Narrative:

We were given a descent from 310000 ft by center. The aircraft was on autoplt with LNAV and VNAV engaged, using the FMC and area navigation. I was the PF. The first officer set mode control panel altitude to 28000 ft with the autoplt engaged. The FMC did not accept 28000 ft into the program and it took 3 entries to activate it. After entering the altitude in the FMC, I looked at the instruments and thought that the altimeter had failed because it was showing a climb through 31600 ft. Shortly thereafter, center called for our altitude as I was taking the aircraft off autoplt and correcting the climb. We were close to 31900 ft before we could level and start down manually. The first officer was involved in paperwork and was caught by surprise also. It appeared that when the FMC would not accept 28000 ft that the VNAV lost the altitude input. This probably caused the autoplt to trip from command to control wheel steering pitch, which was the indication when I took over manually. Why the autoplt went into a climb when tripped to control wheel steering pitch is a mystery. Neither the first officer nor myself had felt the aircraft go into a climb. Our normal altitude warning did not give any signal in this case because the mode control panel had been set to 28000 and we had entered a climb out of 31000 instead of a descent. The autoplt did not give an aural warning because it did not trip off completely, but only switched to control wheel steering in pitch mode. On autoflt aircraft, any problem with programming the FMC can distract the pilots' attention from the flight instruments. Usually, normal instrument scan or any one of the warning devices would have brought my attention to the error in the flight control before altitude could change by 600 ft. Supplemental information from acn 252364: the captain (the PF) had the autoplt engaged, in the 'control wheel steering' mode. We received and acknowledged a descent clearance to FL280. We were also asked to keep our speed up. The captain selected a higher speed in the autoplt mode control panel, then proceeded to lean down over the computer to set in the lower altitude. Meanwhile, with the faster speed dialed in, the autothrottles advanced, which must have pitched the nose of the airplane up and caused it to climb. The 'control wheel steering' mode of the autoplt only holds whatever flight attitude the aircraft is presently holding. Never use 'control wheel steering' mode of the autoplt under normal line operations.

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Original NASA ASRS Text

Title: AN ACR MLG CLBED AFTER RECEIVING A CLRNC TO DSND.

Narrative: WE WERE GIVEN A DSCNT FROM 310000 FT BY CTR. THE ACFT WAS ON AUTOPLT WITH LNAV AND VNAV ENGAGED, USING THE FMC AND AREA NAV. I WAS THE PF. THE FO SET MODE CTL PANEL ALT TO 28000 FT WITH THE AUTOPLT ENGAGED. THE FMC DID NOT ACCEPT 28000 FT INTO THE PROGRAM AND IT TOOK 3 ENTRIES TO ACTIVATE IT. AFTER ENTERING THE ALT IN THE FMC, I LOOKED AT THE INSTS AND THOUGHT THAT THE ALTIMETER HAD FAILED BECAUSE IT WAS SHOWING A CLB THROUGH 31600 FT. SHORTLY THEREAFTER, CTR CALLED FOR OUR ALT AS I WAS TAKING THE ACFT OFF AUTOPLT AND CORRECTING THE CLB. WE WERE CLOSE TO 31900 FT BEFORE WE COULD LEVEL AND START DOWN MANUALLY. THE FO WAS INVOLVED IN PAPERWORK AND WAS CAUGHT BY SURPRISE ALSO. IT APPEARED THAT WHEN THE FMC WOULD NOT ACCEPT 28000 FT THAT THE VNAV LOST THE ALT INPUT. THIS PROBABLY CAUSED THE AUTOPLT TO TRIP FROM COMMAND TO CTL WHEEL STEERING PITCH, WHICH WAS THE INDICATION WHEN I TOOK OVER MANUALLY. WHY THE AUTOPLT WENT INTO A CLB WHEN TRIPPED TO CTL WHEEL STEERING PITCH IS A MYSTERY. NEITHER THE FO NOR MYSELF HAD FELT THE ACFT GO INTO A CLB. OUR NORMAL ALT WARNING DID NOT GIVE ANY SIGNAL IN THIS CASE BECAUSE THE MODE CTL PANEL HAD BEEN SET TO 28000 AND WE HAD ENTERED A CLB OUT OF 31000 INSTEAD OF A DSCNT. THE AUTOPLT DID NOT GIVE AN AURAL WARNING BECAUSE IT DID NOT TRIP OFF COMPLETELY, BUT ONLY SWITCHED TO CTL WHEEL STEERING IN PITCH MODE. ON AUTOFLT ACFT, ANY PROB WITH PROGRAMMING THE FMC CAN DISTRACT THE PLTS' ATTN FROM THE FLT INSTS. USUALLY, NORMAL INST SCAN OR ANY ONE OF THE WARNING DEVICES WOULD HAVE BROUGHT MY ATTN TO THE ERROR IN THE FLT CTL BEFORE ALT COULD CHANGE BY 600 FT. SUPPLEMENTAL INFO FROM ACN 252364: THE CAPT (THE PF) HAD THE AUTOPLT ENGAGED, IN THE 'CTL WHEEL STEERING' MODE. WE RECEIVED AND ACKNOWLEDGED A DSCNT CLRNC TO FL280. WE WERE ALSO ASKED TO KEEP OUR SPD UP. THE CAPT SELECTED A HIGHER SPD IN THE AUTOPLT MODE CTL PANEL, THEN PROCEEDED TO LEAN DOWN OVER THE COMPUTER TO SET IN THE LOWER ALT. MEANWHILE, WITH THE FASTER SPD DIALED IN, THE AUTOTHROTTLES ADVANCED, WHICH MUST HAVE PITCHED THE NOSE OF THE AIRPLANE UP AND CAUSED IT TO CLB. THE 'CTL WHEEL STEERING' MODE OF THE AUTOPLT ONLY HOLDS WHATEVER FLT ATTITUDE THE ACFT IS PRESENTLY HOLDING. NEVER USE 'CTL WHEEL STEERING' MODE OF THE AUTOPLT UNDER NORMAL LINE OPS.

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.