Narrative:

ZLA cleared us to 12000 ft. At 13000 ft, I called out the 1000 ft to go report to the captain. At 12600 ft, we switched to las vegas approach. Las approach told us to reduce speed to 250 KTS. The autoplt of our MD80 was engaged and the altitude was armed for capture. We were currently at 320 KTS for the descent. Captain used the IAS pitch wheel on autoplt to reduce our speed. Airspeed gradually started to reduce. As the aircraft reached 12000 ft, I saw the autoplt did not capture altitude. I called out '12000 ft -- we're cleared to 12000 ft.' captain acknowledged at 11900 ft 'we were cleared for 12000 ft?' 'yes 12000 ft' I responded. Captain disconnected autoplt and recovered at 11600 ft and leveled immediately off at 12000 ft. Total deviation 400 ft for 1 second. (I forgot to mention we were cleared to cross 40 NM on las R-060 at 12000 ft.) the MD80 is notorious for the autoplt not capturing the armed altitude if the IAS or vertical speed pitch wheel is changed prior to the assigned altitude. Especially if a large deviation is required (ie, 70 KTS difference in the above example) the autoplt cannot do both capture altitude and reduce a large difference in airspeed. If las approach would have cleared us to reduce speed to 250 KTS then descend to 12000 ft or level at 12000 ft then reduce to 250 KTS our autoplt would have captured the assigned altitude.

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Original NASA ASRS Text

Title: FLC OF AN MD80 OVERSHOT DSCNT ALT DUE TO THE INCORRECT OP OF THE AUTOPLT. THE ACFT WAS IMMEDIATELY RETURNED TO ASSIGNED ALT.

Narrative: ZLA CLRED US TO 12000 FT. AT 13000 FT, I CALLED OUT THE 1000 FT TO GO RPT TO THE CAPT. AT 12600 FT, WE SWITCHED TO LAS VEGAS APCH. LAS APCH TOLD US TO REDUCE SPD TO 250 KTS. THE AUTOPLT OF OUR MD80 WAS ENGAGED AND THE ALT WAS ARMED FOR CAPTURE. WE WERE CURRENTLY AT 320 KTS FOR THE DSCNT. CAPT USED THE IAS PITCH WHEEL ON AUTOPLT TO REDUCE OUR SPD. AIRSPD GRADUALLY STARTED TO REDUCE. AS THE ACFT REACHED 12000 FT, I SAW THE AUTOPLT DID NOT CAPTURE ALT. I CALLED OUT '12000 FT -- WE'RE CLRED TO 12000 FT.' CAPT ACKNOWLEDGED AT 11900 FT 'WE WERE CLRED FOR 12000 FT?' 'YES 12000 FT' I RESPONDED. CAPT DISCONNECTED AUTOPLT AND RECOVERED AT 11600 FT AND LEVELED IMMEDIATELY OFF AT 12000 FT. TOTAL DEV 400 FT FOR 1 SECOND. (I FORGOT TO MENTION WE WERE CLRED TO CROSS 40 NM ON LAS R-060 AT 12000 FT.) THE MD80 IS NOTORIOUS FOR THE AUTOPLT NOT CAPTURING THE ARMED ALT IF THE IAS OR VERT SPD PITCH WHEEL IS CHANGED PRIOR TO THE ASSIGNED ALT. ESPECIALLY IF A LARGE DEV IS REQUIRED (IE, 70 KTS DIFFERENCE IN THE ABOVE EXAMPLE) THE AUTOPLT CANNOT DO BOTH CAPTURE ALT AND REDUCE A LARGE DIFFERENCE IN AIRSPD. IF LAS APCH WOULD HAVE CLRED US TO REDUCE SPD TO 250 KTS THEN DSND TO 12000 FT OR LEVEL AT 12000 FT THEN REDUCE TO 250 KTS OUR AUTOPLT WOULD HAVE CAPTURED THE ASSIGNED ALT.

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.