Autoplt began a rapid descent when the flight crew attempted to level off at assigned altitude.

Attributes
ACN	82481
Time
Date	198802
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : bdf
State Reference	IL
Altitude	msl bound lower : 23500 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 100 flight time total : 6200
ASRS Report	82481
Person 2
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

The aircraft altitude alert system was inoperative so we were extra aware of altitude. During descent, on autoplt, I was leveling off at FL240, making a small correction on vertical speed, when the autoplt started a rapid descent. The descent went to 23550', approximately, and I recovered it back to FL240. Center didn't say anything to us about it. I don't know how to do anything different if it occurred again.

Original NASA ASRS Text

Title: AUTOPLT BEGAN A RAPID DESCENT WHEN THE FLT CREW ATTEMPTED TO LEVEL OFF AT ASSIGNED ALT.

Narrative: THE ACFT ALT ALERT SYSTEM WAS INOP SO WE WERE EXTRA AWARE OF ALT. DURING DESCENT, ON AUTOPLT, I WAS LEVELING OFF AT FL240, MAKING A SMALL CORRECTION ON VERTICAL SPEED, WHEN THE AUTOPLT STARTED A RAPID DESCENT. THE DESCENT WENT TO 23550', APPROX, AND I RECOVERED IT BACK TO FL240. CENTER DIDN'T SAY ANYTHING TO US ABOUT IT. I DON'T KNOW HOW TO DO ANYTHING DIFFERENT IF IT OCCURRED AGAIN.

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.