B737 crew had the pitch channel of the autoplt command an increased nose down rate.

Attributes
ACN	461011
Time
Date	200001
Day	Wed
Local Time Of Day	0601 To 1200
Place
State Reference	MD
Altitude	msl bound lower : 27600 msl bound upper : 28400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude descent : vacating altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 250 flight time total : 14300 flight time type : 1800
ASRS Report	461011
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Descending to FL280 with autoplt engaged. 1200 FPM descent rate selected on autoplt MCP. At approximately FL284, aircraft went into a steep dive, attaining a descent rate in excess of 2500 FPM. I disconnected autoplt. Aircraft descended to FL276 before descent could be arrested. Returned to FL280. No queries were made by ATC. Used autoplt on captain's side for remainder of flight. Remainder of flight uneventful.

Original NASA ASRS Text

Title: B737 CREW HAD THE PITCH CHANNEL OF THE AUTOPLT COMMAND AN INCREASED NOSE DOWN RATE.

Narrative: DSNDING TO FL280 WITH AUTOPLT ENGAGED. 1200 FPM DSCNT RATE SELECTED ON AUTOPLT MCP. AT APPROX FL284, ACFT WENT INTO A STEEP DIVE, ATTAINING A DSCNT RATE IN EXCESS OF 2500 FPM. I DISCONNECTED AUTOPLT. ACFT DSNDED TO FL276 BEFORE DSCNT COULD BE ARRESTED. RETURNED TO FL280. NO QUERIES WERE MADE BY ATC. USED AUTOPLT ON CAPT'S SIDE FOR REMAINDER OF FLT. REMAINDER OF FLT UNEVENTFUL.

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.