A saab 340 flight crew changed to the #2 transponder to correct the altitude readout at ZFW.

Attributes
ACN	436742
Time
Date	199905
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : mlc.vortac
State Reference	OK
Altitude	msl single value : 19000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zfw.artcc
Operator	common carrier : air carrier
Make Model Name	SF 340A
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 12000 flight time type : 2000
ASRS Report	436742
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial
Experience	flight time last 90 days : 100 flight time total : 2400 flight time type : 800
ASRS Report	436734
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera
Resolutory Action	aircraft : equipment problem dissipated flight crew : overcame equipment problem
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

During cruise flight, ATC advised transponder indicated 600 ft over assigned altitude. We doublechked and confirmed we were at correct altitude. After manipulating transponder #1 (and getting several altitude readings), we switched to transponder #2 and had no further problems. Entered discrepancy in aircraft log, and maintenance replaced faulty transponder.

Original NASA ASRS Text

Title: A SAAB 340 FLC CHANGED TO THE #2 XPONDER TO CORRECT THE ALT READOUT AT ZFW.

Narrative: DURING CRUISE FLT, ATC ADVISED XPONDER INDICATED 600 FT OVER ASSIGNED ALT. WE DOUBLECHKED AND CONFIRMED WE WERE AT CORRECT ALT. AFTER MANIPULATING XPONDER #1 (AND GETTING SEVERAL ALT READINGS), WE SWITCHED TO XPONDER #2 AND HAD NO FURTHER PROBS. ENTERED DISCREPANCY IN ACFT LOG, AND MAINT REPLACED FAULTY XPONDER.

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.