ALTDEV; AN ACR CREW OVERSHOT ITS ASSIGNED ALT ENTERING A HOLDING PATTERN.

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DURING AN AUTOPLT/FLT DIRECTOR COUPLED ENRTE DSCNT; THE AUTOPLT ALT SELECT MODE DISENGAGED UNBEKNOWNST TO THE FLC AND THE ACFT DSNDED BELOW ASSIGNED ALT. FLT WAS CLRED TO 18000 FT AND TOLD TO EXPECT HOLDING AT RACKK INTXN. A CLRNC TO HOLD AS PUBLISHED AT RACKK; EXCEPT 15 DME LEGS; THEN FOLLOWED. SHORTLY AFTER; THE FLT WAS CLRED TO 9000 FT WITH A XING RESTR OF 11000 FT AT RACKK. AS THE FLT ENTERED THE HOLD AND THE ATC RPT WAS MADE (WITH THE MANDATORY ALT CALL); IT BECAME APPARENT THAT THE ALT SELECT HAD FAILED TO CAPTURE AT 9000 FT AND WE HAD DSNDED BELOW ASSIGNED ALT. CORRECTIVE ACTION WAS TAKEN IMMEDIATELY; HOWEVER THE ACFT DID DSND SEVERAL HUNDRED FEET BELOW ASSIGNED ALT; TO APPROX 8300 FT. CONTRIBUTING FACTORS: HIGH RATE DSCNT TO MEET XING RESTR. THIS GAVE THE FLC LESS TIME TO DISCOVER THE PROB; AND EVEN AFTER DISCOVERY; INCREASED THE ALTDEV BEFORE CORRECTIVE ACTIONS TOOK PLACE. CREW DISTR FROM COMPLICATED CLRNC INCLUDING 2 ALT ASSIGNMENTS; XING RESTR AND HOLDING PROCS. I WAS CONCERNED WITH DSNDING RAPIDLY TO MEET THE XING RESTR AND WAS MONITORING ALT AND DSCNT RATE CLOSELY. ONCE I FELT CERTAIN THE XING RESTR WOULD BE MET; MY FOCUS SHIFTED TO THE HOLD ENTRY; ESPECIALLY MAX AIRSPD; AND LESS SO ON ALT. FLC INATTN TO AUTOPLT; DESPITE BEST EFFORTS; THERE IS A TENDENCY TO NOT MONITOR THE ACFT AS CLOSELY AS ONE SHOULD WHEN ON AUTOPLT; ESPECIALLY AFTER SEVERAL HRS OF UNEVENTFUL FLYING. UNUSUAL XING ALT; 8000 FT IS THE NORMAL CLRNC ALT IN THIS AREA. WHEN THE CAPT FIRST NOTED THE ACFT WAS BELOW THE ALT ON THE ALT SELECT; HE THOUGHT IT HAD BEEN SET INCORRECTLY. THE DEV WAS ONLY 200 FT AT THAT POINT. MALFUNCTIONING AUTOPLT. THE ALT SELECT MODE DISENGAGED AT SOME POINT IN THE DSCNT. WE HAD NOT HAD ANY PROBS WITH THE AUTOPLT UNTIL THIS POINT; THE ALT HOLD FUNCTION DISENGAGED 2 MORE TIMES WITHIN A 5 MIN TIME SPAN RIGHT AFTER THIS INCIDENT. COMPANY MAINT WAS NOTIFIED. LACK OF OBVIOUS ADVISORIES TO FLC ON DISENGAGEMENT; AN AUTOPLT DISCONNECT RESULTS IN A 2 SECOND 'CAVALRY CHARGE' AUDIO TONE. HOWEVER; THE ONLY INDICATION OF AN ALT HOLD OR SELECT DISENGAGEMENT IS A SMALL 'ALT' CAPTION EXTINGUISHING ON THE TOP CATHODE RAY TUBE; MUCH MORE DIFFICULT TO NOTICE. A SIMILAR AUDIO TONE WOULD BE GREAT. FAILURE OF FLT CREW TO ADHERE TO COMPANY CALLOUTS; '1000 FT ABOVE; 100 FT ABOVE' WOULD HAVE PREVENTED THIS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING THE JETSTREAM 41 AS FO. HE HAS RECEIVED AN LOI FROM THE FAA REGARDING THIS ALTDEV. APPARENTLY THERE WAS AN ACFT IN THE HOLDING PATTERN AT 8000 FT AND THE RPTR VIOLATED HIS AIRSPACE. THE RPTR HAS RETAINED AN ATTORNEY AS HE UNDERSTANDS THAT THE UNION ATTORNEY IS OVERWORKED AND NOT VERY INTERESTED IN THIS CASE. THE RPTR HAS NOT FOLLOWED UP ON THE AUTOPLT PROB. HE ACKNOWLEDGES THAT HE MAY HAVE CHANGED THE PITCH COMMAND AFTER SELECTING AN ALT AND MAY NOT HAVE RE- SELECTED 9000 FT. NO HEARING HAS YET BEEN SCHEDULED.