also confirming our imbal. Our fuel log predicted landing with just under 4000 pounds of fuel; 600 pounds above our far reserve of 3300 pounds. We continued to ZZZ. After switching frequencys; the low sector controller wanted us to turn for spacing. To get expeditious handling we declared an emergency and requested crash fire rescue equipment for arrival. We told the flight attendants about the situation. I briefed them on the probable outcome; landing and normal deplane and if we evacuate/evacuationed; evacuate/evacuation to the right side only. I also told the passenger we had a fuel leak and that the landing would be normal but fire trucks would be at the runway and would check to see if we were leaking fuel before we went to the gate. Normal approach. After touchdown I shut down the #1 engine and used #2 thrust reverser and brakes to stop aircraft. Our gate was occupied so I parked aircraft behind an aircraft on gate and shut down #2 engine. The crash fire rescue equipment chief inspected left engine and saw very little fuel dripping from #1 engine. After aircraft pushed; we were towed to the gate. Normal deplaning. Deviation from QRH. When we first discovered the imbal; we had no other indications that fuel was actually leaking. We both agreed that shutting down an engine at that point was not the best course of action. When the other carrier and the yoke confirmed that we were leaking fuel; I still did not want to shut down the #1 engine because: 1) we had enough fuel in the left tank even with the leak to land in ZZZ with fuel left over. At this point we were 240 mi from ZZZ and 180 degrees from ZZZ1. 2) diverting to ZZZ1 with a single engine (nearest suitable) was less safe than continuing to ZZZ with the fuel leak. 3) if something happened to the #2 engine; I did not want to try to start the #1 engine with a fuel leak. If the leak was at the fuel control; could it affect the relight? 4) I did not want to slow down (single engine speed 220/210 KTS) and delay arrival. By the time I figured the fuel burn with 1 engine plus if the fuel leak continued after shutdown just seemed a lot simpler to continue with the current confign. 5) I felt as long as we a) kept airflow (ie; in-flight) around the cowl; the fuel would dump overboard without ignition if the fuel leak was in the engine compartment. B) shut down the engine as soon as the wheels were on the ground and we knew we didn't have to go around. C) did not use the #1 thrust reverser. Also; the first officer suggested not using any lights on that side of the aircraft. We did this. I believe we followed the safest course of action. I do not know if the engine manufacturer can say how much fuel can be dumped into the cowl before there is a problem in-flight. Supplemental information from acn 704984: we noticed a 500-700 pound fuel imbal on the left side. This was odd because 10-15 mins prior we just completed a xfer of fuel in the center tank. We start to monitor to see if it was a possible gauge problem or fuel leak. Monitoring all instruments and fuel flows; no other indications were observed. The aileron trim did not move to support this imbal. I started taking 10 min comparison readings; while the PF was monitoring the system and asking one of the flight attendants to see if a stream of fuel could be seen from in the cabin. Report came back negative. We pulled out the QRH to review possible options/actions. Still no confirmations of fuel leak. We started reviewing possible diverts if practical. We advised dispatch of this event via ACARS. We now noticed a large change in arrival fuel at our destination and began a mental math calculation of fuel remain if in fact this was a leak. We concluded 4.5 at ZZZ. After 3 readings or 30 mins; we realized we were losing 200 pounds of fuel every 10 mins. To receive special handling into ZZZ we declared minimum fuel; requested direct to ZZZ and remain at FL360 after descending from FL400. Center kept us high in altitude to conserve. About the same time frame; another carrier's jet overheard this call and had just passed below us in the opposite direction and reported 1 contrail was viewed. Hearing this; we declared an emergency about the same time approach would not flow us direct to the airport. We contacted maintenance and dispatch for a possible tow to the gate. We secured all lights to the wings and informed our crew and the passenger via the captain about our situation. Requested fire truck support at ZZZ and landed flaps 30 degrees. During rollout; secured left engine. Exited and was met by fire crew. They visually inspected the left motor and we taxied near our gate. We secured both motors behind a nearby gate and were towed in to the gate. We ended with 3.6 on the fuel at gate with no other problems.

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Original NASA ASRS Text

Title: A B737-700 DEVELOPED A FUEL LEAK IN CRUISE. THE CREW DECLARED AN EMER AND LANDED AT THEIR ORIGINAL INTENDED DEST; WHICH WAS ALSO THE NEAREST SUITABLE.

