Narrative:

Three months ago I flew an aircraft on which the alternate stabilizer trim switches (the manual switches on the center pedestal; left of the throttles) were inoperative. I occasionally use those thumb switched to trim nose up or down when hand flying the plane. These were completely dead and I wrote them up when we landed at our destination. They were repaired; but there is no telling how long they had been inoperative. Since that event; I've begun using those switches or the manual stabilizer trim levers; as installed; prior to taxi to check operation of the system. On this day; I ran into another malfunctioning stab trim system simply by operating the levers as they were designed. After running the after start checklist; I moved the stabilizer trim levers forward and aft to watch the stab trim index move as it should. When I did; and in both directions; we received an unsched stabilizer trim light on the overhead panel with the corresponding EICAS note and aural warning. (This is the correct response on this aircraft when the autopilot is engaged.) we then received a mach speed trim light on the overhead panel and corresponding EICAS message and aural warning. After discussing the problem on the phone with maintenance control and pulling and resetting circuit breakers at his direction; the problem remained and we returned to gate where line maintenance was unable to troubleshoot the problem and the plane was taken out of service. I spoke to maintenance control the next day and it was discovered a wire was broken/disconnected in the center pedestal which was necessary to send a signal to the stab trim control box (at least that's how I remember it being explained). Again; this is the second aircraft in three months on which I had an issue with this system and there's no telling how long this system had been broken when I found it. Two planes in three months is a pretty small sample size.

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Original NASA ASRS Text

Title: B757 Captain reports discovering two aircraft in three months with malfunctioning alternate horizontal stabilizer trim switches.

Narrative: Three months ago I flew an aircraft on which the alternate stabilizer trim switches (the manual switches on the center pedestal; left of the throttles) were inoperative. I occasionally use those thumb switched to trim nose up or down when hand flying the plane. These were completely dead and I wrote them up when we landed at our destination. They were repaired; but there is no telling how long they had been inoperative. Since that event; I've begun using those switches or the manual STAB TRIM levers; as installed; prior to taxi to check operation of the system. On this day; I ran into another malfunctioning stab trim system simply by operating the levers as they were designed. After running the After Start checklist; I moved the STAB TRIM levers forward and aft to watch the stab trim index move as it should. When I did; and in both directions; we received an UNSCHED STAB TRIM light on the overhead panel with the corresponding EICAS note and aural warning. (This is the correct response on this aircraft when the autopilot is engaged.) We then received a MACH SPD TRIM light on the overhead panel and corresponding EICAS message and aural warning. After discussing the problem on the phone with Maintenance Control and pulling and resetting circuit breakers at his direction; the problem remained and we returned to gate where line maintenance was unable to troubleshoot the problem and the plane was taken out of service. I spoke to Maintenance Control the next day and it was discovered a wire was broken/disconnected in the center pedestal which was necessary to send a signal to the stab trim control box (at least that's how I remember it being explained). Again; this is the second aircraft in three months on which I had an issue with this system and there's no telling how long this system had been broken when I found it. Two planes in three months is a pretty small sample size.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.