Aircraft position altitude - air carrier transponder read lower than actual altitude.

Attributes
ACN	299110
Time
Date	199503
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : hch
State Reference	TN
Altitude	msl bound lower : 24000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : ztl
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 : j89
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 11000 flight time type : 3000
ASRS Report	299110
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 6000 flight time type : 2700
ASRS Report	298737
Events
Anomaly	aircraft equipment problem : less severe 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:

Approximately 10-12 mins after level off at FL310 the center controller informed us that there was no conflict and that the mode 'C' readout on us showed FL306. We told him we were switching xponders and promptly did so. After switching, the controller said he showed us at FL310. A few mins later we were given a descent to FL240 and switched to the next controller. After level off at FL240 we switched back to #2 transponder and asked the controller what our mode 'C' indicated. Initially he said it showed 100 ft low. As we continued the descent, subsequent readouts at intermediate level offs reflected accurate altitude readings. No further problems were encountered.

Original NASA ASRS Text

Title: ACFT POS ALT - ACR XPONDER READ LOWER THAN ACTUAL ALT.

Narrative: APPROX 10-12 MINS AFTER LEVEL OFF AT FL310 THE CTR CTLR INFORMED US THAT THERE WAS NO CONFLICT AND THAT THE MODE 'C' READOUT ON US SHOWED FL306. WE TOLD HIM WE WERE SWITCHING XPONDERS AND PROMPTLY DID SO. AFTER SWITCHING, THE CTLR SAID HE SHOWED US AT FL310. A FEW MINS LATER WE WERE GIVEN A DSCNT TO FL240 AND SWITCHED TO THE NEXT CTLR. AFTER LEVEL OFF AT FL240 WE SWITCHED BACK TO #2 XPONDER AND ASKED THE CTLR WHAT OUR MODE 'C' INDICATED. INITIALLY HE SAID IT SHOWED 100 FT LOW. AS WE CONTINUED THE DSCNT, SUBSEQUENT READOUTS AT INTERMEDIATE LEVEL OFFS REFLECTED ACCURATE ALT READINGS. NO FURTHER PROBS WERE ENCOUNTERED.

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.