Flight crew deviates from assigned altitude.

Attributes
ACN	321175
Time
Date	199511
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : ott
State Reference	MD
Altitude	msl bound lower : 31000 msl bound upper : 31400
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zdc tower : mex
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise 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 : 180 flight time total : 15500 flight time type : 1200
ASRS Report	321175
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : flight engineer pilot : commercial
Experience	flight time last 90 days : 200 flight time total : 8500 flight time type : 4800
ASRS Report	320951
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Passenger flight mco-lga, approaching ott under center control at FL310 with autoplt altitude hold engaged. We were configuring the radios for the nanci arrival to lga, and when I looked back at my altimeter it read 400 ft high (31400 ft). Immediately checked the altitude hold, which was still on, then disengaged the autoplt, and felt the altitude hold switch click off. At the same time, center called to ask if we were level at FL310. The first officer looked at his altimeter, which was reading within the normal difference for an uncorrected altimeter (150 ft high) and replied that we were at FL310. I initiated a slight descent, and turned 'off' the correction to my altimeter, at which time it read the same as the first officer's altimeter (plus 150 ft) and leveled on it at FL310. On the way inbound to lga we attempted to troubleshoot the problem, but when I tried to apply correction to my altimeter, it began a slow climb by itself. We suspect, based on prior experience, that this was a failure of the captain's air data computer which, among other things, corrects the captain's altimeter and determines 'mode C' altitude readout. We wrote it up as such upon arrival in lga and turned it over to maintenance for correction.

Original NASA ASRS Text

Title: FLC DEVIATES FROM ASSIGNED ALT.

Narrative: PAX FLT MCO-LGA, APCHING OTT UNDER CTR CTL AT FL310 WITH AUTOPLT ALT HOLD ENGAGED. WE WERE CONFIGURING THE RADIOS FOR THE NANCI ARR TO LGA, AND WHEN I LOOKED BACK AT MY ALTIMETER IT READ 400 FT HIGH (31400 FT). IMMEDIATELY CHKED THE ALT HOLD, WHICH WAS STILL ON, THEN DISENGAGED THE AUTOPLT, AND FELT THE ALT HOLD SWITCH CLICK OFF. AT THE SAME TIME, CTR CALLED TO ASK IF WE WERE LEVEL AT FL310. THE FO LOOKED AT HIS ALTIMETER, WHICH WAS READING WITHIN THE NORMAL DIFFERENCE FOR AN UNCORRECTED ALTIMETER (150 FT HIGH) AND REPLIED THAT WE WERE AT FL310. I INITIATED A SLIGHT DSCNT, AND TURNED 'OFF' THE CORRECTION TO MY ALTIMETER, AT WHICH TIME IT READ THE SAME AS THE FO'S ALTIMETER (PLUS 150 FT) AND LEVELED ON IT AT FL310. ON THE WAY INBOUND TO LGA WE ATTEMPTED TO TROUBLESHOOT THE PROB, BUT WHEN I TRIED TO APPLY CORRECTION TO MY ALTIMETER, IT BEGAN A SLOW CLB BY ITSELF. WE SUSPECT, BASED ON PRIOR EXPERIENCE, THAT THIS WAS A FAILURE OF THE CAPT'S AIR DATA COMPUTER WHICH, AMONG OTHER THINGS, CORRECTS THE CAPT'S ALTIMETER AND DETERMINES 'MODE C' ALT READOUT. WE WROTE IT UP AS SUCH UPON ARR IN LGA AND TURNED IT OVER TO MAINT FOR CORRECTION.

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.