An MDS80 captain responded to a TCASII RA, which took him below the GS at sea.

Attributes
ACN	453623
Time
Date	199910
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : sea.airport
State Reference	WA
Altitude	msl bound lower : 1800 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tower : sea.tower
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 34r
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tower : sea.tower
Operator	common carrier : air carrier
Make Model Name	Light Transport, High Wing, 2 Turboprop Eng
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 34l
Flight Phase	descent : approach
Route In Use	approach : visual arrival : vfr
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	453623
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	conflict : airborne critical non adherence : required legal separation non adherence : published procedure
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action
Consequence	other
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

While flying a visual approach to runway 34R at seattle, we responded to an RA that required a descent below the GS. We started the descent and asked the controller for traffic in the area. We were told that the only traffic was a commuter aircraft at our 9 O'clock position on a visual approach for runway 34L. As we became more concerned about terrain clearance, we concluded that the RA was for the commuter aircraft. We recaptured the GS and continued for a normal approach and landing. I am concerned that the commuter aircraft was allowed to come so close to our aircraft without our reporting it in sight. Also, I am uncertain as to correct procedure when a descending RA is received while flying an ILS approach.

Original NASA ASRS Text

Title: AN MDS80 CAPT RESPONDED TO A TCASII RA, WHICH TOOK HIM BELOW THE GS AT SEA.

Narrative: WHILE FLYING A VISUAL APCH TO RWY 34R AT SEATTLE, WE RESPONDED TO AN RA THAT REQUIRED A DSCNT BELOW THE GS. WE STARTED THE DSCNT AND ASKED THE CTLR FOR TFC IN THE AREA. WE WERE TOLD THAT THE ONLY TFC WAS A COMMUTER ACFT AT OUR 9 O'CLOCK POS ON A VISUAL APCH FOR RWY 34L. AS WE BECAME MORE CONCERNED ABOUT TERRAIN CLRNC, WE CONCLUDED THAT THE RA WAS FOR THE COMMUTER ACFT. WE RECAPTURED THE GS AND CONTINUED FOR A NORMAL APCH AND LNDG. I AM CONCERNED THAT THE COMMUTER ACFT WAS ALLOWED TO COME SO CLOSE TO OUR ACFT WITHOUT OUR RPTING IT IN SIGHT. ALSO, I AM UNCERTAIN AS TO CORRECT PROC WHEN A DSNDING RA IS RECEIVED WHILE FLYING AN ILS APCH.

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.