ATC equipment problem radar mode C altitude reporting.

Attributes
ACN	265749
Time
Date	199403
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : tpa
State Reference	FL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : lax
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	arrival other enroute airway : tpa
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller military : 5 controller radar : 11
ASRS Report	265749
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : unable
Consequence	faa : investigated
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other
Situations
Navigational Aid	Unspecified

Narrative:

Air carrier X on the dades arrival to tpa was descending to 11000 ft (assigned) as air carrier X converged with Y. 10000 ft assigned, X mode C changed to 10400 ft. I had to reverify the mode C and assigned altitudes of both aircraft. About 10-15 mins later, a BE33 overflt at 8000 ft on V7 to cty, changed mode C readout to 10000 ft when he converged with an opposite direction overflt Z at 7000 ft. I am out of confidence that this radar system is reliable enough to be used for IFR separation.

Original NASA ASRS Text

Title: ATC EQUIP PROB RADAR MODE C ALT RPTING.

Narrative: ACR X ON THE DADES ARR TO TPA WAS DSNDING TO 11000 FT (ASSIGNED) AS ACR X CONVERGED WITH Y. 10000 FT ASSIGNED, X MODE C CHANGED TO 10400 FT. I HAD TO REVERIFY THE MODE C AND ASSIGNED ALTS OF BOTH ACFT. ABOUT 10-15 MINS LATER, A BE33 OVERFLT AT 8000 FT ON V7 TO CTY, CHANGED MODE C READOUT TO 10000 FT WHEN HE CONVERGED WITH AN OPPOSITE DIRECTION OVERFLT Z AT 7000 FT. I AM OUT OF CONFIDENCE THAT THIS RADAR SYS IS RELIABLE ENOUGH TO BE USED FOR IFR SEPARATION.

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.