Altitude deviation altitude overshot in descent. Non adherence to ATC clearance.

Attributes
ACN	305970
Time
Date	199505
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	msl bound lower : 8800 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 8000 flight time type : 7000
ASRS Report	305970
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
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:

The FMC was all loaded correctly with the proper data to cross rhome intersection at 9000 ft at 250 KTS. The altitude capture window was also set at 9000 ft. The altimeters and all checklists were completed and we were well ahead of the aircraft. Upon reaching about 9200 ft the VNAV switched over to VNAV path indicating a leveloff. However, I (the PF) was watching the vsi at 9000 ft and saw a 200-300 FPM descent. At 8800 ft I assumed the aircraft was continuing its descent to an unknown altitude for an unknown reason and disconnected the autoplt and manually climbed back to 9000 ft. I'm not sure if a deviation is even considered, but I felt an event like this should be brought to attention, because we were (are) taught that no matter the altitude in the FMC, it will not descend below the setting of the automatic flight director system panel altitude setting on a B-757.

Original NASA ASRS Text

Title: ALTDEV ALT OVERSHOT IN DSCNT. NON ADHERENCE TO ATC CLRNC.

Narrative: THE FMC WAS ALL LOADED CORRECTLY WITH THE PROPER DATA TO CROSS RHOME INTXN AT 9000 FT AT 250 KTS. THE ALT CAPTURE WINDOW WAS ALSO SET AT 9000 FT. THE ALTIMETERS AND ALL CHKLISTS WERE COMPLETED AND WE WERE WELL AHEAD OF THE ACFT. UPON REACHING ABOUT 9200 FT THE VNAV SWITCHED OVER TO VNAV PATH INDICATING A LEVELOFF. HOWEVER, I (THE PF) WAS WATCHING THE VSI AT 9000 FT AND SAW A 200-300 FPM DSCNT. AT 8800 FT I ASSUMED THE ACFT WAS CONTINUING ITS DSCNT TO AN UNKNOWN ALT FOR AN UNKNOWN REASON AND DISCONNECTED THE AUTOPLT AND MANUALLY CLBED BACK TO 9000 FT. I'M NOT SURE IF A DEV IS EVEN CONSIDERED, BUT I FELT AN EVENT LIKE THIS SHOULD BE BROUGHT TO ATTN, BECAUSE WE WERE (ARE) TAUGHT THAT NO MATTER THE ALT IN THE FMC, IT WILL NOT DSND BELOW THE SETTING OF THE AUTO FLT DIRECTOR SYS PANEL ALT SETTING ON A B-757.

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.