Flight crew of a B727 found that the #1 transponder was giving wrong altitude information to ATC after ARTCC advised that their altitude indicated that they were 500 ft high during cruise.

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Attributes
ACN	373012
Time
Date	199706
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : bna
State Reference	TN
Altitude	msl bound lower : 31000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	other other : other pilot : flight engineer pilot : commercial pilot : atp
Experience	flight time last 90 days : 210 flight time total : 37000 flight time type : 19800
ASRS Report	373012
Person 2
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : instrument pilot : flight engineer pilot : private pilot : commercial pilot : atp
Experience	flight time last 90 days : 130 flight time total : 4000 flight time type : 1500
ASRS Report	372600
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

At altitude in cruise, ATC asked our altitude which I replied FL310. ATC said our transponder showed FL315. I switched to #2 transponder which ATC said showed FL310. For troubleshooting we went back to #1 transponder. ATC indicated the altitude readout was intermittent. Shortly after the circuit breaker popped on #1 transponder, so continued on #2 transponder.

Original NASA ASRS Text

Title: FLC OF A B727 FOUND THAT THE #1 XPONDER WAS GIVING WRONG ALT INFO TO ATC AFTER ARTCC ADVISED THAT THEIR ALT INDICATED THAT THEY WERE 500 FT HIGH DURING CRUISE.

Narrative: AT ALT IN CRUISE, ATC ASKED OUR ALT WHICH I REPLIED FL310. ATC SAID OUR XPONDER SHOWED FL315. I SWITCHED TO #2 XPONDER WHICH ATC SAID SHOWED FL310. FOR TROUBLESHOOTING WE WENT BACK TO #1 XPONDER. ATC INDICATED THE ALT READOUT WAS INTERMITTENT. SHORTLY AFTER THE CIRCUIT BREAKER POPPED ON #1 XPONDER, SO CONTINUED ON #2 XPONDER.

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.