B737-800 flight crew reported a safety concern regarding a flight control system operation as described in the aircraft flight manual; conflicting with line maintenance troubleshooting of the system while on the ground.
Synopsis
B737-800 flight crew reported a safety concern regarding a flight control system operation as described in the aircraft flight manual; conflicting with line maintenance troubleshooting of the system while on the ground.
Narrative
On Day 0; while operating a flight; the Captain and I noted the aileron trim did not stop when commanded while conducting the aileron trim check during the before start checklist flow. We made an logbook entry and ended up plane swapping. I followed the airplane for several days (out of curiosity because I had never encountered this maintenance item previously). The plane ended up being taken out of service for several days while they replaced the aileron quadrant springs (737-800).On Day 37; while operating a flight; the Captain and I noticed the aileron trim continued to move after releasing the aileron trim switches during the trim check. We made an logbook entry and ended up plane swapping. I mentioned the previous event to both the Captain and the mechanics - making clear to differentiate that the Day 0 event was a NG (Next Generation). The plane remained out of service for approximately 48 hrs while the aileron tap centering unit cam bearing was replaced (737-MAX 8).On Day 37; after plane swapping; we again found ourselves on another aircraft where the aileron trim did not stop when commanded while conducting the aileron trim check. We made another logbook entry. A mechanic reluctantly agreed to sign off the entry as 'normal' after a maintenance supervisor boarded the aircraft. The supervisor stated he 'checked 4 other 737 Max aircraft and they all behave the same. Therefore; it must be normal.' The logbook closure included a maintenance reference so the Captain and I accepted the aircraft. The rest of the flight was conducted normally (737-MAX 8).On Day 38; while conducting a flight the aileron trim behaved completely normally during the trim check - stopping when commanded and not drifting; continuing; etc (737-MAX 8).Having been on the fleet for 2 years; it has not been a normal occurrence for the aileron trim to continue to move once the switches are released during the trim check. We are taught to trust our mechanics. However; it is extremely disconcerting 3 separate; yet identical; maintenance events can result is 2 wildly different outcomes (1- aircraft taken out of service for days to replace parts or 2- told the situation is completely normal). If the situation is indeed normal; then why were the other 2 aircraft taken out of service for the same issue as the aircraft that was not removed from service? If it is normal for the aileron trim to continue to move after releasing the switches; then please update the aircraft operating manual accordingly.
Second reporter narrative
On Day 0 the aileron trim randomly coasted for .5 to 1 units after the trim switch was released during the preflight trim check. The aircraft went out service and replaced by another one; which had the exact same problem. A mechanic reluctantly signed off the issue after a maintenance supervisor walked onto the aircraft. The maintenance supervisor said he checked 4 other 737s and the aileron trim coasted on each one; therefore it was a normal event. The sign-off referenced a maintenance document; so I accepted the aircraft. The next day; I flew on another aircraft which did not have any coasting of the aileron trim. If it is normal for the aileron trim to randomly coast after the activation has stopped them place that info in the 737 operating manual.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.