ALTDEV ALT EXCURSION.

Narrative

DEPARTING OAKLAND; A NORMAL CLB PROFILE WAS USED. AFTER LEAVING 10000 FT; I ACCELERATED FROM 250 TO 320 KTS. REACHING 320 KTS; I ENGAGED THE AUTOPLT. IT IS IMPORTANT TO NOTE THAT THE AUTOPLT ON THIS ACFT HAS A PITCH TRIM WHEEL WHICH IS USED TO SET VERT SPD. WHEN THE AUTOPLT IS NOT IN USE; THIS PITCH TRIM WHEEL IS SUPPOSED TO FOLLOW THE VERT SPD OF THE ACFT; SO THAT WHEN THE AUTOPLT IS ENGAGED; A PITCH CHANGE WON'T RESULT. ANYWAY; WHEN I ENGAGED THE AUTOPLT; A PITCH CHANGE DID RESULT. THE ACFT PITCHED DOWN. I QUICKLY CHKED THE POS OF THE AUTOPLT PITCH TRIM WHEEL AND NOTICED THAT IT WAS POSITIONED TO COMMAND A 1500 FPM DSCNT. I THEN REPOSITIONED THE PITCH TRIM WHEEL MANUALLY TO ABOUT 2000 FPM CLB AND A POSITIVE RATE OF CLB WAS MAINTAINED AT ALL TIMES. FROM THIS POINT; I MAINTAINED 320 KTS WITH THE PITCH TRIM WHEEL UNTIL REACHING ABOUT FL180. I THEN ENGAGED THE AIRSPD HOLD FEATURE OF THE AUTOPLT; AND I ALSO ENGAGED THE ALT SELECT AND HDG FEATURE. BY THIS TIME WE HAD BEEN CLRED TO FL290 AND FL290 WAS SET INTO THE ALT ALERTER. I AND THE CAPT FLY THIS RTE OFTEN. THE ACFT DIFFERENCES INVOLVED WITH THE ACFT WE FLEW THIS NIGHT CREATED A LITTLE EXTRA WORKLOAD. IN AND OF ITSELF; THIS WOULDN'T HAVE BEEN A PROB. WE HAD BRIEFED THE DIFFERENCES PRIOR TO LEAVING THE RAMP; BUT WE (OR AT LEAST I) FOUND THAT THE DIFFERENCES WERE ENOUGH TO INTERRUPT SOME FAIRLY INGRAINED FLOW PATTERNS. ALSO; ADD TO THAT THE FACT THAT THE UPCOMING DEV OCCURRED AT XAAM IN THE MORNING; NORMAL WORKING HRS FOR US; BUT STILL YOUR BRAIN DOESN'T EXACTLY LEAP INTO GEAR AT THAT HR. NOW; BACK TO WHAT HAPPENED. AS WE WERE APCHING THE PALMDALE VOR; WE WERE GIVEN A 'DSND AT PLT'S DISCRETION' CLRNC TO FL240. WE WERE GIVEN A FREQ CHANGE AND A NEW CLRNC FOR THE ZIGGY 3 ARR; CROSS MAJEK AT 14000 FT; DSND TO 13000 FT. THE CAPT NOTICED ON HIS ALTIMETER THAT WE WERE AT FL298 AND CALLED IT TO MY ATTN. I CHKED MY ALTIMETER AND IT SAID FL299. I ALSO LOOKED AT THE ALT ALERTER WHICH HAD OUR 'PLT'S DISCRETION' ALT OF FL24O DIALED IN. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND BEGAN A DSCNT TO OUR ASSIGNED CRUISE ALT OF FL290. WE THEN DIALED IN 14000 FT WHICH REFLECTED OUR NEW CLRNC TO CROSS MAJEK AT THAT ALT. WITH THE CAPT'S CONCURRENCE; WE CONDUCTED A TEST. SURE ENOUGH; WHEN WE LEVELED OFF AT 14000 FT; AND LATER AT 13000 FT; THE ALT SELECT CAPTURED THE ALT. BUT; THE PITCH/TRIM WHEEL ON THE AUTOPLT STAYED IN A PITCH DOWN CONDITION. FURTHERMORE; WHEN WE SELECTED A NEW ALT ON THE ALT ALERTER; THE ALT CAPTURE GREEN LIGHT CHANGED FROM GREEN TO AMBER; AND THE STUCK PITCH TRIM WHEEL THEN CAUSED THE ACFT TO DSND. THE OPPOSITE WOULD HAVE HAPPENED AFTER LEVELING OFF FROM A CLB. THE ALT SELECT WOULD HAVE LEVELED THE ACFT AT FL290; BUT THE MALFUNCTIONING PITCH TRIM WHEEL WOULD HAVE REMAINED IN A CLB CONDITION. WHEN THE NEW LOWER ALT OF FL240 WAS SELECTED INTO THE ALT ALERTER; THE ALT SELECT LIGHT WOULD HAVE GONE FROM GREEN TO AMBER; AND THE STUCK TRIM WHEEL; COMMANDING A CLB; WOULD HAVE CAUSED THE ACFT TO CLB. WE DID GET A GREEN ALT SELECT LIGHT ON THE AUTOPLT CONSOLE; HOWEVER. AND WE MONITORED THE ACFT'S ALT AND NOTICED THAT IT MAINTAINED FL290 NORMALLY. IN FACT; WE LATER FOUND THAT THE ALT HOLD LIGHT WILL NOT COME ON UNLESS THE AUTOPLT THUMB WHEEL IS DETENTED IN THE NEUTRAL 0 RATE OF CLB POS; AND BECAUSE IT WASN'T; IT DIDN'T. QUITE SIMPLY; THE ALT SELECT WAS MAINTAINING OUR ALT; NOT THE ALT HOLD. TO SUM UP; AN ALTDEV OCCURRED BECAUSE OF SEVERAL EVENTS: 1) MECHANICAL FAILURE OF THE AUTOMATIC VERT SPD FOLLOWING SYS IN THE AUTOPLT THUMB WHEEL. 2) INTERRUPTED FLOW PATTERNS CAUSED BY A NONSTANDARD COCKPIT CONFIGN. 3) SEVERAL UNRELATED ELECTRICAL GLITCHES; LIKE REPEATED FALSE AUTOPLT DISCONNECT WARNINGS. 4) THE TIME OF DAY; AROUND XYAM IN THE MORNING; AND THE NORMAL DEGRADATION IN PERFORMANCE ASSOCIATED WITH IT.