B737 flight crew reported a leading edge flap malfunction during cruise flight. The crew diverted the flight to an alternate airport and landed.
Synopsis
B737 flight crew reported a leading edge flap malfunction during cruise flight. The crew diverted the flight to an alternate airport and landed.
Narrative
Approximately one hour into our trip; during cruise at .79 Mach; we had coordinated a bathroom break and the First Officer had taken both radios and flying duties. The Captain was not in the flight deck during the first malfunction occurrence. Another First Officer was in the flight deck Jumpseat and saw the amber Leading Edge in Transit light briefly illuminate. The First Officer noticed the speed bug drop to approximately 240 knots to protect the flap integrity and the auto throttle started moving the thrust levers toward idle. Within 2 seconds the Leading Edge in Transit light extinguished; the speed bug jumped back up to .79 and the thrust levers returned to the appropriate cruise position.Upon entering the flight deck and back on headsets; the First Officer communicated the oddity to the Captain. We observed the systems and a few minutes later a momentary repeat event occurred. After a third occurrence we coordinated a decent with ATC to 20;000 feet; our max flaps extended altitude and slowed to 240 knots should the leading edge slats actually be deploying uncommanded. The light briefly illuminated a few more times but did not affect the speed bug as we had opened the speed window.The Captain transferred flight controls and radios to the First Officer during decent and started running checklists. The checklists only continued for one step for the uncommanded deployment if leading edge slats or the indicator illuminating as the light extinguished. We read through the checklist anyway while coordinating a call with dispatch and Maintenance Control. A joint decision was made to divert to ZZZ1 to swap aircraft for the trip to ZZZ2.We decided to 12;000 feet and deployed flaps and gear to burn fuel to meet landing weight requirements in ZZZ1 and held until landing weights were assured. We held for about an hour. We kept speed no faster than 240 and left the flaps 1 position in place after the hold as a more conservative decision in case there was a true leading edge device malfunction. Dispatch checked landing distance numbers should we have a flap issue. The approach and landing were normal.The assistance from the jumpseater with workload management; observing the issue; backing up checklists; checking weather at the alternate; assisting with fuel burn calculations and more must be noted. Having the jump seater was an asset in this situation.Once at the gate the Captain held all the appropriate calls with company and maintenance and the First Officer assisted the flight attendants to show a face to the passengers while deplaning. The flight attendants were excellent with the passengers both by calming them; and translating all our announcements into Spanish.
Second reporter narrative
Approximately 1 hour into our flight to ZZZ1 while in cruise; I called the flight attendants to set up a bathroom break. While I was outside of the flight deck; I was told by the flight deck jump seater and my First Officer that there was a momentary Leading Edge (LE) flaps amber light indication on the First Officer's inboard display. This caused the speed bug to significantly reduce to protect the integrity of the flaps. Shortly after the message extinguished causing the Autothrottle system to accelerate again to the original speed of .790 Mach. We continued to monitor the system and shortly after I noticed the LE flaps amber light indication came on. A few seconds later it extinguished again. The occurrence rate significantly increased during this time. As a crew we decided to descend to 20;000 feet and reduce speed to 240 knots to comply with Airplane limitations. While descending I transferred flight controls to the First Officer and Ran the non normal checklist which the Jump seater had pull up already. We only got to step 1 on the checklist since the LE flaps transit light was no longer on. While the first Officer was flying; I reached out to Dispatch and Maintenance Control and as a crew we decided it was best to divert to ZZZ2. We held for about an hour in order to burn enough fuel to land within landing weight limits. During this time we were able to brief and set up for the arrival into ZZZ2 and make consecutive PA's to our passengers to keep them informed of the situation. Our flight attendants did a great job keeping our customers calmed and informed as well. Our jump seater took an active approach to help us and reduce our workload drastically; providing latest weather and monitoring systems in the flight deck. We were able to land under normal conditions without the need to declare an emergency.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.