Altitude deviation altitude overshot during descent procedure on autoplt.

Attributes
ACN	220363
Time
Date	199209
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : atl
State Reference	GA
Altitude	msl bound lower : 9600 msl bound upper : 10000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : atl
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other descent 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 : 180 flight time total : 18000 flight time type : 250
ASRS Report	220363
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Events
Anomaly	altitude deviation : overshoot inflight encounter : weather non adherence : clearance other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Avoiding thunderstorms on arrival into atl we were cleared by approach to turn right a maximum of 175 degree heading and descend to 10000 ft. We penetrated a cumulu nimbus with moderate turbulence in the turn and descent profile. During this sequence, the autoplt disengaged from cmd (command) to cws (control wheel steering) position. Upon realization that we weren't leveling off at 10000 ft, disengaged the autoplt completely and leveled off at 9600 ft. I started to recover to 10000 and approach notified us of our restriction and then cleared us to 9000 ft. Cause: assume an automatic malfunction. Prevention: a C10R (report) being submitted to the company prevention. No logbook write up at that time because of the confusion of the events. The widebody transport manual states that altitude protection is still protected in cws. The first officer and I, as a habit, verbally confirm on an altitude change that the annunciator panel 'altitude alarm' is illuminated. We both felt strongly that in this instance, it was armed. I surmise that the autoplt dropped to the cws position during the altitude capture mode and this disruption cancelled the level off protection. I can find nothing in the prm addressing this situation. Note: an audio warning of the autoplt dropping from cmd to cws position possibly would alert a flight crew of a potential hazardous situation.

Original NASA ASRS Text

Title: ALTDEV ALT OVERSHOT DURING DSCNT PROC ON AUTOPLT.

Narrative: AVOIDING TSTMS ON ARR INTO ATL WE WERE CLRED BY APCH TO TURN R A MAX OF 175 DEG HDG AND DSND TO 10000 FT. WE PENETRATED A CUMULU NIMBUS WITH MODERATE TURB IN THE TURN AND DSCNT PROFILE. DURING THIS SEQUENCE, THE AUTOPLT DISENGAGED FROM CMD (COMMAND) TO CWS (CTL WHEEL STEERING) POS. UPON REALIZATION THAT WE WEREN'T LEVELING OFF AT 10000 FT, DISENGAGED THE AUTOPLT COMPLETELY AND LEVELED OFF AT 9600 FT. I STARTED TO RECOVER TO 10000 AND APCH NOTIFIED US OF OUR RESTRICTION AND THEN CLRED US TO 9000 FT. CAUSE: ASSUME AN AUTO MALFUNCTION. PREVENTION: A C10R (RPT) BEING SUBMITTED TO THE COMPANY PREVENTION. NO LOGBOOK WRITE UP AT THAT TIME BECAUSE OF THE CONFUSION OF THE EVENTS. THE WDB MANUAL STATES THAT ALT PROTECTION IS STILL PROTECTED IN CWS. THE FO AND I, AS A HABIT, VERBALLY CONFIRM ON AN ALT CHANGE THAT THE ANNUNCIATOR PANEL 'ALT ALARM' IS ILLUMINATED. WE BOTH FELT STRONGLY THAT IN THIS INSTANCE, IT WAS ARMED. I SURMISE THAT THE AUTOPLT DROPPED TO THE CWS POS DURING THE ALT CAPTURE MODE AND THIS DISRUPTION CANCELLED THE LEVEL OFF PROTECTION. I CAN FIND NOTHING IN THE PRM ADDRESSING THIS SITUATION. NOTE: AN AUDIO WARNING OF THE AUTOPLT DROPPING FROM CMD TO CWS POS POSSIBLY WOULD ALERT A FLC OF A POTENTIAL HAZARDOUS SITUATION.

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.