B737 flight crew reported a fuel imbalance developed while at cruise. After the imbalance worsened; the flight crew performed an in flight engine shut down and continued to the destination airport.
Synopsis
B737 flight crew reported a fuel imbalance developed while at cruise. After the imbalance worsened; the flight crew performed an in flight engine shut down and continued to the destination airport.
Narrative
The event took place enroute from ZZZ to ZZZ1. Approximately 4 hour and 45 mins into the flight we started to notice a growing imbalance in between the main fuel tanks one and two. The number two side was decreasing in quantity at a much faster rate than the number one side. We got the fuel imbalance caution associated with the issue and as it continued to increase we went to the QRH and settled on the 'fuel leak engine' checklist as it appeared to us that the imbalance could be from a potential leak. As we went through the process the Captain was in conversations with both Flight Operations and Dispatch about where to proceed next. By the time the conversations and checklist had been gone through we were in the best position to continue to ZZZ1. As we made our descent and continued to follow the checklist we secured the engine; as the parameters for doing so were present based on the checklist. We had already [requested priority handling] and continued to land safely at ZZZ1 on one engine per the single engine landing checklist.In the end it appears that the cross feed valve had failed but because there was no power to the valve it displayed itself in the closed position. During the fuel leak checklist it specifies to check the valve is closed which we did; but per the note in the checklist stated that if the circuit breaker was tripped that there would not be an indication of whether the valve was open or closed. I think that creating an item to check the circuit breaker as a set as opposed to just a note would help increase a successful recognition of that possible situation for other crews going forward. Because of the possible time critical situation we were dealing with we followed the steps that we believed to be the correct course of action.
Second reporter narrative
Flight was with scheduled departure from ZZZ and arrival at ZZZ1. Captain was pilot flying. First Officer was monitoring. Event began approximately 4.75 hours into the flight and one hour from touchdown at cruise altitude of FL380 abeam ZZZ2 to the north and ZZZ3 to the south. First indication of problem was Master Caution light and fuel imbalance indication showing #2 main fuel tank (right) approximately 1000 pounds less than #1 main tank (left). Engine indications showed the fuel flows matched; as did the total fuel used in each engine during the flight. The right tank continued to decrease at a much faster rate than the left; which led us to begin the 'Fuel Leak - Engine' QRH procedure. First Officer now became pilot flying. Captain became monitoring pilot and began the checklist. Step Y of this procedure was a decision point. The first bullet point has an 'and' conjunction between phrases. The second bullet point has an 'and/or' conjunction between the phrases. While the total fuel planned for landing at ZZZ1 remained constant during the entire event; the total quantity was decreasing at an abnormal rate in the right tank. We paused the checklist here to contact both Dispatch and the Flight Operations for input. Dispatch provided guidance that ZZZ2 was the better divert field and weather / NOTAMS / operations supported that decision. A continued discussion was accomplished by all parties at this part of the checklist. It appeared that we had a leak on the right engine; but planned fuel was remaining constant for our arrival at ZZZ1. Because we viewed the 'total fuel decreasing at an abnormal rate' in the right tank; I opted to proceed to step Z. Flight attendants were notified of the situation. At this point they are told to prepare to divert into ZZZ2. ZZZ1 Captain Name; who was deadheading on the flight; performed a visual check of the right engine and did not see any indication of fuel leak or vapor spray. However; our imbalance continued to increase towards 2;000 pounds between the main tanks. Step Z indicates a continued increasing imbalance should be viewed as confirmation of an engine fuel leak. This decision took us to step A; which begins the engine shutdown process. Again we paused the checklist; as we can see that the fuel imbalance is not linear in intensity. It has now slowed significantly. Right tank still had over 4;000 pounds of fuel. Left tank had over 6;000 pounds of fuel. Total fuel remained close to our planned arrival fuel. We discussed whether the safer course of action was shutting down the engine or allowing it to run. Total fuel remained constant; but the imbalance was slowly getting worse. By this time; we are now equal distance from ZZZ1 and ZZZ2. Weather is good at both locations. We elected to continue to ZZZ1 to minimize the risks of an unplanned descent into the divert field. Fuel calculations told us that even if we shut down the right engine and lose all remaining fuel in the right tank; we would be landing at ZZZ1 with