Reporter responds to TCASII RA commands which contradicted controller commands.

Attributes
ACN	319657
Time
Date	199510
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iad
State Reference	VA
Altitude	msl bound lower : 5700 msl bound upper : 6400
Environment
Flight Conditions	Mixed
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : iad
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	HS 125 Series 1-600
Operating Under FAR Part	Part 91
Navigation In Use	Other Other
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 : 250 flight time total : 6500 flight time type : 2000
ASRS Report	319657
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 12000 vertical : 0
Supplementary
Air Traffic Incident	other

Narrative:

We were west of dulles airport descending through approximately 6200 ft on ATC vectors to ILS runway 12 at iad (heading about 150 degrees). ATC advised immediate left turn to 060 degrees and immediate descent to 3000 ft. Almost simultaneously our TCASII commanded 'descend, descend.' I pushed over to TCASII indicated descent rate and turned. At about 5700 ft TCASII commanded climb, climb now and I pulled up to TCASII indicated climb rate. First officer then spotted other aircraft and took controls. Normal descent was continued after approximately 6400 ft altitude. Other aircraft appeared to be approximately northbound corporate hawker or such. We were on heading, descending normally when event happened and have no idea why it did. Also: with such a large change in heading and altitude so close to the field it was difficult and disorienting getting lined up with the ILS. Worse, the ATIS had indicated good VFR conditions. We broke out of the clouds on about 2 mi final in low visibility in heavy rain.

Original NASA ASRS Text

Title: RPTR RESPONDS TO TCASII RA COMMANDS WHICH CONTRADICTED CTLR COMMANDS.

Narrative: WE WERE W OF DULLES ARPT DSNDING THROUGH APPROX 6200 FT ON ATC VECTORS TO ILS RWY 12 AT IAD (HDG ABOUT 150 DEGS). ATC ADVISED IMMEDIATE L TURN TO 060 DEGS AND IMMEDIATE DSCNT TO 3000 FT. ALMOST SIMULTANEOUSLY OUR TCASII COMMANDED 'DSND, DSND.' I PUSHED OVER TO TCASII INDICATED DSCNT RATE AND TURNED. AT ABOUT 5700 FT TCASII COMMANDED CLB, CLB NOW AND I PULLED UP TO TCASII INDICATED CLB RATE. FO THEN SPOTTED OTHER ACFT AND TOOK CTLS. NORMAL DSCNT WAS CONTINUED AFTER APPROX 6400 FT ALT. OTHER ACFT APPEARED TO BE APPROX NBOUND CORPORATE HAWKER OR SUCH. WE WERE ON HDG, DSNDING NORMALLY WHEN EVENT HAPPENED AND HAVE NO IDEA WHY IT DID. ALSO: WITH SUCH A LARGE CHANGE IN HDG AND ALT SO CLOSE TO THE FIELD IT WAS DIFFICULT AND DISORIENTING GETTING LINED UP WITH THE ILS. WORSE, THE ATIS HAD INDICATED GOOD VFR CONDITIONS. WE BROKE OUT OF THE CLOUDS ON ABOUT 2 MI FINAL IN LOW VISIBILITY IN HVY RAIN.

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.