A B777 horizontal trim was inoperative when operated from the yoke trim switches during climb but operated normally from the Alt Stab Trim Levers. At cruise the yoke trim switch's input was normal but in the descent for landing the yoke switches again became intermittent. A horizontal trim tab ACE was changed.
Synopsis
A B777 horizontal trim was inoperative when operated from the yoke trim switches during climb but operated normally from the Alt Stab Trim Levers. At cruise the yoke trim switch's input was normal but in the descent for landing the yoke switches again became intermittent. A horizontal trim tab ACE was changed.
Narrative
While climbing out; at about 1;000 FT; I called for flap retraction when I noticed that excessive forward trim was required. I mentioned this to the Captain. A few thousand feet higher I still required forward pressure on the yoke to maintain my desired pitch attitude. While trimming; I looked at the stab trim setting and the indicator wasn't moving. Next I used the Alternate Stab Trim levers on the center console and was able to adequately trim the aircraft. I mentioned this to the Captain and suggested he attempt trimming from his side. Initially he couldn't trim from the the yoke trim switches either. Eventually the yoke trim switch did start functioning again after using the Alternate Stab Trim Levers. After consulting with Dispatch; Maintenance; and amongst ourselves over the systems; we continued to our planned destination. We checked every hour to ensure the yoke trim worked. At top of descent the trim was checked and worked but at about 8;000 FT the trim worked intermittently again. On final; the trim worked normally.
Second reporter narrative
During flap retraction the First Officer discovered that the aircraft would not trim nose forward using the yoke trim switches. He reverted to the alternate trim levers. He handed the aircraft to the Captain and the Captain was also unable to trim using the yoke trim switches. After several attempts at trimming nose up and nose down with the yoke trim switches; the yoke trim returned to normal operation. A call ensued with Dispatch and Maintenance Control and aircraft systems information was downloaded for analysis and a decision to continue. A collaborative decision was made to continue. The autopilot; yoke and alternative trim were all working properly. A writeup was submitted. Several times during the flight we disconnected the autopilot to check for trim and trim function. Everything was working perfectly. On approach the yoke trim again failed to work properly and the fault was intermittent. The flight landed safely without further difficulty. We hope maintenance was able to find the cause of this intermittent fault.
NASA callback
The Reporter stated that Maintenance reportedly changed an actuator control electronics unit (ACE) for the horizontal stabilizer trim to correct the intermittent trim problem.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.