Aircraft equipment problem. Faulty transponder altitude reporting.

Attributes
ACN	315610
Time
Date	199509
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ttn
State Reference	NJ
Altitude	msl bound lower : 16500 msl bound upper : 17000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Operator	common carrier : air taxi
Make Model Name	Learjet 24
Operating Under FAR Part	Part 135
Flight Phase	descent other
Route In Use	enroute airway : zny
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 6000 flight time type : 3000
ASRS Report	315610
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	none taken : unable other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We had been having trouble with our mode 'C' (both xponders) for several trips. Sometimes the transponder showed 300-500 ft high to ATC, then after adjustments by the mechanic, they would show 300-500 ft low. We were level at 17000 ft with the proper barometer pressure and phl approach control said we were showing 16500 ft. We were told to stop squawking mode 'C' and that was that. I guess the only thing I could do is stop flying for the company until the problem is fixed.

Original NASA ASRS Text

Title: ACFT EQUIP PROB. FAULTY XPONDER ALT RPTING.

Narrative: WE HAD BEEN HAVING TROUBLE WITH OUR MODE 'C' (BOTH XPONDERS) FOR SEVERAL TRIPS. SOMETIMES THE XPONDER SHOWED 300-500 FT HIGH TO ATC, THEN AFTER ADJUSTMENTS BY THE MECH, THEY WOULD SHOW 300-500 FT LOW. WE WERE LEVEL AT 17000 FT WITH THE PROPER BAROMETER PRESSURE AND PHL APCH CTL SAID WE WERE SHOWING 16500 FT. WE WERE TOLD TO STOP SQUAWKING MODE 'C' AND THAT WAS THAT. I GUESS THE ONLY THING I COULD DO IS STOP FLYING FOR THE COMPANY UNTIL THE PROB IS FIXED.

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.