Medium large transport on automatic controls given speed reduction and heading change. Automatic controls extend slats, causing descent below assigned.

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Attributes
ACN	174957
Time
Date	199104
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : cvg
State Reference	OH
Altitude	msl bound lower : 6700 msl bound upper : 7000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : cvg tower : pbi
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 11000 flight time type : 1100
ASRS Report	174957
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Descending on autoplt and autothrottles from 8000 to 7000' MSL, fairly gentle descent rate. Five speed reduction from about 210 KTS to 180 KTS, and a heading change at same time. Aircraft in 250 degree bank, about 300' above level off, when 'slats extend' (medium large transport) called for and selected. Aircraft descended slowly to as low as 6725' (275' low). As I (PNF) pulled back on yoke to override autoplt (no disconnect occurred, but that was because I was pulling only hard enough to assist the autoplt back to 7000'--any harder would have disconnected the autoplt with a jolt), the altitude warning went off (as advertised). I have seen this event occur at least 7 times now (over a 1 yr span), and have been trying to get the word out to the 'pwrs that be' that the medium large transport autoplt often cannot handle a configuring, turning level-off. You do not have to be slam-dunking for this to happen! I would have caught the bust under normal circumstances, but we were simultaneously attempting to steer clear of a red radar return and got fixated on the screen.

Original NASA ASRS Text

Title: MLG ON AUTO CTLS GIVEN SPEED REDUCTION AND HEADING CHANGE. AUTO CTLS EXTEND SLATS, CAUSING DESCENT BELOW ASSIGNED.

Narrative: DSNDING ON AUTOPLT AND AUTOTHROTTLES FROM 8000 TO 7000' MSL, FAIRLY GENTLE DSNT RATE. FIVE SPD REDUCTION FROM ABOUT 210 KTS TO 180 KTS, AND A HDG CHANGE AT SAME TIME. ACFT IN 250 DEG BANK, ABOUT 300' ABOVE LEVEL OFF, WHEN 'SLATS EXTEND' (MLG) CALLED FOR AND SELECTED. ACFT DSNDED SLOWLY TO AS LOW AS 6725' (275' LOW). AS I (PNF) PULLED BACK ON YOKE TO OVERRIDE AUTOPLT (NO DISCONNECT OCCURRED, BUT THAT WAS BECAUSE I WAS PULLING ONLY HARD ENOUGH TO ASSIST THE AUTOPLT BACK TO 7000'--ANY HARDER WOULD HAVE DISCONNECTED THE AUTOPLT WITH A JOLT), THE ALT WARNING WENT OFF (AS ADVERTISED). I HAVE SEEN THIS EVENT OCCUR AT LEAST 7 TIMES NOW (OVER A 1 YR SPAN), AND HAVE BEEN TRYING TO GET THE WORD OUT TO THE 'PWRS THAT BE' THAT THE MLG AUTOPLT OFTEN CANNOT HANDLE A CONFIGURING, TURNING LEVEL-OFF. YOU DO NOT HAVE TO BE SLAM-DUNKING FOR THIS TO HAPPEN! I WOULD HAVE CAUGHT THE BUST UNDER NORMAL CIRCUMSTANCES, BUT WE WERE SIMULTANEOUSLY ATTEMPTING TO STEER CLR OF A RED RADAR RETURN AND GOT FIXATED ON THE SCREEN.

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.