Narrative: DEPARTED NORMAL. AT CRUISE; FL400; WE NOTICED A FUEL IMBAL OF APPROX 800 LBS. WE THOUGHT IT WAS A POSSIBLE GAUGE PROB BECAUSE BOTH OF US JUST LOOKED AT THE FUEL LESS THAN 5 MINS BEFORE AND IT WAS ONLY 200 LBS. WE STARTED A FUEL LOG AND 10 MINS LATER CALCULATED WE WERE BURNING OVER 1000 LBS MORE PER HR FROM THE L ENG. WE ALSO LOOKED AT THE FUEL FLOW AND SAW A VERY SMALL DIFFERENCE WHICH WOULD HAVE LITTLE EFFECT. WE DID THE FUEL LEAK CHKLIST AND ASKED THE FLT ATTENDANT TO LOOK TO SEE IF FUEL WAS LEAKING FROM THE L ENG/WING THEY SAW NOTHING. SINCE WE WERE NOT SURE IF THE FUEL WAS LEAKING OR IF IT WAS A GAUGE MALFUNCTION; WE DID NOT SHUT DOWN THE #1 ENG. AT THIS TIME WE HAD NO OTHER INDICATIONS -- THE YOKE WAS CTRED. AS WE CONTINUED TOWARD ZZZ WE CONTINUED TO MONITOR THE FUEL BURN AND AN ESTIMATED ARR FUEL IF INDEED IT WAS A FUEL LEAK. APPROX 240 MI FROM LAS WE WERE ASKED TO DSND TO FL330. WE THEN DECLARED 'MINIMUM FUEL' TO STAY HIGH TO CONSERVE FUEL JUST IN CASE THE CAUSE WAS CORRECT. WE EXPLAINED OUR SITUATION TO CTR AND THEY GAVE US FL360. ANOTHER CARRIER ACFT ASKED IF WE WERE THE AIRPLANE THAT JUST OVERFLEW THEM (WE WERE) AND THEY RPTED WE HAD ONLY 1 CONTRAIL. AT THIS POINT WE NOW KNEW WE WERE LEAKING FUEL. ALSO; THE YOKE SHOWED A SLIGHT L DEFLECTION (LESS THAN 1 FT); AT THIS POINT ALSO CONFIRMING OUR IMBAL. OUR FUEL LOG PREDICTED LNDG WITH JUST UNDER 4000 LBS OF FUEL; 600 LBS ABOVE OUR FAR RESERVE OF 3300 LBS. WE CONTINUED TO ZZZ. AFTER SWITCHING FREQS; THE LOW SECTOR CTLR WANTED US TO TURN FOR SPACING. TO GET EXPEDITIOUS HANDLING WE DECLARED AN EMER AND REQUESTED CFR FOR ARR. WE TOLD THE FLT ATTENDANTS ABOUT THE SITUATION. I BRIEFED THEM ON THE PROBABLE OUTCOME; LNDG AND NORMAL DEPLANE AND IF WE EVACED; EVAC TO THE R SIDE ONLY. I ALSO TOLD THE PAX WE HAD A FUEL LEAK AND THAT THE LNDG WOULD BE NORMAL BUT FIRE TRUCKS WOULD BE AT THE RWY AND WOULD CHK TO SEE IF WE WERE LEAKING FUEL BEFORE WE WENT TO THE GATE. NORMAL APCH. AFTER TOUCHDOWN I SHUT DOWN THE #1 ENG AND USED #2 THRUST REVERSER AND BRAKES TO STOP ACFT. OUR GATE WAS OCCUPIED SO I PARKED ACFT BEHIND AN ACFT ON GATE AND SHUT DOWN #2 ENG. THE CFR CHIEF INSPECTED L ENG AND SAW VERY LITTLE FUEL DRIPPING FROM #1 ENG. AFTER ACFT PUSHED; WE WERE TOWED TO THE GATE. NORMAL DEPLANING. DEV FROM QRH. WHEN WE FIRST DISCOVERED THE IMBAL; WE HAD NO OTHER INDICATIONS THAT FUEL WAS ACTUALLY LEAKING. WE BOTH AGREED THAT SHUTTING DOWN AN ENG AT THAT POINT WAS NOT THE BEST COURSE OF ACTION. WHEN THE OTHER CARRIER AND THE YOKE CONFIRMED THAT WE WERE LEAKING FUEL; I STILL DID NOT WANT TO SHUT DOWN THE #1 ENG BECAUSE: 1) WE HAD ENOUGH FUEL IN THE L TANK EVEN WITH THE LEAK TO LAND IN ZZZ WITH FUEL LEFT OVER. AT THIS POINT WE WERE 240 MI FROM ZZZ AND 180 DEGS FROM ZZZ1. 2) DIVERTING TO ZZZ1 WITH A SINGLE ENG (NEAREST SUITABLE) WAS LESS SAFE THAN CONTINUING TO ZZZ WITH THE FUEL LEAK. 3) IF SOMETHING HAPPENED TO THE #2 ENG; I DID NOT WANT TO TRY TO START THE #1 ENG WITH A FUEL LEAK. IF THE LEAK WAS AT THE FUEL CTL; COULD IT AFFECT THE RELIGHT? 