Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 121; Flight Crew Type 6000
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance; General Maintenance Action
Aircraft; Human Factors
I was Pilot Flying on a vectored heading to intercept localizer final; level at 2500. The autopilot was on and holding altitude. Just prior to course intercept; we got a Stab out of Trim warning. I looked back at my instruments and the aircraft was descending and about 100 feet low with the autopilot on. I immediately disconnected the autopilot to manually climb back up to altitude while intercepting the Localizer. The trim was heavy nose down; and electric trim was inop while the aircraft continued to descend until I could get enough muscle on the controls; and manual trim; to climb back to altitude. We were about 400 feet low and Approach called us and notified us he had a Low Altitude Alert on us; and gave the min vectoring altitude 2100 MSL for our location. We maintained localizer; climbed back up to 2500 MSL; captured the glideslope; ran the QRH and continued the approach using manual trim. We talked about a missed approach; but decided it would be safer to continue and land. We broke out at about 700 ft to a normal landing using manual trim. After clearing the runway; the Captain tried his electric trim and it was operational; so I tried mine and now it worked. The Captain wrote up the trim; Contract Maintenance came out; we ran the trim but it would just stop on its own. Maintenance determined the trim motor was going bad and the trim was intermittent.I don't know of anything that we could change to keep this from happening again. We don't use manual trim very often; and there is learning curve on how much you have to spin the wheel and how much force is required to move it.
B737-NG First Officer reported receiving a low altitude alert from ATC on approach when the stab trim failed leaving them with manual trim only.