ARTCC controller advised crew of medium large transport that radar showed them 500 ft high during cruise. Subsequently, radar showed them on altitude after crew had switched to #2 transponder.

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Attributes
ACN	502980
Time
Date	200102
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : jhw.vor
State Reference	NY
Altitude	msl bound lower : 31000 msl bound upper : 31500
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	Regional Jet CL65, Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	enroute airway : j29.airway
Flight Plan	IFR
Person 1
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial pilot : cfi
Experience	flight time last 90 days : 240 flight time total : 4120 flight time type : 1920
ASRS Report	502980
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	atc equipment other atc equipment : radar aircraft equipment other aircraft equipment : transponder/tcas other controllera
Resolutory Action	aircraft : equipment problem dissipated controller : issued alert
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	ATC Facility Aircraft
Primary Problem	Aircraft

Narrative:

We had been in level cruising flight for quite some time with the autoplt on and all indications showing FL310. Then ATC called and asked us to verify our altitude. I reported level at FL310. ATC said our radar tag showed 500 ft high. We doublechked our equipment, verified all 3 altimeters were set to 29.92 and switched to our #2 transponder. Both xponders were showing us at FL310 on our TCASII screen. I called ATC to ask if radar still showed us 500 ft high and ATC said that we now indicated FL310. The controller added that it was probably just a transponder or radar glitch as we passed directly below another aircraft.

Original NASA ASRS Text

Title: ARTCC CTLR ADVISED CREW OF MLG THAT RADAR SHOWED THEM 500 FT HIGH DURING CRUISE. SUBSEQUENTLY, RADAR SHOWED THEM ON ALT AFTER CREW HAD SWITCHED TO #2 XPONDER.

Narrative: WE HAD BEEN IN LEVEL CRUISING FLT FOR QUITE SOME TIME WITH THE AUTOPLT ON AND ALL INDICATIONS SHOWING FL310. THEN ATC CALLED AND ASKED US TO VERIFY OUR ALT. I RPTED LEVEL AT FL310. ATC SAID OUR RADAR TAG SHOWED 500 FT HIGH. WE DOUBLECHKED OUR EQUIP, VERIFIED ALL 3 ALTIMETERS WERE SET TO 29.92 AND SWITCHED TO OUR #2 XPONDER. BOTH XPONDERS WERE SHOWING US AT FL310 ON OUR TCASII SCREEN. I CALLED ATC TO ASK IF RADAR STILL SHOWED US 500 FT HIGH AND ATC SAID THAT WE NOW INDICATED FL310. THE CTLR ADDED THAT IT WAS PROBABLY JUST A XPONDER OR RADAR GLITCH AS WE PASSED DIRECTLY BELOW ANOTHER ACFT.

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.