Air carrier flight crew encounters apparent mode C transponder error.

Attributes
ACN	461561
Time
Date	200001
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : crl.vortac
State Reference	MI
Altitude	msl single value : 31000
Environment
Flight Conditions	VMC
Weather Elements	Turbulence
Light	Dawn
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : commercial pilot : cfi pilot : atp pilot : instrument
Experience	flight time last 90 days : 120 flight time total : 6000 flight time type : 800
ASRS Report	461290
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 220 flight time total : 14500 flight time type : 9000
ASRS Report	461561
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera
Resolutory Action	controller : issued advisory flight crew : overcame equipment problem
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

The center was advised that both ATC xponders were going to be recycled (selected off and on, and in this case, circuit breakers for ATC #1 and ATC #2 pulled and reset). This procedure did not correct the altitude reporting error as confirmed by ZOB and later ZAU. The altitude reporting mode was selected off to prevent ATC conflicts. Dispatch and maintenance were notified. No additional recommendations were available from maintenance and the aircraft was operated for the remainder of the flight to msp without altitude reporting. ATC was advised of change in operational status. Mode C transponder and captain's electric servo altimeter replaced to correct altitude reporting error. Supplemental information from acn 461290: ZOB questioned our assigned altitude of FL310. We confirmed we were at FL310. In response to his question, we asked ZOB if they showed something different. ZOB answered yes -- we were indicating FL315. We verified that we were indeed at FL310 with all altimeters set to 29.92. After several attempts at troubleshooting the problem, ZOB still had us indicating FL315. We were then given a block altitude FL310-FL330. Our company dispatch and maintenance were informed of our situation. To our knowledge, no conflict had occurred.

Original NASA ASRS Text

Title: ACR FLC ENCOUNTERS APPARENT MODE C XPONDER ERROR.

Narrative: THE CTR WAS ADVISED THAT BOTH ATC XPONDERS WERE GOING TO BE RECYCLED (SELECTED OFF AND ON, AND IN THIS CASE, CIRCUIT BREAKERS FOR ATC #1 AND ATC #2 PULLED AND RESET). THIS PROC DID NOT CORRECT THE ALT RPTING ERROR AS CONFIRMED BY ZOB AND LATER ZAU. THE ALT RPTING MODE WAS SELECTED OFF TO PREVENT ATC CONFLICTS. DISPATCH AND MAINT WERE NOTIFIED. NO ADDITIONAL RECOMMENDATIONS WERE AVAILABLE FROM MAINT AND THE ACFT WAS OPERATED FOR THE REMAINDER OF THE FLT TO MSP WITHOUT ALT RPTING. ATC WAS ADVISED OF CHANGE IN OPERATIONAL STATUS. MODE C XPONDER AND CAPT'S ELECTRIC SERVO ALTIMETER REPLACED TO CORRECT ALT RPTING ERROR. SUPPLEMENTAL INFO FROM ACN 461290: ZOB QUESTIONED OUR ASSIGNED ALT OF FL310. WE CONFIRMED WE WERE AT FL310. IN RESPONSE TO HIS QUESTION, WE ASKED ZOB IF THEY SHOWED SOMETHING DIFFERENT. ZOB ANSWERED YES -- WE WERE INDICATING FL315. WE VERIFIED THAT WE WERE INDEED AT FL310 WITH ALL ALTIMETERS SET TO 29.92. AFTER SEVERAL ATTEMPTS AT TROUBLESHOOTING THE PROB, ZOB STILL HAD US INDICATING FL315. WE WERE THEN GIVEN A BLOCK ALT FL310-FL330. OUR COMPANY DISPATCH AND MAINT WERE INFORMED OF OUR SIT. TO OUR KNOWLEDGE, NO CONFLICT HAD OCCURRED.

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.