B737 crew had altitude deviation.

Attributes
ACN	461415
Time
Date	200001
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : petty
State Reference	WI
Altitude	msl bound lower : 18640 msl bound upper : 19000
Environment
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	461415
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : commercial
ASRS Report	461480
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : company policies
Independent Detector	aircraft equipment other aircraft equipment : alt.warning
Resolutory Action	flight crew : returned to assigned altitude
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Autoplt malfunction. After leveling at FL190, I engaged the autoplt in the VNAV mode. I looked down to figure our landing time and then heard the altitude alert. I looked up as we were beginning a rapid descent and disengaged the autoplt at FL187. We descended to FL186.4 before I began a manual climb back to FL190. ZAU made no comment and I re-engaged the autoplt and tried to figure what caused the descent. Both the first officer and myself could not come up with any operational error to cause this. I am certain the 'a' autoplt engaged in command mode and in VNAV path. Later during approach maneuvering the autoplt reverted to control wheel steering during a heading change so I turned it off for the rest of the flight. Dtw maintenance was informed but I do not know the outcome of any checks made. This drives home the point that all autoplts must be constantly monitored during flight.

Original NASA ASRS Text

Title: B737 CREW HAD ALTDEV.

Narrative: AUTOPLT MALFUNCTION. AFTER LEVELING AT FL190, I ENGAGED THE AUTOPLT IN THE VNAV MODE. I LOOKED DOWN TO FIGURE OUR LNDG TIME AND THEN HEARD THE ALT ALERT. I LOOKED UP AS WE WERE BEGINNING A RAPID DSCNT AND DISENGAGED THE AUTOPLT AT FL187. WE DSNDED TO FL186.4 BEFORE I BEGAN A MANUAL CLB BACK TO FL190. ZAU MADE NO COMMENT AND I RE-ENGAGED THE AUTOPLT AND TRIED TO FIGURE WHAT CAUSED THE DSCNT. BOTH THE FO AND MYSELF COULD NOT COME UP WITH ANY OPERROR TO CAUSE THIS. I AM CERTAIN THE 'A' AUTOPLT ENGAGED IN COMMAND MODE AND IN VNAV PATH. LATER DURING APCH MANEUVERING THE AUTOPLT REVERTED TO CTL WHEEL STEERING DURING A HDG CHANGE SO I TURNED IT OFF FOR THE REST OF THE FLT. DTW MAINT WAS INFORMED BUT I DO NOT KNOW THE OUTCOME OF ANY CHKS MADE. THIS DRIVES HOME THE POINT THAT ALL AUTOPLTS MUST BE CONSTANTLY MONITORED DURING FLT.

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.