Mode C showing altitude deviation at the center.

Attributes
ACN	163437
Time
Date	199011
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : cic
State Reference	CA
Altitude	msl bound lower : 7000 msl bound upper : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa tower : hsv
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 160 flight time total : 3650
ASRS Report	163437
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera
Resolutory Action	aircraft : equipment problem dissipated
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While en route from chico to san francisco, approximately 25 mi southwest of the chico VOR, we were told by ZOA on frequency 132.2 to maintain 7000'. While level at 7000' the center controller asked us to verify our altitude. I replied 7000'. After a few mins, the controller urgently asked us to verify our altitude. I again replied 7000'. He then stated that our mode C readout was 7200' and that there was VFR traffic at 7500' ahead of us and conflicting. I then switched to our second transponder and asked center to verify our altitude. After a few mins, center told us to switch back to our other transponder as he was not receiving our second one. We switched back to our first transponder and asked the next center controller on 124.85 to verify our mode C readout. The controller on 125.85 stated our readout was fine. Note: during the time of the incident we were always at 7000'. We also sighted the traffic and was approximately 500' below it and about 1/2 mi away from it.

Original NASA ASRS Text

Title: MODE C SHOWING ALT DEVIATION AT THE CTR.

Narrative: WHILE ENRTE FROM CHICO TO SAN FRANCISCO, APPROX 25 MI SW OF THE CHICO VOR, WE WERE TOLD BY ZOA ON FREQ 132.2 TO MAINTAIN 7000'. WHILE LEVEL AT 7000' THE CTR CTLR ASKED US TO VERIFY OUR ALT. I REPLIED 7000'. AFTER A FEW MINS, THE CTLR URGENTLY ASKED US TO VERIFY OUR ALT. I AGAIN REPLIED 7000'. HE THEN STATED THAT OUR MODE C READOUT WAS 7200' AND THAT THERE WAS VFR TFC AT 7500' AHEAD OF US AND CONFLICTING. I THEN SWITCHED TO OUR SEC XPONDER AND ASKED CTR TO VERIFY OUR ALT. AFTER A FEW MINS, CTR TOLD US TO SWITCH BACK TO OUR OTHER XPONDER AS HE WAS NOT RECEIVING OUR SEC ONE. WE SWITCHED BACK TO OUR FIRST XPONDER AND ASKED THE NEXT CTR CTLR ON 124.85 TO VERIFY OUR MODE C READOUT. THE CTLR ON 125.85 STATED OUR READOUT WAS FINE. NOTE: DURING THE TIME OF THE INCIDENT WE WERE ALWAYS AT 7000'. WE ALSO SIGHTED THE TFC AND WAS APPROX 500' BELOW IT AND ABOUT 1/2 MI AWAY FROM IT.

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.