Captain of an MD88 descended below assigned due to autoplt failure to trim aircraft to hold altitude due to the high rate of slowing the aircraft for a speed reduction.

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Attributes
ACN	368720
Time
Date	199705
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : atl
State Reference	GA
Altitude	msl bound lower : 10700 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : atl
Operator	common carrier : air carrier
Make Model Name	MD-88
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other cruise other
Route In Use	arrival other arrival star : star enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 15500 flight time type : 4500
ASRS Report	368720
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On arrival into atl on the rome arrival we were level at 11000 ft and we were slowing from 320 KTS to 210 KIAS. As the aircraft was slowing down and trimming up, I was doing an approach briefing. I had my commercial chart book in front of me and was briefing the approach. I did not notice, but the aircraft started a gradual descent because the autoplt could not trim fast enough for the airspeed change. We lost 300 ft before I noticed and I immediately corrected back to altitude. No other aircraft were in the area and ATC made no comment.

Original NASA ASRS Text

Title: CAPT OF AN MD88 DSNDED BELOW ASSIGNED DUE TO AUTOPLT FAILURE TO TRIM ACFT TO HOLD ALT DUE TO THE HIGH RATE OF SLOWING THE ACFT FOR A SPD REDUCTION.

Narrative: ON ARR INTO ATL ON THE ROME ARR WE WERE LEVEL AT 11000 FT AND WE WERE SLOWING FROM 320 KTS TO 210 KIAS. AS THE ACFT WAS SLOWING DOWN AND TRIMMING UP, I WAS DOING AN APCH BRIEFING. I HAD MY COMMERCIAL CHART BOOK IN FRONT OF ME AND WAS BRIEFING THE APCH. I DID NOT NOTICE, BUT THE ACFT STARTED A GRADUAL DSCNT BECAUSE THE AUTOPLT COULD NOT TRIM FAST ENOUGH FOR THE AIRSPD CHANGE. WE LOST 300 FT BEFORE I NOTICED AND I IMMEDIATELY CORRECTED BACK TO ALT. NO OTHER ACFT WERE IN THE AREA AND ATC MADE NO COMMENT.

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.