Narrative:

I was flying. We were given a descent clearance to 4000 ft. As we approached 4000 ft, I placed the altitude hold wheel to the engaged position. The aircraft continued to descend slightly which is normal (not instantaneous). When the aircraft reached 3900 ft MSL, I made sure that the altitude hold function was engaged (it was). The aircraft continued to descend. At 3800 ft MSL, I turned off the autoplt and manually climbed to 4000 ft MSL. I think that the reason for the poor leveloff was caused by a late turn onto the localizer by the controller requiring a 30 degree bank turn by the autoplt. I have noticed that the autoplt does not operate well in altitude function when there is a change in lift/pitch caused by steep turns.

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Original NASA ASRS Text

Title: AN ACR DC9 FLC DSNDED BELOW THEIR ASSIGNED ALT WHILE OBSERVING A KNOWN ACFT AUTOFLT SYS PERFORMANCE CHARACTERISTIC BEHAVE AS THEY EXPECTED.

Narrative: I WAS FLYING. WE WERE GIVEN A DSCNT CLRNC TO 4000 FT. AS WE APCHED 4000 FT, I PLACED THE ALT HOLD WHEEL TO THE ENGAGED POS. THE ACFT CONTINUED TO DSND SLIGHTLY WHICH IS NORMAL (NOT INSTANTANEOUS). WHEN THE ACFT REACHED 3900 FT MSL, I MADE SURE THAT THE ALT HOLD FUNCTION WAS ENGAGED (IT WAS). THE ACFT CONTINUED TO DSND. AT 3800 FT MSL, I TURNED OFF THE AUTOPLT AND MANUALLY CLBED TO 4000 FT MSL. I THINK THAT THE REASON FOR THE POOR LEVELOFF WAS CAUSED BY A LATE TURN ONTO THE LOC BY THE CTLR REQUIRING A 30 DEG BANK TURN BY THE AUTOPLT. I HAVE NOTICED THAT THE AUTOPLT DOES NOT OPERATE WELL IN ALT FUNCTION WHEN THERE IS A CHANGE IN LIFT/PITCH CAUSED BY STEEP TURNS.

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.