Mode C displayed aircraft as 600 ft higher than ATC assigned altitude.

Attributes
ACN	309800
Time
Date	199507
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : djb
State Reference	OH
Altitude	msl bound lower : 12000 msl bound upper : 12600
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob tower : bhm
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
Route In Use	enroute airway : zob
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 300 flight time total : 11000 flight time type : 3500
ASRS Report	309800
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude 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:

On jul/fri/95 at approximately XA30Z at 12000 ft, ATC advised that our altitude showed 600 ft higher than our actual altitude. I suspected a fault with our mode C transponder and switched to backup transponder. No further deviation in mode C readout occurred. After an uneventful landing at ind, maintenance was notified and took appropriate corrective action.

Original NASA ASRS Text

Title: MODE C DISPLAYED ACFT AS 600 FT HIGHER THAN ATC ASSIGNED ALT.

Narrative: ON JUL/FRI/95 AT APPROX XA30Z AT 12000 FT, ATC ADVISED THAT OUR ALT SHOWED 600 FT HIGHER THAN OUR ACTUAL ALT. I SUSPECTED A FAULT WITH OUR MODE C XPONDER AND SWITCHED TO BACKUP XPONDER. NO FURTHER DEV IN MODE C READOUT OCCURRED. AFTER AN UNEVENTFUL LNDG AT IND, MAINT WAS NOTIFIED AND TOOK APPROPRIATE CORRECTIVE ACTION.

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.