over 4;000 pounds of fuel in the left tank. With concurrence of dispatch and the Flight Operations; we committed to ZZZ1 as the destination. Flight attendants were notified of the change in plans. [ATC was advised] at the top of descent to allow for priority handling into ZZZ1. At this point; we thought that a cross feed valve problem or a fuel gauge problem was more likely than a fuel leak. During descent; the 'non-linear' fuel imbalance began to get worse. The decision was made to shut down the right engine. Not so much to stop the possibility of a fuel leak; but to prevent an unplanned shutdown of that engine on final approach due to fuel starvation. Better to slowly and methodically secure the engine now than rush to do so later in the approach. The checklist was continued at step A and completed. The One Engine Inoperative Landing checklist was completed. A detailed PA was made to the passengers at 18;000 feet explaining both the fuel issue and the single engine situation. RNP (RNAV) approach to runway XXR was flown and landed by the First Officer. Aircraft was taxied clear of the runway at taxiway and inspected on taxiway by the Crash Fire Rescue team. We requested an evaluation of any fuel leaks around the right wing and right engine prior to proceeding to gate. No sign of any fuel leakage; so aircraft continued to the gate for normal deplaning process. No passenger questions or concerns are noted by the flight attendants. CFR chief has a few questions that are answered. Flight attendants; pilots and DH Captain Name had a crew debrief to address any questions; concerns; and improvements in communication. Flight Operations and Dispatcher were called individually by phone to debrief and collect feedback. Fuel imbalance at the gate was 2;600 pounds. No additional information. 1) Checklist choice: Two checklist options that I considered using during the event: Fuel Leak - Engine and Fuel Quantity Indicator Inoperative. However; the appropriate checklist to begin with would have been simply the indication warning itself: Fuel Imbalance. Using this checklist would have forced us to confirm proper operation of the cross feed valve; which ended up being the root cause of the problem. However; the rapidly decreasing quantity in the right tank (initially 10 pounds every six seconds) caused me to psychologically think 'fuel leak' or 'fuel gauge' problem. A perceived time critical situation caused me to not evaluate each option carefully in the Fuel - Abnormal Checklist section. 2)Fuel Leak - Engine checklist step Y: This decision point determines whether a fuel leak is probable or not. A decision must be made as to whether the 'total fuel quantity is decreasing at a normal rate' or 'total fuel quantity is decreasing at an abnormal rate'. During the event; I read this statement as total fuel quantity in the right tank decreasing at an abnormal rate. This evaluation caused me to continue the checklist at step Z and eventually shutdown the engine. But I can now see that this could be interrupted as total fuel remaining on the aircraft in all remaining fuel tanks. In that case; our total aircraft fuel was not decreasing at an abnormal rate and this checklist would have ended much differently. 3) Fuel Leak - Engine checklist step X: The step simply confirms the cross feed valve is closed. The step was accomplished; but not enough consideration was given to the 'caution' and the 'note' following this step. At this point in the event; I simply should have cycled the cross feed valve to verify it's proper operation. That would have taken the event in a very different direction (Cross Feed Selector Inoperative checklist). Ironically; my past experience has me take the extra step each flight of cycling the crossfeed value during the Captain's Before Start flow. This holdover step from my past provides confirmation that this valve will work 'if needed' later on during the flight. Unfortunately; I did not check it again at cruise when it truly mattered during this event. 4) Satellite Communication: The particular aircraft did not have SAT COM capability. Communication with Dispatch and the Flight Operations was difficult due to phone patch problems with ZZZ2 operations and ZZZ4. Our descriptions of the problem and resolution suggestions were not clearly heard and broken up; needing to be repeated for clarification. Perhaps with a clear line of communication more information would have been exchanged between the three parties. The addition of SAT COM capability to all aircraft would be helpful. 5) Pilot Flying / Pilot Monitoring: As stated in the irregularity narrative; the First Officer began the flight as the Pilot Monitoring. Once the event began and the crew saw that checklist usage was needed; the roles of the pilots were reversed. By having the FO take control of both the aircraft and the ATC communication; I was able to focus on both the checklist decisions and communication with all others. It was also the logical decision to have the FO continue the single engine approach to landing. This worked well and I highly encourage the training department to continue these defined roles during an event.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.