Narrative:

On a flight from knoxville to atlanta, the aircraft was leveled at FL190 with the autoplt engaged. The autoplt completed the level off and the altitude hold function was engaged. Airspeed was 350 KTS and power was set at the cruise setting. Approximately 3 mins after level off, ZTL called traffic at 11 O'clock, 15 mi, and FL200. All 3 crew members cleared for traffic. The second officer called traffic in sight and the first officer noticed the stabilizer trim amber light illuminated. This indicates that the elevator trim was not set properly for the autoplt input. The captain started to manually trim the aircraft. With the autoplt still engaged and indicating altitude hold engaged, the aircraft pitched up 500'. First officer disconnected autoplt and reestablished altitude at FL190. Captain continued to monitor the traffic. Traffic passed to left and behind our aircraft after we releveled at FL190. Although autoplt indicated coupled and the altitude warning was functional, there were no aural indications of an altitude deviation. Due to the insidious nature of the malfunction (no aural warning of altitude deviation or autoplt failure) and the distraction of the opposing traffic, I feel that it may be prudent to treat out of trim lights as an autoplt failure. The aircraft should be immediately disconnected from the autoplt, retrimmed and reset on autoplt.

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Original NASA ASRS Text

Title: ACR LGT FLT CREW RECEIVES TRIM WARNING LIGHT WHICH CAUSES CAPT TO ATTEMPT TO MANUALLY TRIM ACFT WHICH RESULTING IN AUTOPLT INDUCED PITCH UP WITH A RESULTANT ALT DEVIATION OF 500'.

Narrative: ON A FLT FROM KNOXVILLE TO ATLANTA, THE ACFT WAS LEVELED AT FL190 WITH THE AUTOPLT ENGAGED. THE AUTOPLT COMPLETED THE LEVEL OFF AND THE ALT HOLD FUNCTION WAS ENGAGED. AIRSPEED WAS 350 KTS AND POWER WAS SET AT THE CRUISE SETTING. APPROX 3 MINS AFTER LEVEL OFF, ZTL CALLED TFC AT 11 O'CLOCK, 15 MI, AND FL200. ALL 3 CREW MEMBERS CLRED FOR TFC. THE S/O CALLED TFC IN SIGHT AND THE F/O NOTICED THE STAB TRIM AMBER LIGHT ILLUMINATED. THIS INDICATES THAT THE ELEVATOR TRIM WAS NOT SET PROPERLY FOR THE AUTOPLT INPUT. THE CAPT STARTED TO MANUALLY TRIM THE ACFT. WITH THE AUTOPLT STILL ENGAGED AND INDICATING ALT HOLD ENGAGED, THE ACFT PITCHED UP 500'. F/O DISCONNECTED AUTOPLT AND REESTABLISHED ALT AT FL190. CAPT CONTINUED TO MONITOR THE TFC. TFC PASSED TO LEFT AND BEHIND OUR ACFT AFTER WE RELEVELED AT FL190. ALTHOUGH AUTOPLT INDICATED COUPLED AND THE ALT WARNING WAS FUNCTIONAL, THERE WERE NO AURAL INDICATIONS OF AN ALT DEVIATION. DUE TO THE INSIDIOUS NATURE OF THE MALFUNCTION (NO AURAL WARNING OF ALT DEVIATION OR AUTOPLT FAILURE) AND THE DISTR OF THE OPPOSING TFC, I FEEL THAT IT MAY BE PRUDENT TO TREAT OUT OF TRIM LIGHTS AS AN AUTOPLT FAILURE. THE ACFT SHOULD BE IMMEDIATELY DISCONNECTED FROM THE AUTOPLT, RETRIMMED AND RESET ON AUTOPLT.

Data retrieved from NASA's ASRS site as of August 2007 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.