4) I DID NOT WANT TO SLOW DOWN (SINGLE ENG SPD 220/210 KTS) AND DELAY ARR. BY THE TIME I FIGURED THE FUEL BURN WITH 1 ENG PLUS IF THE FUEL LEAK CONTINUED AFTER SHUTDOWN JUST SEEMED A LOT SIMPLER TO CONTINUE WITH THE CURRENT CONFIGN. 5) I FELT AS LONG AS WE A) KEPT AIRFLOW (IE; INFLT) AROUND THE COWL; THE FUEL WOULD DUMP OVERBOARD WITHOUT IGNITION IF THE FUEL LEAK WAS IN THE ENG COMPARTMENT. B) SHUT DOWN THE ENG AS SOON AS THE WHEELS WERE ON THE GND AND WE KNEW WE DIDN'T HAVE TO GO AROUND. C) DID NOT USE THE #1 THRUST REVERSER. ALSO; THE FO SUGGESTED NOT USING ANY LIGHTS ON THAT SIDE OF THE ACFT. WE DID THIS. I BELIEVE WE FOLLOWED THE SAFEST COURSE OF ACTION. I DO NOT KNOW IF THE ENG MANUFACTURER CAN SAY HOW MUCH FUEL CAN BE DUMPED INTO THE COWL BEFORE THERE IS A PROB INFLT. SUPPLEMENTAL INFO FROM ACN 704984: WE NOTICED A 500-700 LB FUEL IMBAL ON THE L SIDE. THIS WAS ODD BECAUSE 10-15 MINS PRIOR WE JUST COMPLETED A XFER OF FUEL IN THE CTR TANK. WE START TO MONITOR TO SEE IF IT WAS A POSSIBLE GAUGE PROB OR FUEL LEAK. MONITORING ALL INSTS AND FUEL FLOWS; NO OTHER INDICATIONS WERE OBSERVED. THE AILERON TRIM DID NOT MOVE TO SUPPORT THIS IMBAL. I STARTED TAKING 10 MIN COMPARISON READINGS; WHILE THE PF WAS MONITORING THE SYS AND ASKING ONE OF THE FLT ATTENDANTS TO SEE IF A STREAM OF FUEL COULD BE SEEN FROM IN THE CABIN. RPT CAME BACK NEGATIVE. WE PULLED OUT THE QRH TO REVIEW POSSIBLE OPTIONS/ACTIONS. STILL NO CONFIRMATIONS OF FUEL LEAK. WE STARTED REVIEWING POSSIBLE DIVERTS IF PRACTICAL. WE ADVISED DISPATCH OF THIS EVENT VIA ACARS. WE NOW NOTICED A LARGE CHANGE IN ARR FUEL AT OUR DEST AND BEGAN A MENTAL MATH CALCULATION OF FUEL REMAIN IF IN FACT THIS WAS A LEAK. WE CONCLUDED 4.5 AT ZZZ. AFTER 3 READINGS OR 30 MINS; WE REALIZED WE WERE LOSING 200 LBS OF FUEL EVERY 10 MINS. TO RECEIVE SPECIAL HANDLING INTO ZZZ WE DECLARED MINIMUM FUEL; REQUESTED DIRECT TO ZZZ AND REMAIN AT FL360 AFTER DSNDING FROM FL400. CENTER KEPT US HIGH IN ALT TO CONSERVE. ABOUT THE SAME TIME FRAME; ANOTHER CARRIER'S JET OVERHEARD THIS CALL AND HAD JUST PASSED BELOW US IN THE OPPOSITE DIRECTION AND RPTED 1 CONTRAIL WAS VIEWED. HEARING THIS; WE DECLARED AN EMER ABOUT THE SAME TIME APCH WOULD NOT FLOW US DIRECT TO THE ARPT. WE CONTACTED MAINT AND DISPATCH FOR A POSSIBLE TOW TO THE GATE. WE SECURED ALL LIGHTS TO THE WINGS AND INFORMED OUR CREW AND THE PAX VIA THE CAPT ABOUT OUR SITUATION. REQUESTED FIRE TRUCK SUPPORT AT ZZZ AND LANDED FLAPS 30 DEGS. DURING ROLLOUT; SECURED L ENG. EXITED AND WAS MET BY FIRE CREW. THEY VISUALLY INSPECTED THE L MOTOR AND WE TAXIED NEAR OUR GATE. WE SECURED BOTH MOTORS BEHIND A NEARBY GATE AND WERE TOWED IN TO THE GATE. WE ENDED WITH 3.6 ON THE FUEL AT GATE WITH NO OTHER PROBS.

Data retrieved from NASA's ASRS site as of January 2009 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.