A DA20 descending at 4000 FPM leveled off at 15000 ft. The center radar controller stated that the mode C indicated 13800 ft. The reporter verified level at 15000 ft.

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Attributes
ACN	376460
Time
Date	199707
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : yyz
State Reference	ON
Altitude	msl bound lower : 13800 msl bound upper : 15000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : czyz
Operator	common carrier : air carrier
Make Model Name	Falcon 20FJF/20C/20D/20E/20F
Operating Under FAR Part	Part 135
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 4100 flight time type : 600
ASRS Report	376460
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other

Narrative:

Descending into toronto I had just leveled at 15000 ft from a 4000 FPM descent when center said my mode C showed 13800 ft. I confirmed my altitude at 15000 ft but the controller did not seem convinced. Question: if the center loses your mode C in a descent does their computer go into a prediction mode? Has this happened before or is the problem in my equipment?

Original NASA ASRS Text

Title: A DA20 DSNDING AT 4000 FPM LEVELED OFF AT 15000 FT. THE CTR RADAR CTLR STATED THAT THE MODE C INDICATED 13800 FT. THE RPTR VERIFIED LEVEL AT 15000 FT.

Narrative: DSNDING INTO TORONTO I HAD JUST LEVELED AT 15000 FT FROM A 4000 FPM DSCNT WHEN CTR SAID MY MODE C SHOWED 13800 FT. I CONFIRMED MY ALT AT 15000 FT BUT THE CTLR DID NOT SEEM CONVINCED. QUESTION: IF THE CTR LOSES YOUR MODE C IN A DSCNT DOES THEIR COMPUTER GO INTO A PREDICTION MODE? HAS THIS HAPPENED BEFORE OR IS THE PROB IN MY EQUIP?

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.