Hawker 800XP crew had an autoplt uncommanded pitch over that resulted in a 500 ft altitude deviation...

Attributes
ACN	597695
Time
Date	200310
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : cmh.airport
State Reference	OH
Altitude	msl single value : 39000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zid.artcc
Operator	general aviation : corporate
Make Model Name	BAe 125 Series 800
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 75 flight time total : 6600 flight time type : 2200
ASRS Report	597695
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued alert flight crew : returned to assigned altitude
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

While in cruise flight at FL390 in smooth air we had an uncommanded pitch down of the autoplt. Within an instant the aircraft started a 1600 FPM descent. I disconnected the autoplt, but not before we lost 500 ft. ATC queried us about our altitude before we could get back up to FL390. A block altitude was given until things were stabilized.

Original NASA ASRS Text

Title: HAWKER 800XP CREW HAD AN AUTOPLT UNCOMMANDED PITCH OVER THAT RESULTED IN A 500 FT ALTDEV.

Narrative: WHILE IN CRUISE FLT AT FL390 IN SMOOTH AIR WE HAD AN UNCOMMANDED PITCH DOWN OF THE AUTOPLT. WITHIN AN INSTANT THE ACFT STARTED A 1600 FPM DSCNT. I DISCONNECTED THE AUTOPLT, BUT NOT BEFORE WE LOST 500 FT. ATC QUERIED US ABOUT OUR ALT BEFORE WE COULD GET BACK UP TO FL390. A BLOCK ALT WAS GIVEN UNTIL THINGS WERE STABILIZED.

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.