Transponder transmitting erroneous altitude.

Attributes
ACN	300780
Time
Date	199503
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : stl
State Reference	MO
Altitude	msl bound lower : 31000 msl bound upper : 33700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zkc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
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 : 220 flight time total : 14500 flight time type : 5500
ASRS Report	300780
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : unable other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were level at 31000 ft en route to a final altitude of 35000 ft. ATC informed us that there was slow traffic 12 O'clock at 35000 ft. ATC cleared to 'turn left 10 degrees and climb to 33000 ft.' as we leveled at 33000 ft ATC informed us our altitude showed 33700 ft. We informed ATC our altitude was 33000 ft. The traffic did not show up on TCASII. Several times en route to phx, the encoder was reported as intermittent. Upon arriving at phx, the encoder was written up and maintenance was called.

Original NASA ASRS Text

Title: XPONDER XMITTING ERRONEOUS ALT.

Narrative: WE WERE LEVEL AT 31000 FT ENRTE TO A FINAL ALT OF 35000 FT. ATC INFORMED US THAT THERE WAS SLOW TFC 12 O'CLOCK AT 35000 FT. ATC CLRED TO 'TURN L 10 DEGS AND CLB TO 33000 FT.' AS WE LEVELED AT 33000 FT ATC INFORMED US OUR ALT SHOWED 33700 FT. WE INFORMED ATC OUR ALT WAS 33000 FT. THE TFC DID NOT SHOW UP ON TCASII. SEVERAL TIMES ENRTE TO PHX, THE ENCODER WAS RPTED AS INTERMITTENT. UPON ARRIVING AT PHX, THE ENCODER WAS WRITTEN UP AND MAINT WAS CALLED.

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.