Air carrier flight crew experiences malfunctioning altitude capture hold function of autoplt.

Attributes
ACN	129515
Time
Date	198911
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	msl bound lower : 3000 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zfw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 59 flight time total : 5300 flight time type : 645
ASRS Report	129515
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 80 flight time total : 6500 flight time type : 25
ASRS Report	129516
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Aircraft was cleared in step dscnts from 37000 to 24000'. Dscnts were made with autoplt on, with all cleared altitudes set and armed. Autoplt did not capture 24000'. Other pilot was turning on engine and wing anti-ice. Aural altitude warning sounded. Altitude hold was engaged and aircraft was returned to 24000'. Aircraft was then cleared to 9000' and descent was made on autoplt, with 9000' set and armed. During this descent, pilots discussed altitude excursion at 24000', and could not determine what caused it. Aircraft was cleared to 6000' with 100' level off required at 9000'. Autoplt did not capture 6000'. Pilots were also slowing, configuring aircraft for landing and running landing checklist. Aural altitude warning sounded, altitude hold was engaged and aircraft was returned to 6000'. Altitude deviation 200-300' each time. Autoplt was disengaged at 3000' when it did not capture 3000' set and armed during cleared ILS approach to dfw 35R. Hand-flown ILS and landing. Autoplt written up in aircraft maintenance log as malfunctioning in altitude capture.

Original NASA ASRS Text

Title: ACR FLT CREW EXPERIENCES MALFUNCTIONING ALT CAPTURE HOLD FUNCTION OF AUTOPLT.

Narrative: ACFT WAS CLRED IN STEP DSCNTS FROM 37000 TO 24000'. DSCNTS WERE MADE WITH AUTOPLT ON, WITH ALL CLRED ALTS SET AND ARMED. AUTOPLT DID NOT CAPTURE 24000'. OTHER PLT WAS TURNING ON ENG AND WING ANTI-ICE. AURAL ALT WARNING SOUNDED. ALT HOLD WAS ENGAGED AND ACFT WAS RETURNED TO 24000'. ACFT WAS THEN CLRED TO 9000' AND DSCNT WAS MADE ON AUTOPLT, WITH 9000' SET AND ARMED. DURING THIS DSCNT, PLTS DISCUSSED ALT EXCURSION AT 24000', AND COULD NOT DETERMINE WHAT CAUSED IT. ACFT WAS CLRED TO 6000' WITH 100' LEVEL OFF REQUIRED AT 9000'. AUTOPLT DID NOT CAPTURE 6000'. PLTS WERE ALSO SLOWING, CONFIGURING ACFT FOR LNDG AND RUNNING LNDG CHKLIST. AURAL ALT WARNING SOUNDED, ALT HOLD WAS ENGAGED AND ACFT WAS RETURNED TO 6000'. ALT DEVIATION 200-300' EACH TIME. AUTOPLT WAS DISENGAGED AT 3000' WHEN IT DID NOT CAPTURE 3000' SET AND ARMED DURING CLRED ILS APCH TO DFW 35R. HAND-FLOWN ILS AND LNDG. AUTOPLT WRITTEN UP IN ACFT MAINT LOG AS MALFUNCTIONING IN ALT CAPTURE.

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.