B737-800 flight crew reported returning to departure airport after experiencing flight control issues related to Mach Trim anomalies.
Synopsis
B737-800 flight crew reported returning to departure airport after experiencing flight control issues related to Mach Trim anomalies.
Narrative
Normal takeoff from Runway XXR on a 270 heading. During climbout while Captain was flying pilot through approximately 8000 ft with the autopilot off at 250 kts I noticed the Master Caution illuminate. On the overhead panel I saw MACH TRIM FAIL light was illuminated. Captain called 'my aircraft' while I opened the QRH. The procedure only says to fly below 280 Kts/.82. ATC then asked us to accelerate to 300 kts. I told them 280 was as fast as we could do and that we were troubleshooting a flight control malfunction. CA (Captain) then tried to engage the autopilot but CMD A or CMD B would not take. After climbing through 12;000 feet the jump seater advised that the mach trim fail light had extinguished. The Captain was then able to engage the autopilot. After passing approx; 18;000 [ft.] the mach trim fail light returned as we continued the climb with the autopilot on and the autopilot began to capture FL240; the CA noticed the aircraft was potentially going to continue through FL240 and then decided to disconnect the autopilot. He described feeling a lot of forward control pressure for the level off and what he described as an abnormal amount of nose up trim was required. At that point we decided to return to ZZZ1 based on the abnormal trim situation he described to me. CA then gave me the aircraft and radios while he advised the FA's (Flight Attendants); Dispatch; and passenger while also running the non routine landing checklist and overweight landing checklist. ATC asked if we needed any assistance; I relayed the message to the CA and he advised me that we would not need any assistance and I advised ATC. We continued and set up for an overweight landing. On final the Captain took the aircraft back for the landing which had a touchdown that was less than 100ft/min. The 280kts/.82 restriction per the QRH was never exceeded and the autopilot was never re-engaged.I'm unaware of any other factors that may have contributed to this event.
Second reporter narrative
Normal takeoff and climb. At 8000 [ft.] MSL the Master Caution & Mach Trim lights illuminated. The CA (Captain) stated; 'my aircraft' and continued to fly while the FO (First Officer) opened the QRH. It advised not to exceed 280 KIAS or .82 mach. The CA tried both autopilots but neither CMD A or B would engage. We continued climb past 12000 [ft.] MSL and the Jumpseat Pilot told us that the Mach Trim light extinguished. The CMD B continued to not work but the CMD A autopilot engaged. Passing approximately FL180 the Master Caution and Mach Trim lights illuminated again. CA and FO continued climb out while considering options to the destination. Leveling at FL240 the aircraft failed to level off and overshot altitude by 200 [ft]. The CA disconnected the autopilot and immediately felt a large amount of nose down trim; which pitched the nose down and the crew noticed a flash of the Stab Trim light. The CA recovered the momentary nose low condition with yoke aft application and used normal stabilizer trim to continue level flight. The Flight Attendants in the rear galley area experienced a 'light headed feeling' during the recovery. The crew decided to return the aircraft to ZZZ1 without use of the autopilot. We maintained a speed well below 280 KIAS and requested a return with ZZZ Center. The CA had the FO fly the descent to approach while accomplishing the descent; Non-Routine Landing and Overweight Landing checklists. The CA chose to land the aircraft. The aircraft landed overweight; at 152.5K; with a normal touchdown and approximately 100 FPM sink. Taxi back was normal.The aircraft had been recently returned to service after a long storage.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.