False TCASII logic caused air carrier X TCASII RA with VFR aircraft. Evasive action taken.

Attributes
ACN	270943
Time
Date	199405
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : gon
State Reference	CT
Altitude	msl bound lower : 9500 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : maf
Operator	common carrier : air carrier
Make Model Name	SF 340A
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	enroute : direct enroute airway : bdl
Flight Plan	IFR
Aircraft 2
Operator	general aviation : personal
Make Model Name	MU-2
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	VFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller radar : 8
ASRS Report	270943
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 0 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Working air carrier X JF34 level flight 10000 ft IFR, issued traffic, aircraft Y, mu-2, opposite direction VFR 9500 ft. X told me he had a TCASII alert and was climbing. When I questioned the climb as traffic was separated by 500 ft, he told me TCASII showed mu-2 in a climb. I never saw a change of mu-2's mode C and mu-2 pilot confirmed with me verbally he was in level flight.

Original NASA ASRS Text

Title: FALSE TCASII LOGIC CAUSED ACR X TCASII RA WITH VFR ACFT. EVASIVE ACTION TAKEN.

Narrative: WORKING ACR X JF34 LEVEL FLT 10000 FT IFR, ISSUED TFC, ACFT Y, MU-2, OPPOSITE DIRECTION VFR 9500 FT. X TOLD ME HE HAD A TCASII ALERT AND WAS CLBING. WHEN I QUESTIONED THE CLB AS TFC WAS SEPARATED BY 500 FT, HE TOLD ME TCASII SHOWED MU-2 IN A CLB. I NEVER SAW A CHANGE OF MU-2'S MODE C AND MU-2 PLT CONFIRMED WITH ME VERBALLY HE WAS IN LEVEL FLT.

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.