Flight crew of an mdt descended below assigned approach vector altitude in response to a TCASII RA.

Attributes
ACN	291200
Time
Date	199412
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dca
State Reference	DC
Altitude	msl bound lower : 4300 msl bound upper : 5000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dca
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
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 : 220 flight time total : 11000 flight time type : 2800
ASRS Report	291200
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 1500 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

While being vectored for approach into washington national runway 21, approach called traffic VFR at 5500 ft and 11 O'clock (frequency was 128.35) our aircraft equipped with TCASII, first we got a traffic alert then a very strong RA to descend. Autoplt was disengaged and a descend at 1500 FPM was made to avoid intruder aircraft. ATC was notified that we had an RA. We descended to 4300 ft, intruder aircraft slowed 800 ft above. This would be 100 ft above our assigned altitude.

Original NASA ASRS Text

Title: FLC OF AN MDT DSNDED BELOW ASSIGNED APCH VECTOR ALT IN RESPONSE TO A TCASII RA.

Narrative: WHILE BEING VECTORED FOR APCH INTO WASHINGTON NATIONAL RWY 21, APCH CALLED TFC VFR AT 5500 FT AND 11 O'CLOCK (FREQ WAS 128.35) OUR ACFT EQUIPPED WITH TCASII, FIRST WE GOT A TFC ALERT THEN A VERY STRONG RA TO DSND. AUTOPLT WAS DISENGAGED AND A DSND AT 1500 FPM WAS MADE TO AVOID INTRUDER ACFT. ATC WAS NOTIFIED THAT WE HAD AN RA. WE DSNDED TO 4300 FT, INTRUDER ACFT SLOWED 800 FT ABOVE. THIS WOULD BE 100 FT ABOVE OUR ASSIGNED ALT.

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.