B737 pilots in descent instructed to maintain visual separation from paralleling traffic 1000 ft below, receive a TCASII RA when the traffic makes a turn toward their path.

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Attributes
ACN	493764
Time
Date	200011
Day	Thu
Local Time Of Day	0601 To 1200
Place
State Reference	PA
Altitude	msl single value : 18000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zny.artcc tracon : n90.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	descent : vacating altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zny.artcc
Operator	common carrier : air carrier
Make Model Name	Dash 8 Series Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 70 flight time total : 13000 flight time type : 11000
ASRS Report	493764
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	other spatial deviation
Independent Detector	aircraft equipment : tcas other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : returned to original clearance none taken : detected after the fact
Miss Distance	horizontal : 6000 vertical : 500
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

We were at FL180, other plane was at 17000 ft. ATC gave us the traffic 4 mi right and parallel. We acknowledged and said 'in sight.' controller then cleared us for a visual descent to 13000 ft. I started down, and the other aircraft made a turn toward our path. TCASII gave a TA then RA 'monitor vertical descent.' I turned right and climbed to FL180 behind the aircraft. In hindsight, we should not have been given a visual descent. I did not know the intent of the other aircraft. I was expecting to parallel his course in descent. We were given instructions to 'maintain visual separation,' 'descend to 13000 ft.' controller seemed busy with other traffic. 'Vibes' was our next crossing point (13000 ft) on the STAR (mid) arrival to lga.

Original NASA ASRS Text

Title: B737 PLTS IN DSCNT INSTRUCTED TO MAINTAIN VISUAL SEPARATION FROM PARALLELING TFC 1000 FT BELOW, RECEIVE A TCASII RA WHEN THE TFC MAKES A TURN TOWARD THEIR PATH.

Narrative: WE WERE AT FL180, OTHER PLANE WAS AT 17000 FT. ATC GAVE US THE TFC 4 MI R AND PARALLEL. WE ACKNOWLEDGED AND SAID 'IN SIGHT.' CTLR THEN CLRED US FOR A VISUAL DSCNT TO 13000 FT. I STARTED DOWN, AND THE OTHER ACFT MADE A TURN TOWARD OUR PATH. TCASII GAVE A TA THEN RA 'MONITOR VERT DSCNT.' I TURNED R AND CLBED TO FL180 BEHIND THE ACFT. IN HINDSIGHT, WE SHOULD NOT HAVE BEEN GIVEN A VISUAL DSCNT. I DID NOT KNOW THE INTENT OF THE OTHER ACFT. I WAS EXPECTING TO PARALLEL HIS COURSE IN DSCNT. WE WERE GIVEN INSTRUCTIONS TO 'MAINTAIN VISUAL SEPARATION,' 'DSND TO 13000 FT.' CTLR SEEMED BUSY WITH OTHER TFC. 'VIBES' WAS OUR NEXT XING POINT (13000 FT) ON THE STAR (MID) ARR TO LGA.

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.