AN ACR MLG CLBED AFTER RECEIVING A CLRNC TO DSND.

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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.