Flight crew of a DC9 descended off cruise altitude when the autoplt suddenly pitched the aircraft down. Flight crew disconnected autoplt and immediately return to their assigned altitude.

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Attributes
ACN	492544
Time
Date	200011
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : bae.vortac
State Reference	WI
Altitude	msl bound lower : 34650 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp.artcc tower : zzz.tower
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : atp
Experience	flight time last 90 days : 230 flight time total : 3000 flight time type : 800
ASRS Report	492544
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to assigned altitude other
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Our aircraft autoplt performed an uncommanded pitch down and thus losing 350 ft before we manually kicked it off and recovered the aircraft. Center said no bells went off, but did notice our deviation. We made a maintenance write-up in the logbook for a fix of the servo.

Original NASA ASRS Text

Title: FLC OF A DC9 DSNDED OFF CRUISE ALT WHEN THE AUTOPLT SUDDENLY PITCHED THE ACFT DOWN. FLC DISCONNECTED AUTOPLT AND IMMEDIATELY RETURN TO THEIR ASSIGNED ALT.

Narrative: OUR ACFT AUTOPLT PERFORMED AN UNCOMMANDED PITCH DOWN AND THUS LOSING 350 FT BEFORE WE MANUALLY KICKED IT OFF AND RECOVERED THE ACFT. CTR SAID NO BELLS WENT OFF, BUT DID NOTICE OUR DEV. WE MADE A MAINT WRITE-UP IN THE LOGBOOK FOR A FIX OF THE SERVO.

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.