Piper arrow aircraft transponder mode C erroneous altitude readout. ATC was reading 2500 ft when air...

Attributes
ACN	368150
Time
Date	199705
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : n90 airport : owd
State Reference	NY
Altitude	msl bound lower : 1000 msl bound upper : 2500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : zny
Operator	general aviation : personal
Make Model Name	PA-28 Cherokee Arrow IV
Operating Under FAR Part	Part 91
Flight Phase	climbout : takeoff cruise other landing other
Flight Plan	None
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private
Experience	flight time last 90 days : 20 flight time total : 680 flight time type : 492
ASRS Report	368150
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Departed 2n8, flew north up the hudson river VFR corridor at 1000 ft to owd. Upon arriving at owd I was told to call the radar supervisor. He advised me that the radar showed I was at 2500 ft over the river. Therefore I entered ewr's and lga's airspace. I explained to him I was definitely not over 1000 ft and I had work done to the avionics. He then told me to have my transponder and encoder checked. Upon returning to 2n8 I called lga for a transponder check. I was told at 1000 ft the transponder was reporting 2500 ft. They advised me to turn off mode C and proceed south down the river. On may/xx/97 I took the aircraft to FBO, where they found the encoder to read wrong. In further troubleshooting they found the B2 line from the encoder was inoperative. This causes the transponder to read 2500 ft when the aircraft is actually at 1000 ft. The equipment is now being repaired. I believe a real problem exists with mode C. At any time a malfunction may occur and report erroneous altitudes. The pilot has no way of knowing when this happens, and this may cause a violation. That never really occurred.

Original NASA ASRS Text

Title: PIPER ARROW ACFT XPONDER MODE C ERRONEOUS ALT READOUT. ATC WAS READING 2500 FT WHEN ACFT WAS AT 1000 FT AND ATC THOUGHT RPTR PLT HAD ENTERED CLASS B AIRSPACE.

Narrative: DEPARTED 2N8, FLEW N UP THE HUDSON RIVER VFR CORRIDOR AT 1000 FT TO OWD. UPON ARRIVING AT OWD I WAS TOLD TO CALL THE RADAR SUPVR. HE ADVISED ME THAT THE RADAR SHOWED I WAS AT 2500 FT OVER THE RIVER. THEREFORE I ENTERED EWR'S AND LGA'S AIRSPACE. I EXPLAINED TO HIM I WAS DEFINITELY NOT OVER 1000 FT AND I HAD WORK DONE TO THE AVIONICS. HE THEN TOLD ME TO HAVE MY XPONDER AND ENCODER CHKED. UPON RETURNING TO 2N8 I CALLED LGA FOR A XPONDER CHK. I WAS TOLD AT 1000 FT THE XPONDER WAS RPTING 2500 FT. THEY ADVISED ME TO TURN OFF MODE C AND PROCEED S DOWN THE RIVER. ON MAY/XX/97 I TOOK THE ACFT TO FBO, WHERE THEY FOUND THE ENCODER TO READ WRONG. IN FURTHER TROUBLESHOOTING THEY FOUND THE B2 LINE FROM THE ENCODER WAS INOP. THIS CAUSES THE XPONDER TO READ 2500 FT WHEN THE ACFT IS ACTUALLY AT 1000 FT. THE EQUIP IS NOW BEING REPAIRED. I BELIEVE A REAL PROB EXISTS WITH MODE C. AT ANY TIME A MALFUNCTION MAY OCCUR AND RPT ERRONEOUS ALTS. THE PLT HAS NO WAY OF KNOWING WHEN THIS HAPPENS, AND THIS MAY CAUSE A VIOLATION. THAT NEVER REALLY OCCURRED.

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.

Piper arrow aircraft transponder mode C erroneous altitude readout. ATC was reading 2500 ft when aircraft was at 1000 ft and ATC thought reporter pilot had entered class B airspace.