Narrative:

We were approaching dfw on the boids 9 arrival with clearance to cross pivot intersection at 11000' and 250 KTS. In descent the aircraft was on autoplt in the IAS mode and 11000' armed. At 12500' ATC asked us to expedite descent. I selected vertical speed on the flight guidance panel and increased our rate of descent from 1500 FPM to 2500 FPM. 11000' should have remained armed during this procedure. It did not and I did not notice it drop out of the autoplt annunciator until approximately 11200'. The aircraft was still in a 2500 FPM descent when hit altitude hold. The aircraft did not stop its descent until 10600'. I immediately climbed and leveled at 11000'. This is not the first time the medium large transport autoplt has dropped the armed altitude in my experience, and I should have noticed it in the annunciator. However, we were on an arrival 25 mi from dfw during a peak traffic period on a VFR day. I had diverted my attention outside to watch for traffic for no more than 30 seconds, which was enough to miss the altitude. The altitude capture function of this autoplt should be redesigned to make it more difficult to disarm so more attention can be focused outside.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT INTO DFW.

Narrative: WE WERE APCHING DFW ON THE BOIDS 9 ARR WITH CLRNC TO CROSS PIVOT INTXN AT 11000' AND 250 KTS. IN DSCNT THE ACFT WAS ON AUTOPLT IN THE IAS MODE AND 11000' ARMED. AT 12500' ATC ASKED US TO EXPEDITE DSCNT. I SELECTED VERT SPD ON THE FLT GUIDANCE PANEL AND INCREASED OUR RATE OF DSCNT FROM 1500 FPM TO 2500 FPM. 11000' SHOULD HAVE REMAINED ARMED DURING THIS PROC. IT DID NOT AND I DID NOT NOTICE IT DROP OUT OF THE AUTOPLT ANNUNCIATOR UNTIL APPROX 11200'. THE ACFT WAS STILL IN A 2500 FPM DSCNT WHEN HIT ALT HOLD. THE ACFT DID NOT STOP ITS DSCNT UNTIL 10600'. I IMMEDIATELY CLBED AND LEVELED AT 11000'. THIS IS NOT THE FIRST TIME THE MLG AUTOPLT HAS DROPPED THE ARMED ALT IN MY EXPERIENCE, AND I SHOULD HAVE NOTICED IT IN THE ANNUNCIATOR. HOWEVER, WE WERE ON AN ARR 25 MI FROM DFW DURING A PEAK TFC PERIOD ON A VFR DAY. I HAD DIVERTED MY ATTN OUTSIDE TO WATCH FOR TFC FOR NO MORE THAN 30 SECS, WHICH WAS ENOUGH TO MISS THE ALT. THE ALT CAPTURE FUNCTION OF THIS AUTOPLT SHOULD BE REDESIGNED TO MAKE IT MORE DIFFICULT TO DISARM SO MORE ATTN CAN BE FOCUSED OUTSIDE.

Data retrieved from NASA's ASRS site as of August 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.