AUTO EQUIP FAILED TO LEVEL OFF PRECISELY. OVERSHOT. ON RECOVERY OVERSHOT AGAIN BEFORE FLC LEVELED THE ACFT.

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ACFT WAS CLBING AT 1800 FPM THROUGH FL285 FOR FL330 WITH AUTOPLT AND PERFORMANCE MGMNT SYS ENGAGED. DUE TO UNFAVORABLE RIDE RPTS ALOFT; CLRNC TO MAINTAIN FL290 WAS RECEIVED. PMS BEGAN TO CAPTURE FL290 BUT WENT TO FL292 BEFORE REVERSING AND DSNDING AT 1800 FPM. AT THIS POINT THE PMS DISENGAGED WITH NO VISUAL OR AURAL WARNING/INDICATION (SUCH AS THAT GENERATED WHEN THE AUTOPLT DISENGAGES ITSELF). THE AUTOPLT REMAINED ENGAGED; MAINTAINED THE 1800 FPM DSCNT; DISARMED THE ALT CAPTURE FUNCTION AND MADE NO ATTEMPT TO CAPTURE FL290. FLC ATTEMPTED TO GIVE NEW COMMANDS TO THE AUTOPLT TO CLB TO AND CAPTURE FL290. WITHOUT AN IMMEDIATE RESPONSE FROM THE AUTOPLT; THE FLC DISENGAGED THE AUTOPLT; BUT NOT UNTIL THE ACFT REACHED FL286. FL290 WAS PROMPTLY REATTAINED AND AUTOPLT WAS REENGAGED WITHOUT FURTHER INCIDENT. 3 ITEMS CONTRIBUTED TO THIS EVENT. THE PMS WAS TOO SLOW TO REACT TO A LEVEL OFF REQUEST. THERE WAS NO WARNING TO FLC OF A CHANGE IN THE SOURCE OF COMMANDS FOR AUTOMATIC CTL OF THE ACFT. WHEN THE CTL OF THE ACFT TRANSFERRED FROM THE PMS TO THE AUTOPLT; THERE WAS NO TRANSFER OF THE ALT CAPTURE COMMAND. ALTHOUGH THE COCKPIT ENVIRONMENT IS ALREADY HEAVILY LOADED WITH BELLS AND LIGHTS; A MAJOR; SELF- GENERATED CHANGE IN THE OPERATING STATUS OF AUTOFLT (SUCH AS A PMS DISENGAGE) MERITS MORE THAN THE CURRENT ANNUNCIATIONS OF ATS; PITCH AND ROLL MODES. CURRENT PMS/FGS PROGRAMMING DOES NOT CAUSE THE AUTOMATIC REASSIGNMENT OF ALT ARMING/CAPTURE FUNCTIONS FROM 1 SYS TO ANOTHER. WITH TODAY'S SOPHISTICATED SOFTWARE DEVELOPMENTS; THE DISENGAGING OF A PMS ATTEMPTING A LEVEL-OFF SHOULD NOT RESULT IN THE FGS JUST BLINDLY RESUMING THE INSTANTANEOUS CONDITION OF FLT HANDED TO IT.