Narrative:

Speed 240 KTS directed by ATC (approach) control to descend from 6000 to 5000'. Autoplt on using IAS/mach mode on autoflt system autothrottle on. Descent rate 1500 FPM. Orange altitude armed light illuminated on FMA (flight mode annunciator). Passing 5500' approach asked for our speed. We acknowledged. Approach directed us to slow from 240 to 180 KTS and turn from 340 to 120 degrees, a 140 degree heading change. At approximately 5150' FMA displayed 'altitude cap' and orange 'altitude' light out. Aircraft continued descent as autoplt trim slowly tried to bring the nose up with the combined pitch down of altitude capture, reducing speed with throttle idle and 30 degrees of bank angle. It is not unusual for normal autoflt to go 100' below altitude and return to selected altitude when capturing from above with 'altitude cap' showing. At 4750' altitude warning sounded and descent rate was still 1500 to 1800 FPM. Disconnected autoplt. Nose pitched down to 2500 FPM. I pulled back with autoplt and autothrottles disconnected and stopped descent between 4600 and 4500' and immediately climbed back to 5000'. Captain was flying aircraft. From 5000 to 4500' took less than 15 seconds and return to 5000' about 30 seconds. The setting of the autoflt controls occurred at about the same time the autoplt was attempted to capture the selected 5000' altitude. The altitude set knob was not touched during the 'altitude cap' phase. Unfortunately if it is, this autoflt system will disengage altitude capture. That was not the case in this incident. The pitch wheel will also disengage 'altitude cap' if moved during this phase. Again, this was not the case.

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Original NASA ASRS Text

Title: ALT OVERSHOT ON DESCENT WHEN AUTOPLT FAILED TO CAPTURE.

Narrative: SPD 240 KTS DIRECTED BY ATC (APCH) CTL TO DSND FROM 6000 TO 5000'. AUTOPLT ON USING IAS/MACH MODE ON AUTOFLT SYS AUTOTHROTTLE ON. DSCNT RATE 1500 FPM. ORANGE ALT ARMED LIGHT ILLUMINATED ON FMA (FLT MODE ANNUNCIATOR). PASSING 5500' APCH ASKED FOR OUR SPD. WE ACKNOWLEDGED. APCH DIRECTED US TO SLOW FROM 240 TO 180 KTS AND TURN FROM 340 TO 120 DEGS, A 140 DEG HDG CHANGE. AT APPROX 5150' FMA DISPLAYED 'ALT CAP' AND ORANGE 'ALT' LIGHT OUT. ACFT CONTINUED DSCNT AS AUTOPLT TRIM SLOWLY TRIED TO BRING THE NOSE UP WITH THE COMBINED PITCH DOWN OF ALT CAPTURE, REDUCING SPD WITH THROTTLE IDLE AND 30 DEGS OF BANK ANGLE. IT IS NOT UNUSUAL FOR NORMAL AUTOFLT TO GO 100' BELOW ALT AND RETURN TO SELECTED ALT WHEN CAPTURING FROM ABOVE WITH 'ALT CAP' SHOWING. AT 4750' ALT WARNING SOUNDED AND DSCNT RATE WAS STILL 1500 TO 1800 FPM. DISCONNECTED AUTOPLT. NOSE PITCHED DOWN TO 2500 FPM. I PULLED BACK WITH AUTOPLT AND AUTOTHROTTLES DISCONNECTED AND STOPPED DSCNT BTWN 4600 AND 4500' AND IMMEDIATELY CLBED BACK TO 5000'. CAPT WAS FLYING ACFT. FROM 5000 TO 4500' TOOK LESS THAN 15 SECS AND RETURN TO 5000' ABOUT 30 SECS. THE SETTING OF THE AUTOFLT CONTROLS OCCURRED AT ABOUT THE SAME TIME THE AUTOPLT WAS ATTEMPTED TO CAPTURE THE SELECTED 5000' ALT. THE ALT SET KNOB WAS NOT TOUCHED DURING THE 'ALT CAP' PHASE. UNFORTUNATELY IF IT IS, THIS AUTOFLT SYS WILL DISENGAGE ALT CAPTURE. THAT WAS NOT THE CASE IN THIS INCIDENT. THE PITCH WHEEL WILL ALSO DISENGAGE 'ALT CAP' IF MOVED DURING THIS PHASE. AGAIN, THIS WAS NOT THE CASE.

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.