ALT EXCURSION IN ZTL AIRSPACE.

Narrative

WHILE IN CRUISE FLT AT FL350; NWBOUND; DIRECT FROM ATL- SBN; PARALLELING J89 USING GPS NAV; THE AIR CREW WAS ENGAGED IN CONVERSATION WITH A PAX INQUIRING ABOUT THE FLT SCHEDULE AND REQUESTING A CHANGE IN CABIN TEMP. WHILE WE WERE MOMENTARILY DISTR; THE AP103 AUTOPLT ALT HOLD FUNCTION APPARENTLY DISENGAGED WITHOUT OUR KNOWLEDGE UNTIL THE 300 FT WARNING BELL ACTIVATED AT FL347. I IMMEDIATELY DISENGAGED THE AUTOPLT COMPLETELY AND RE- ESTABLISHED FLT AT FL350. THE CTR CTLR ASKED US TO CHK OUR ALT; AND WE RPTED FL349; AS WE WERE ALREADY NEARLY BACK TO FL350. THE EXCURSION WAS SO GRADUAL AS TO BE UNNOTICED; AND THE ACFT DID NOT DSND BELOW FL347. THIS TYPE OF PERFORMANCE IS SO COMMON WITH THE AP103 AUTOPLT; THAT IT IS NOT MNPS CERTIFIABLE. A VERY HIGH LEVEL OF VIGILANCE IS REQUIRED TO PREVENT A RECURRENCE OF THIS TYPE INCIDENT; AND WE HAVE ALTERED OUR CREW PROCS ACCORDINGLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT RPTS THAT HE HAS SPOKEN TO OTHER OPERATORS OF THIS ACFT WHICH ARE EQUIPPED WITH THE AP103 AUTOPLT. ALL RPT THAT THIS AUTOPLT WILL DRIFT OFF ALT WHILE THE ALT HOLD FEATURE IS ENGAGED.