Flight Crew Air Transport Pilot (ATP)
Confusion; Distraction; Troubleshooting
Aircraft Equipment Problem Critical
Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft; Weather
After cruising at FL380 with an oat of -33 C; we started a descent and observed an AP TRIM MIS COMP annunciator along with a Master Caution. Upon running the Abnormal Checklist we discovered the elevator trim to be frozen; would not move in either direction electrically or manually; so our speed was reduced to about 250 KTS to achieve a neutral trim setting in the descent. It wasn't until about 8;000 FT with an OAT of +5 C that the elevator trim became unstuck. The only thing we could not comply with was an increased assigned speed by ATC. We landed uneventfully.
The reporter stated that the aircraft's maintenance log indicated the trim actuators were replaced but he and his First Officer suspected ice someplace in the system. His suspicion is based on his having heard of this happening before and because of the fact that the trim returned to normal with warming temperatures during descent. He and his First Officer had flown this aircraft on several short flights the previous day but had not flown above the mid FL200's. Also the aircraft had not been exposed to precipitation and the daylight ground temperatures in the flight areas were in the mid 60's.
A CE-560XL AP TRIM MIS COMP and Master Caution alerted as the descent was begun from FL380 with a -33 OAT so the aircraft was slowed to descend and at about 8;000 FT with a +5 OAT the trim began normal operation so the crew suspected icing somewhere in the trim system.