Altitude of air carrier medium large transport is questioned by ARTCC because transponder was indicating off cruise altitude. Switched transponder correcting the problem. Aircraft logbook write up for corrective action.

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Attributes
ACN	187581
Time
Date	199108
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : ewc
State Reference	PA
Altitude	msl bound lower : 34500 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute airway : j64
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi pilot : flight engineer
Experience	flight time last 90 days : 210 flight time total : 11000 flight time type : 2900
ASRS Report	187581
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 152 flight time total : 10000 flight time type : 3000
ASRS Report	187430
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While in cruise at FL350 cle center inquired about our altitude. We stated level 350. Center altitude readout was 34.5, 500 ft low. We switched to #2 transponder and altitude reporting. Center advised we were now showing 350, with fluctuations of 100 ft to 200 ft. Company maintenance was advised, item entered into aircraft log. We took no action. 3 altimeters read FL350. This was an equipment failure which could have triggered a TCASII RA.

Original NASA ASRS Text

Title: ALT OF ACR MLG IS QUESTIONED BY ARTCC BECAUSE TRANSPONDER WAS INDICATING OFF CRUISE ALT. SWITCHED TRANSPONDER CORRECTING THE PROBLEM. ACFT LOGBOOK WRITE UP FOR CORRECTIVE ACTION.

Narrative: WHILE IN CRUISE AT FL350 CLE CENTER INQUIRED ABOUT OUR ALT. WE STATED LEVEL 350. CENTER ALT READOUT WAS 34.5, 500 FT LOW. WE SWITCHED TO #2 TRANSPONDER AND ALT RPTING. CENTER ADVISED WE WERE NOW SHOWING 350, WITH FLUCTUATIONS OF 100 FT TO 200 FT. COMPANY MAINT WAS ADVISED, ITEM ENTERED INTO ACFT LOG. WE TOOK NO ACTION. 3 ALTIMETERS READ FL350. THIS WAS AN EQUIP FAILURE WHICH COULD HAVE TRIGGERED A TCASII RA.

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.