Air carrier descending on approach has TCASII RA. Responds.

Attributes
ACN	198593
Time
Date	199201
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bwi
State Reference	MD
Altitude	msl bound lower : 3500 msl bound upper : 4600
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : bwi
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	general aviation : personal
Make Model Name	Small Aircraft
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 270 flight time total : 8700
ASRS Report	198593
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

I was acting as captain on flight from NAS to bwi. Aircraft type was an medium large transport TCASII equipped. Between 15-20 mi from bwi while being vectored by approach control for landing at bwi we were given a descent to 4000 ft. Passing 5000 ft descending we received a TCASII advisory of 'monitor rate of descent.' an aircraft symbol was displayed in our 10 O'clock position at 3500 ft, 3 mi and level. While looking for the traffic visually, I had my first officer ask approach control about the traffic. Prior to their response, we received a 'climb, climb now' command on TCASII. We reacted as commanded via TCASII to climb at 1500 FPM. Since we could not attain the intruder visually we began a climb as per FAA regulation. We noticed at this time our altitude was 4300 ft descending and the intruder had started a climb. We advised ATC that we were responding to the TCASII command and about the same time received a 'clear of conflict.' highest altitude obtained was 4600 ft. We never did see the intruder. ATC advised the VFR aircraft was not under their control. ATC also advised us they would file an incident report. 'Pilot responding to TCASII command.' we then descended to at cleared altitude and remainder of approach was uneventful.

Original NASA ASRS Text

Title: ACR DSNDING ON APCH HAS TCASII RA. RESPONDS.

Narrative: I WAS ACTING AS CAPT ON FLT FROM NAS TO BWI. ACFT TYPE WAS AN MLG TCASII EQUIPPED. BTWN 15-20 MI FROM BWI WHILE BEING VECTORED BY APCH CTL FOR LNDG AT BWI WE WERE GIVEN A DSCNT TO 4000 FT. PASSING 5000 FT DSNDING WE RECEIVED A TCASII ADVISORY OF 'MONITOR RATE OF DSCNT.' AN ACFT SYMBOL WAS DISPLAYED IN OUR 10 O'CLOCK POS AT 3500 FT, 3 MI AND LEVEL. WHILE LOOKING FOR THE TFC VISUALLY, I HAD MY FO ASK APCH CTL ABOUT THE TFC. PRIOR TO THEIR RESPONSE, WE RECEIVED A 'CLB, CLB NOW' COMMAND ON TCASII. WE REACTED AS COMMANDED VIA TCASII TO CLB AT 1500 FPM. SINCE WE COULD NOT ATTAIN THE INTRUDER VISUALLY WE BEGAN A CLB AS PER FAA REG. WE NOTICED AT THIS TIME OUR ALT WAS 4300 FT DSNDING AND THE INTRUDER HAD STARTED A CLB. WE ADVISED ATC THAT WE WERE RESPONDING TO THE TCASII COMMAND AND ABOUT THE SAME TIME RECEIVED A 'CLR OF CONFLICT.' HIGHEST ALT OBTAINED WAS 4600 FT. WE NEVER DID SEE THE INTRUDER. ATC ADVISED THE VFR ACFT WAS NOT UNDER THEIR CTL. ATC ALSO ADVISED US THEY WOULD FILE AN INCIDENT RPT. 'PLT RESPONDING TO TCASII COMMAND.' WE THEN DSNDED TO AT CLRED ALT AND REMAINDER OF APCH WAS UNEVENTFUL.

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.