A B737-300 ON 'B' AUTOPLT WITH FL200 SELECTED ON MODE CTL PANEL ALT HOLD. AIRPLANE CLBED TO FL319 UNCOMMANDED.

Narrative

WE WERE AT CRUISE ALT (31000 FT) AND PREPARING TO DSND ON THE PULLMAN ARR INTO ORD. THE ACFT WAS ON THE 'B' AUTOPLT IN VNAV PATH AND LNAV MODES. IN THE FMC; CLAUD INTXN HAD AN ALT RESTR OF FL200 THAT I HAD PUT IN A FEW MINS EARLIER AFTER HEARING ANOTHER ACFT AHEAD RECEIVE THAT CLRNC. CTR GAVE US 'CROSS CLAUD AT FL200; AND WHEN ABLE; MAINTAIN 320 KTS.' THE CAPT ROLLED THE MCP ALT WINDOW TO 20000 FT AND I POINTED AT IT. AT THAT TIME; I DOUBLE CHKED THAT THE ACFT WAS IN VNAV PATH AND NOTED THAT WE WERE STILL ABOUT 20-30 MI FROM THE FMC COMPUTED TOP-OF-DSCNT POINT. I DO NOT RECALL DOUBLE CHKING OUR ACTUAL ALT AT THAT TIME. THIS FINALLY PROMPTED ME TO LOOK AT THE ALT INDICATOR. I WAS SHOCKED TO SEE WE WERE AT 31900 FT IN A SHALLOW CLB (200-300 FPM). FOR SOME REASON; MY IMMEDIATE REACTION WAS TO REACH UP AND ROLL THE VERT SPD WHEEL DOWN. AS I DID THIS; I NOTED THAT THE VERT SPD WINDOW WAS NOT OPEN; SO MY NEXT REACTION WAS TO GRAB THE YOKE AND PUSH ON IT. DURING THE DSCNT; WE DISCUSSED WHAT COULD HAVE CAUSED THE AUTOPLT TO COMMAND A CLB; LEADING ME TO RECALL THE SEQUENCE OF EVENTS WHILE IT WAS STILL VERY FRESH IN MY MIND. WE ALSO RECALLED THAT BEFORE THE FLT; WE WERE DELAYED FOR ABOUT 40 MINS WHILE A MECH CHANGED OUT AN MCP PANEL TO FINALLY GET RID OF A PERSISTENT 'A/P P/RST' LIGHT. WE AGREED THAT THIS WAS SUSPICIOUS. WE CONTINUED THE DSCNT AND LNDG WITHOUT INCIDENT WITH ME HAND FLYING THE REST OF THE WAY. OUR BEST GUESS OF WHAT HAPPENED WAS THE AUTOPLT SIMPLY FLEW AWAY FROM THE ASSIGNED ALT IN VNAV AND BECAUSE THE RATE OF ASCENT WAS SO SHALLOW; NEITHER OF US NOTICED. WE TOLD THIS TO THE MECH WHO MET THE ACFT; AND HE TOLD US THAT HE WOULD PROBABLY JUST DEFER THE VNAV MODE.