A D328 crew responded to a TCASII RA while on approach to phl.

Attributes
ACN	524326
Time
Date	200108
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : phl.airport
State Reference	PA
Altitude	msl bound lower : 3800 msl bound upper : 4000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : phl.tracon
Operator	common carrier : air carrier
Make Model Name	Do 328 TP (Turboprop)
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
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 : 6000 flight time type : 2000
ASRS Report	524326
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	conflict : nmac other anomaly other
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance
Consequence	faa : reviewed incident with flight crew other
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Problem Areas	Airspace Structure ATC Human Performance Flight Crew Human Performance Environmental Factor
Primary Problem	Flight Crew Human Performance

Narrative:

After being handed off to phl final controller at 6000 ft we were told to fly a heading of 060 degrees and descend and maintain 3000 ft. Passing 500 ft a square appeared less than 5 mi and 1500 ft below us climbing. After investigation I advised the first officer that it appeared this aircraft would be in conflict with us and to be ready. On the fourth or fifth interrogation we got a TA which was almost immediately followed by an RA 'monitor vertical speed.' a red arc was set at any vs below 0. I disconnected automatic pilot instinctively at the same time the first officer reached for it and we both brought the aircraft back up to 4000 ft from 3800 ft as the aircraft directly below us within 400 ft to 500 ft, I advised ATC of our action and we were told to maintain 4000 ft and that this aircraft was supposed to be on a different sector. The controller said, 'he just popped up out of nowhere.' we advised him that we came within 400 ft or 500 ft. The rest of the flight was uneventful. The first officer was the flying pilot. We never obtained visual confirmation of this aircraft. This had mid air collision written all over it. ATC was unaware and we never saw him. If it weren't for TCASII and collision avoidance procedures, this would have been tragic.

Original NASA ASRS Text

Title: A D328 CREW RESPONDED TO A TCASII RA WHILE ON APCH TO PHL.

Narrative: AFTER BEING HANDED OFF TO PHL FINAL CTLR AT 6000 FT WE WERE TOLD TO FLY A HDG OF 060 DEGS AND DSND AND MAINTAIN 3000 FT. PASSING 500 FT A SQUARE APPEARED LESS THAN 5 MI AND 1500 FT BELOW US CLBING. AFTER INVESTIGATION I ADVISED THE FO THAT IT APPEARED THIS ACFT WOULD BE IN CONFLICT WITH US AND TO BE READY. ON THE FOURTH OR FIFTH INTERROGATION WE GOT A TA WHICH WAS ALMOST IMMEDIATELY FOLLOWED BY AN RA 'MONITOR VERT SPD.' A RED ARC WAS SET AT ANY VS BELOW 0. I DISCONNECTED AUTO PLT INSTINCTIVELY AT THE SAME TIME THE FO REACHED FOR IT AND WE BOTH BROUGHT THE ACFT BACK UP TO 4000 FT FROM 3800 FT AS THE ACFT DIRECTLY BELOW US WITHIN 400 FT TO 500 FT, I ADVISED ATC OF OUR ACTION AND WE WERE TOLD TO MAINTAIN 4000 FT AND THAT THIS ACFT WAS SUPPOSED TO BE ON A DIFFERENT SECTOR. THE CTLR SAID, 'HE JUST POPPED UP OUT OF NOWHERE.' WE ADVISED HIM THAT WE CAME WITHIN 400 FT OR 500 FT. THE REST OF THE FLT WAS UNEVENTFUL. THE FO WAS THE FLYING PLT. WE NEVER OBTAINED VISUAL CONFIRMATION OF THIS ACFT. THIS HAD MID AIR COLLISION WRITTEN ALL OVER IT. ATC WAS UNAWARE AND WE NEVER SAW HIM. IF IT WEREN'T FOR TCASII AND COLLISION AVOIDANCE PROCS, THIS WOULD HAVE BEEN TRAGIC.

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.