Aircraft equipment problem -- air carrier transponder reads 700 ft high.

Attributes
ACN	316973
Time
Date	199509
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : iiu
State Reference	KY
Altitude	msl bound lower : 29000 msl bound upper : 29700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	common carrier : air carrier
Make Model Name	B737-300
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 : 75 flight time total : 11250 flight time type : 2000
ASRS Report	316973
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 14000 flight time type : 3200
ASRS Report	316853
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While climbing to cruise altitude, ATC instructed to level off at FL290. Later ZTL asked us to confirm our altitude. We confirmed level at FL290. ATC stated that they showed 29700 ft. We changed xponders and ATC then stated they showed us at FL290. The nearest traffic was 15 NM. I do not know if this was a problem with our transponder or another ATC problem with their system. Our transponder checked ok later. No other comments or actions were taken by ATC.

Original NASA ASRS Text

Title: ACFT EQUIP PROB -- ACR XPONDER READS 700 FT HIGH.

Narrative: WHILE CLBING TO CRUISE ALT, ATC INSTRUCTED TO LEVEL OFF AT FL290. LATER ZTL ASKED US TO CONFIRM OUR ALT. WE CONFIRMED LEVEL AT FL290. ATC STATED THAT THEY SHOWED 29700 FT. WE CHANGED XPONDERS AND ATC THEN STATED THEY SHOWED US AT FL290. THE NEAREST TFC WAS 15 NM. I DO NOT KNOW IF THIS WAS A PROB WITH OUR XPONDER OR ANOTHER ATC PROB WITH THEIR SYS. OUR XPONDER CHKED OK LATER. NO OTHER COMMENTS OR ACTIONS WERE TAKEN BY ATC.

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.