Bad mode C readout from air carrier medium large transport.

Attributes
ACN	251059
Time
Date	199309
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : ard
State Reference	PA
Altitude	msl bound lower : 11000 msl bound upper : 13000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Route In Use	arrival other arrival star : star enroute airway : zny
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : commercial pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 310 flight time total : 12255 flight time type : 4300
ASRS Report	251059
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 10000 flight time type : 2500
ASRS Report	250751
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While on the arrival into lga we received a clearance to cross somto at 11000 ft MSL. Shortly after crossing somto at 11000 ft the controller asked us what our altitude was. We were at 11000 ft but his scope was indicating 13000 ft. When we switched to the #2 transponder his readout changed and agreed with our altitude. We wrote the transponder up with maintenance upon landing in lga.

Original NASA ASRS Text

Title: BAD MODE C READOUT FROM ACR MLG.

Narrative: WHILE ON THE ARR INTO LGA WE RECEIVED A CLRNC TO CROSS SOMTO AT 11000 FT MSL. SHORTLY AFTER XING SOMTO AT 11000 FT THE CTLR ASKED US WHAT OUR ALT WAS. WE WERE AT 11000 FT BUT HIS SCOPE WAS INDICATING 13000 FT. WHEN WE SWITCHED TO THE #2 XPONDER HIS READOUT CHANGED AND AGREED WITH OUR ALT. WE WROTE THE XPONDER UP WITH MAINT UPON LNDG IN LGA.

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.