Narrative:

During an autoplt/flight director coupled en route descent, the autoplt altitude select mode disengaged unbeknownst to the flight crew and the aircraft descended below assigned altitude. Flight was cleared to 18000 ft and told to expect holding at rackk intersection. A clearance to hold as published at rackk, except 15 DME legs, then followed. Shortly after, the flight was cleared to 9000 ft with a crossing restr of 11000 ft at rackk. As the flight entered the hold and the ATC report was made (with the mandatory altitude call), it became apparent that the altitude select had failed to capture at 9000 ft and we had descended below assigned altitude. Corrective action was taken immediately, however the aircraft did descend several hundred feet below assigned altitude, to approximately 8300 ft. Contributing factors: high rate descent to meet crossing restr. This gave the flight crew less time to discover the problem, and even after discovery, increased the altitude deviation before corrective actions took place. Crew distraction from complicated clearance including 2 altitude assignments, crossing restr and holding procedures. I was concerned with descending rapidly to meet the crossing restr and was monitoring altitude and descent rate closely. Once I felt certain the crossing restr would be met, my focus shifted to the hold entry, especially maximum airspeed, and less so on altitude. Flight crew inattn to autoplt, despite best efforts, there is a tendency to not monitor the aircraft as closely as one should when on autoplt, especially after several hours of uneventful flying. Unusual crossing altitude, 8000 ft is the normal clearance altitude in this area. When the captain first noted the aircraft was below the altitude on the altitude select, he thought it had been set incorrectly. The deviation was only 200 ft at that point. Malfunctioning autoplt. The altitude select mode disengaged at some point in the descent. We had not had any problems with the autoplt until this point, the altitude hold function disengaged 2 more times within a 5 min time span right after this incident. Company maintenance was notified. Lack of obvious advisories to flight crew on disengagement, an autoplt disconnect results in a 2 second 'cavalry charge' audio tone. However, the only indication of an altitude hold or select disengagement is a small 'altitude' caption extinguishing on the top cathode ray tube, much more difficult to notice. A similar audio tone would be great. Failure of flight crew to adhere to company callouts, '1000 ft above, 100 ft above' would have prevented this. Callback conversation with reporter revealed the following information: the reporter was flying the jetstream 41 as first officer. He has received an loi from the FAA regarding this altitude deviation. Apparently there was an aircraft in the holding pattern at 8000 ft and the reporter violated his airspace. The reporter has retained an attorney as he understands that the union attorney is overworked and not very interested in this case. The reporter has not followed up on the autoplt problem. He acknowledges that he may have changed the pitch command after selecting an altitude and may not have re- selected 9000 ft. No hearing has yet been scheduled.

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Original NASA ASRS Text

Title: ALTDEV, AN ACR CREW OVERSHOT ITS ASSIGNED ALT ENTERING A HOLDING PATTERN.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.