Air carrier medium large transport overshot altitude descent due to autoplt malfunction.

Attributes
ACN	82955
Time
Date	198803
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : das
State Reference	TX
Altitude	msl bound lower : 9700 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zhu
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 125 flight time total : 8940 flight time type : 495
ASRS Report	82955
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Altimeters set correctly to local setting. Descending to 10000' MSL at 250 KIAS. Aircraft is medium large transport. Altitude is armed at 10000', speed select at 250 KTS. When aircraft is about to level at 10000' MSL flight guidance suddenly goes into vertical speed at about 2100' descended. I immediately pressed altitude hold 2 or 3 times, but had no effect. Disconnected autoplt and manually flew back to 10000' MSL. Altitude deviation was 250-300' down from 10000'. Altitude voice from CAWS was heard and also altitude light on altimeter illuminated. Reason for excursion unknown. Flight guidance system just had a mind of its own. Pilot was not touching anything when excursion started. System was set properly and altitude armed.

Original NASA ASRS Text

Title: ACR MLG OVERSHOT ALT DESCENT DUE TO AUTOPLT MALFUNCTION.

Narrative: ALTIMETERS SET CORRECTLY TO LCL SETTING. DSNDING TO 10000' MSL AT 250 KIAS. ACFT IS MLG. ALT IS ARMED AT 10000', SPD SELECT AT 250 KTS. WHEN ACFT IS ABOUT TO LEVEL AT 10000' MSL FLT GUIDANCE SUDDENLY GOES INTO VERT SPD AT ABOUT 2100' DESCENDED. I IMMEDIATELY PRESSED ALT HOLD 2 OR 3 TIMES, BUT HAD NO EFFECT. DISCONNECTED AUTOPLT AND MANUALLY FLEW BACK TO 10000' MSL. ALT DEVIATION WAS 250-300' DOWN FROM 10000'. ALT VOICE FROM CAWS WAS HEARD AND ALSO ALT LIGHT ON ALTIMETER ILLUMINATED. REASON FOR EXCURSION UNKNOWN. FLT GUIDANCE SYS JUST HAD A MIND OF ITS OWN. PLT WAS NOT TOUCHING ANYTHING WHEN EXCURSION STARTED. SYS WAS SET PROPERLY AND ALT ARMED.

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.