MD80 crew had faulty mode C readout on ATC radar.

Attributes
ACN	502300
Time
Date	200102
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : stl.vortac
State Reference	MO
Altitude	msl bound lower : 8500 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : t75.tracon
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : intermediate altitude
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 5000 flight time type : 100
ASRS Report	502300
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	other anomaly other
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	other
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

On a descent to 9000 ft, ATC queried our altitude saying they were showing 8500 ft. We were level at 9000 ft. We recycled our transponder to #2 and ATC said that we lost altitude reporting. We recycled back to #1 transponder and ATC said nothing more. The captain any myself were puzzled by the inquiry. The flight continued uneventfully to a safe landing.

Original NASA ASRS Text

Title: MD80 CREW HAD FAULTY MODE C READOUT ON ATC RADAR.

Narrative: ON A DSCNT TO 9000 FT, ATC QUERIED OUR ALT SAYING THEY WERE SHOWING 8500 FT. WE WERE LEVEL AT 9000 FT. WE RECYCLED OUR XPONDER TO #2 AND ATC SAID THAT WE LOST ALT RPTING. WE RECYCLED BACK TO #1 XPONDER AND ATC SAID NOTHING MORE. THE CAPT ANY MYSELF WERE PUZZLED BY THE INQUIRY. THE FLT CONTINUED UNEVENTFULLY TO A SAFE LNDG.

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.