Phl controller experienced near loss of separation at 8000 when aircraft was descended through occupied altitude; TCAS RA saved legal separation.

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Attributes
ACN	735844
Time
Date	200704
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : phl.airport
State Reference	PA
Altitude	msl single value : 8000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : phl.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : phl.tracon
Operator	general aviation : corporate
Make Model Name	Citation X
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : developmental
ASRS Report	739844
Events
Anomaly	conflict : airborne critical
Independent Detector	atc equipment : conflict alert aircraft equipment : tcas other controllera
Resolutory Action	controller : issued new clearance controller : issued alert flight crew : took evasive action
Miss Distance	vertical : 1000
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

B737 was on a 070 degree heading at 8000 ft. CIT10 was sbound landing ilg at 7000 ft. B737 was instructed to descend to 6000 ft. I noticed B737 could conflict with CIT10 so I issued B737 a turn to the left to establish course divergence. He advised me he was responding to an RA. B737 climbed back up to 8000 ft and I turned him eastbound.

Original NASA ASRS Text

Title: PHL CTLR EXPERIENCED NEAR LOSS OF SEPARATION AT 8000 WHEN ACFT WAS DESCENDED THROUGH OCCUPIED ALT; TCAS RA SAVED LEGAL SEPARATION.

Narrative: B737 WAS ON A 070 DEG HDG AT 8000 FT. CIT10 WAS SBOUND LNDG ILG AT 7000 FT. B737 WAS INSTRUCTED TO DSND TO 6000 FT. I NOTICED B737 COULD CONFLICT WITH CIT10 SO I ISSUED B737 A TURN TO THE L TO ESTABLISH COURSE DIVERGENCE. HE ADVISED ME HE WAS RESPONDING TO AN RA. B737 CLBED BACK UP TO 8000 FT AND I TURNED HIM EBOUND.

Data retrieved from NASA's ASRS site as of January 2009 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.