TRACON controller lost legal separation with two air carrier aircraft when the transponder, mode C, of one aircraft jumped 1300 ft in altitude causing the other aircraft TCASII to issue an RA to descend. Both aircraft missed each other by approximately a half of mi.

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Attributes
ACN	336453
Time
Date	199605
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 5000 msl bound upper : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Beech 1900
Operating Under FAR Part	Part 135
Flight Phase	climbout : intermediate altitude
Route In Use	departure other departure sid : sid
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar pilot : instrument
Experience	controller radar : 11 flight time total : 300
ASRS Report	336453
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 3000 vertical : 500
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Air carrier X was 5 mi southwest of ord on a 360 degree heading level at 6000 ft. Air carrier Y, on ord departure, was on a southwesterly vector climbing off ord to 5000 ft (as per ord 1 SID). Merging traffic advisories were not issued because it appeared that the targets wouldn't merge. Air carrier Y mode C jumped to indicate 6300 ft. Air carrier Y did level at 5000 ft, as verified by pilot report. Air carrier X's TCASII issued an RA, based on the inaccurate mode C, and the pilots descended. The pilots of air carrier X then saw air carrier Y pass below them. The airplanes were separated until the TCASII issued the RA based on the bad mode C indication. The pilots obeyed the RA (as required) and notified ATC during the descent. As a result, the airplanes passed about 1/2 mi apart with 500-600 ft vertical separation. Contributing factors: air carrier Y mode C jumped from 2700 ft to 6300 ft in just a few radar updates. The controller would have to be watching air carrier Y and nothing else to have caught the discrepancy. Position relief briefings were being conducted at both sectors, south and west departure (controling air carrier Y) and north satellite/north departure (controling air carrier X). Both targets were very close together, with possible overlap. This made it difficult to see the discrepancy.

Original NASA ASRS Text

Title: TRACON CTLR LOST LEGAL SEPARATION WITH TWO ACR ACFT WHEN THE XPONDER, MODE C, OF ONE ACFT JUMPED 1300 FT IN ALT CAUSING THE OTHER ACFT TCASII TO ISSUE AN RA TO DSND. BOTH ACFT MISSED EACH OTHER BY APPROX A HALF OF MI.

Narrative: ACR X WAS 5 MI SW OF ORD ON A 360 DEG HDG LEVEL AT 6000 FT. ACR Y, ON ORD DEP, WAS ON A SOUTHWESTERLY VECTOR CLBING OFF ORD TO 5000 FT (AS PER ORD 1 SID). MERGING TFC ADVISORIES WERE NOT ISSUED BECAUSE IT APPEARED THAT THE TARGETS WOULDN'T MERGE. ACR Y MODE C JUMPED TO INDICATE 6300 FT. ACR Y DID LEVEL AT 5000 FT, AS VERIFIED BY PLT RPT. ACR X'S TCASII ISSUED AN RA, BASED ON THE INACCURATE MODE C, AND THE PLTS DSNDED. THE PLTS OF ACR X THEN SAW ACR Y PASS BELOW THEM. THE AIRPLANES WERE SEPARATED UNTIL THE TCASII ISSUED THE RA BASED ON THE BAD MODE C INDICATION. THE PLTS OBEYED THE RA (AS REQUIRED) AND NOTIFIED ATC DURING THE DSCNT. AS A RESULT, THE AIRPLANES PASSED ABOUT 1/2 MI APART WITH 500-600 FT VERT SEPARATION. CONTRIBUTING FACTORS: ACR Y MODE C JUMPED FROM 2700 FT TO 6300 FT IN JUST A FEW RADAR UPDATES. THE CTLR WOULD HAVE TO BE WATCHING ACR Y AND NOTHING ELSE TO HAVE CAUGHT THE DISCREPANCY. POS RELIEF BRIEFINGS WERE BEING CONDUCTED AT BOTH SECTORS, S AND W DEP (CTLING ACR Y) AND N SATELLITE/N DEP (CTLING ACR X). BOTH TARGETS WERE VERY CLOSE TOGETHER, WITH POSSIBLE OVERLAP. THIS MADE IT DIFFICULT TO SEE THE DISCREPANCY.

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.