ATC advised pilot of an small transport twin of altdevs indicated by the aircraft's transponder retu...

Attributes
ACN	307930
Time
Date	199506
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : yqk
State Reference	ON
Altitude	msl bound lower : 7000 msl bound upper : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : czwg artcc : zmp tracon : dlh
Operator	general aviation : personal
Make Model Name	PA-23-250 Aztec
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Route In Use	enroute airway : zmp
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 30 flight time total : 2000 flight time type : 1000
ASRS Report	307930
Person 2
Affiliation	government other
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	other controllera
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Instrument flight plan filed from madison, wi, to duluth, mn, with second leg of trip from duluth, mn to kenora, ontario, canada. Primary transponder had been giving erroneous altitude reports periodically during the flight. Madison approach and ZAU received accurate reports. ZMP and duluth approach received inaccurate reports, changed to standby transponder when this event occurred. I employed the use of the primary transponder when establishing contact with winnipeg center. Upon descent into destination airport (yok), winnipeg advised that altitude deviation existed, immediately changed to standby transponder for balance of trip and the return trip 4 days later. Primary primary transponder has been repaired. '2 decoders for mode C were inoperable.' callback conversation with reporter revealed the following: reporter stated that he was not further contacted by either the foreign government or the FAA in regard to the possible altitude deviation. As he indicated, the encoding transponder was malfunctioning and not always giving a correct altitude readout. He was operating a piper PA23-250 aztec aircraft with an older king transponder with encoding altimeter readout.

Original NASA ASRS Text

Title: ATC ADVISED PLT OF AN SMT TWIN OF ALTDEVS INDICATED BY THE ACFT'S XPONDER RETURN.

Narrative: INST FLT PLAN FILED FROM MADISON, WI, TO DULUTH, MN, WITH SECOND LEG OF TRIP FROM DULUTH, MN TO KENORA, ONTARIO, CANADA. PRIMARY XPONDER HAD BEEN GIVING ERRONEOUS ALT RPTS PERIODICALLY DURING THE FLT. MADISON APCH AND ZAU RECEIVED ACCURATE RPTS. ZMP AND DULUTH APCH RECEIVED INACCURATE RPTS, CHANGED TO STANDBY XPONDER WHEN THIS EVENT OCCURRED. I EMPLOYED THE USE OF THE PRIMARY XPONDER WHEN ESTABLISHING CONTACT WITH WINNIPEG CTR. UPON DSCNT INTO DEST ARPT (YOK), WINNIPEG ADVISED THAT ALTDEV EXISTED, IMMEDIATELY CHANGED TO STANDBY XPONDER FOR BALANCE OF TRIP AND THE RETURN TRIP 4 DAYS LATER. PRIMARY PRIMARY XPONDER HAS BEEN REPAIRED. '2 DECODERS FOR MODE C WERE INOPERABLE.' CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR STATED THAT HE WAS NOT FURTHER CONTACTED BY EITHER THE FOREIGN GOV OR THE FAA IN REGARD TO THE POSSIBLE ALTDEV. AS HE INDICATED, THE ENCODING XPONDER WAS MALFUNCTIONING AND NOT ALWAYS GIVING A CORRECT ALT READOUT. HE WAS OPERATING A PIPER PA23-250 AZTEC ACFT WITH AN OLDER KING XPONDER WITH ENCODING ALTIMETER READOUT.

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.

ATC advised pilot of an small transport twin of altdevs indicated by the aircraft's transponder return.