Air Carrier flight crew reported in flight experiencing a fuel leak resulting in landing at their destination with less fuel than planned for the arrival.
Synopsis
Air Carrier flight crew reported in flight experiencing a fuel leak resulting in landing at their destination with less fuel than planned for the arrival.
Narrative
When cleared for takeoff in ZZZ1; I noted that our fuel was +200lb from planned takeoff fuel.Due to the short nature of the flight; the first 25 minutes was busy. We accomplished a normal climb and cruise. We then set up for the ILS XX into ZZZ. Briefings were accomplished and the 'descent checklist' was accomplished approximately 25-30 minutes from ZZZ. During my briefing I did read the REMF (Remaining Fuel) fuel at 8.6 as part of my standard 'go around/contingency' brief. I also stated that was sufficient fuel for landing; contingency and go-around/air return if necessary. It didn't occur to me at that time that our flight plan planned REMF was actually 10.1.Shortly after accomplishing the 'descent checklist'; we both noticed that the right fuel tank was significantly lower than the left tank and seemed to be decreasing faster…..when we noticed it; the left tank was appx. 5.0; and the right tank was appx. 4.3. A discussion ensued; and we cross checked the REMF from the flight plan. At this time we discovered the discrepancy between the FMC REMF of 8.6 and the flight plan REMF of 10.1. We looked at fuel panel; and the configuration was normal (4 main pumps on; cross feed closed). In the minute or two it took us to discuss and diagnose; the REMF dropped to 8.5. The FMC was loaded direct to ZZZ; with a discontinuity; then a basic ILS approach to XX; so the 1500lb fuel discrepancy didn't make sense. It was at this point that I called for the 'fuel leak' QRH. The FO ran the '(NG) Fuel Leak - Engine QRH' through step 6; which reads: 6 An engine fuel leak is confirmed if one or both of the following are true:-Fuel spray is observed from an engine or strut-A change in fuel imbalance of 500 pounds within 30 minutes or lessIn the time that it took us to run the QRH (appx 1-2 mins) we had lost another 100 lbs. Based that timing; we were losing appx. 100 lbs every 2-3 minutes; and definitely lost the 500 lb imbalance in less than 30 minutes; as that was about how long we had been flying. Referencing QRH step 7; the next function of the QRH is to shut down the engine with the suspected leak. We were getting close to top of descent at this point and I was weighing the risk analysis of shutting down the engine during a late night flight; after a long duty day into an uncontrolled field. We decided at this point it was prudent to [request priority handling] for a suspected fuel leak. The controller copied then cleared us direct to the initial approach fix for ILS XX into ZZZ. The center controller also called the airport ARFF (Airport Rescue and Firefighting) team to scramble for our arrival. We thought this would be prudent in the event of an actual fuel leak possibility. Due to the possibility of compounding problems by shutting down the right engine; I did a calculation of how fast we were losing the fuel number in the right tank. With less than 20 minutes remaining to landing; I calculated that at the current rate of fuel loss; we would land with approximately 3.0 in right tank and 4.2 in left tank; conservatively; which we deemed plenty to continue to landing with 2 engines. By this time in the process; the left tank was reading 4.9 and the right tank was 4.1. REMF was now showing 8.1 on the FMC. We were given descent at pilots discretion when we were cleared for the approach. The decision was made to start our descent using the most efficient descent profile as possible. The theory was that if there in fact was a right engine fuel leak; having reduced power as long as possible would decrease the rate of fuel loss. When we arrived at TOD (Top of Descent) and started down; we did notice a stabilization in the rate of fuel loss. By this time; REMF was 8.0; which is the lowest number we noticed during the descent/arrival. At this time the FMC was programmed direct to a 10 mile initial approach fix for a straight in approach.A normal approach and landing was made. We coordinated with the ARFF crew on CTAF that we would clear the runway; and requested an escort into the gate for inspection before ground service crews approached the plane. The FO tried to contact ops on the ground and advise them of the situation; but the ops frequently was not being monitored.ARFF crews found no evidence of a leak and by the time we ran the parking checklist; the fuel gauges read perfectly balanced with a final fuel (at the gate) of 9.6. I then proceeded to call dispatch and the Chief Pilot full debrief. The ARFF crews found was also debriefed. An ELB entry was made and a full debrief was accomplished with Maintenance Control controller.
Second reporter narrative
Fuel was 16.0. We pushed with 16.1. A few minutes before TOC (Top of Climb) I saw the 'using rsv fuel' indication light up on the FMC which was strange for where we were. Captain likes to set reserve fuel at REMF-1; which would have been 9.1. The 'using rsv fuel' annunciation came up at 10.1; our REMF for landing at ZZZ. We leveled off and I looked at the captain and he had a look of surprise when he saw the fuel. I asked if it seemed like we burned a lot of fuel for flying for only 30 minutes and he said yes. 6000 lbs in 30 minutes was excessive. He then noticed the #2 fuel dropping fairly quickly. He hacked a clock and asked me to pull out the checklist; which I did. I ran through the fuel leak for the ng checklist and when it got to where the checklist wants you to start a clock he stopped me. In the time I had gone through the checklist the imbalance had gone from around 300lbs to 900. We discussed next steps and I advocated for requesting priority handling. At this time we got a descend to 16000' at pilots discretion from ATC. Captain transferred the controls to me and requested priority handling and started the checklist again. We ran the suspected engine fuel leak and stopped at shutting down the engine. As we started our descent the imbalance slowed significantly. We landed with no issues and had ARFF follow us in and inspect us. They reported that they saw no visible leaks. The strangest part was that at the worst; in the air; we were down to 8000 lbs of fuel and a 900 lb imbalance. When we parked at the gate and shut everything down we had 9400 lbs and 0 imbalance.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.