Narrative:

After entering holding as published at ramah intersection at 21000' and at approximately 220 KTS, we made about 2 circuits. The flight computer was programmed to enter and perform the hold, which it did. We were then cleared 'direct kiowa, descend and maintain FL190 and increase your speed to 250 KTS'. I placed 19000' in the altitude window and the captain entered the lower altitude and increase in airspeed in the computer. I then informed the captain that I would be off ATC frequency to get den ATIS and to inform company that we were out of holding and would be on the ground shortly. I was occupied with these tasks for no more than 2-3 mins. When I finished I directed my attention back to the aircraft, its instruments and what the captain was doing. I noticed the automatic throttles were advancing to increase speed, that the captain was looking down at his computer terminal, inserting additional information, and the aircraft was in a 500-600 FPM climb. I shouted the aircraft was climbing, and the captain immediately disconnected the autoplt and descended the aircraft to FL190. The aircraft reached an approximate altitude of 22600'. The VNAV was disconnected, but the autoplt enunciator panel showed altitude hold. All computer commands were for a descent to FL190. I have no honest idea why this aircraft went into a gradual climb when it was supposed to be descending, and the autoplt was showing altitude hold. If anything I would think it would have remained at FL210 with the VNAV disconnected. Since the altitude window was set at FL190 and the aircraft was climbing away and was more than 1000' away, we received no altitude alert warning us of the gradual climb. Was there a temporary failure of the autoplt, or maybe the FMS computer? I don't know, we couldn't get it to duplicate. Everything worked fine the remaining few mins. Could we have encountered an updraft that overpwred the autoplt? There was mountain wave activity and turbulence present. I'm not sure. Whatever, we have reported this incident to our company that plans to take it to XXXX for further evaluation. I'm sure you could say the crew should have been more observant, but we were doing normal cockpit procedures necessary in that phase of flight, yet we missed this deviation. If you have records of anything similar happening in an medium large transport a or medium large transport B I would love to hear from you!

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Original NASA ASRS Text

Title: ALT DEVIATION. EXCURSION FROM ASSIGNED. UNABLE CALLBACK.

Narrative: AFTER ENTERING HOLDING AS PUBLISHED AT RAMAH INTXN AT 21000' AND AT APPROX 220 KTS, WE MADE ABOUT 2 CIRCUITS. THE FLT COMPUTER WAS PROGRAMMED TO ENTER AND PERFORM THE HOLD, WHICH IT DID. WE WERE THEN CLRED 'DIRECT KIOWA, DSND AND MAINTAIN FL190 AND INCREASE YOUR SPEED TO 250 KTS'. I PLACED 19000' IN THE ALT WINDOW AND THE CAPT ENTERED THE LOWER ALT AND INCREASE IN AIRSPEED IN THE COMPUTER. I THEN INFORMED THE CAPT THAT I WOULD BE OFF ATC FREQ TO GET DEN ATIS AND TO INFORM COMPANY THAT WE WERE OUT OF HOLDING AND WOULD BE ON THE GND SHORTLY. I WAS OCCUPIED WITH THESE TASKS FOR NO MORE THAN 2-3 MINS. WHEN I FINISHED I DIRECTED MY ATTN BACK TO THE ACFT, ITS INSTRUMENTS AND WHAT THE CAPT WAS DOING. I NOTICED THE AUTO THROTTLES WERE ADVANCING TO INCREASE SPEED, THAT THE CAPT WAS LOOKING DOWN AT HIS COMPUTER TERMINAL, INSERTING ADDITIONAL INFO, AND THE ACFT WAS IN A 500-600 FPM CLIMB. I SHOUTED THE ACFT WAS CLIMBING, AND THE CAPT IMMEDIATELY DISCONNECTED THE AUTOPLT AND DESCENDED THE ACFT TO FL190. THE ACFT REACHED AN APPROXIMATE ALT OF 22600'. THE VNAV WAS DISCONNECTED, BUT THE AUTOPLT ENUNCIATOR PANEL SHOWED ALT HOLD. ALL COMPUTER COMMANDS WERE FOR A DSCNT TO FL190. I HAVE NO HONEST IDEA WHY THIS ACFT WENT INTO A GRADUAL CLIMB WHEN IT WAS SUPPOSED TO BE DESCENDING, AND THE AUTOPLT WAS SHOWING ALT HOLD. IF ANYTHING I WOULD THINK IT WOULD HAVE REMAINED AT FL210 WITH THE VNAV DISCONNECTED. SINCE THE ALT WINDOW WAS SET AT FL190 AND THE ACFT WAS CLIMBING AWAY AND WAS MORE THAN 1000' AWAY, WE RECEIVED NO ALT ALERT WARNING US OF THE GRADUAL CLIMB. WAS THERE A TEMPORARY FAILURE OF THE AUTOPLT, OR MAYBE THE FMS COMPUTER? I DON'T KNOW, WE COULDN'T GET IT TO DUPLICATE. EVERYTHING WORKED FINE THE REMAINING FEW MINS. COULD WE HAVE ENCOUNTERED AN UPDRAFT THAT OVERPWRED THE AUTOPLT? THERE WAS MOUNTAIN WAVE ACTIVITY AND TURBULENCE PRESENT. I'M NOT SURE. WHATEVER, WE HAVE REPORTED THIS INCIDENT TO OUR COMPANY THAT PLANS TO TAKE IT TO XXXX FOR FURTHER EVALUATION. I'M SURE YOU COULD SAY THE CREW SHOULD HAVE BEEN MORE OBSERVANT, BUT WE WERE DOING NORMAL COCKPIT PROCS NECESSARY IN THAT PHASE OF FLT, YET WE MISSED THIS DEVIATION. IF YOU HAVE RECORDS OF ANYTHING SIMILAR HAPPENING IN AN MLG A OR MLG B I WOULD LOVE TO HEAR FROM YOU!

Data retrieved from NASA's ASRS site as of July 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.