Air carrier X TCASII RA had less than standard separation from air carrier Y. System error.

Attributes
ACN	299976
Time
Date	199503
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : sna
State Reference	CA
Altitude	msl bound lower : 4000 msl bound upper : 4000
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sna
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 9000 flight time type : 2500
ASRS Report	299976
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : nmac non adherence : required legal separation non adherence : published procedure
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 : 0 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X talking to socal approach coming in to orange county, we were given a heading of 160 degrees at 170 KTS, we were flying at 4000 ft, then approach told to descend to 3000 ft and 3000 ft until established cleared for ILS runway 19R approach. At that point in time noticed a target on our TCASII 6 mi scale. We were descending to 3000 ft. I told my first officer to increase the rate down at the same time received the RA to (descend now). I looked up and saw air carrier Y 757 passing over us approximately 400-500 ft above, the mentioned they had to climb to 4300 ft to clear the RA. We just increased our rate of descent. I believe if we knew about them we would have increased our rate right away from 4000 ft to 3000 ft. We were following the GS down to 3000 ft for the localizer intercept. A normal approach and landing was made at orange county. I was not asked by anyone to make a call. I called on my own to socal approach in san diego to report this, and found out the air carrier Y 757 captain did also.

Original NASA ASRS Text

Title: ACR X TCASII RA HAD LTSS FROM ACR Y. SYS ERROR.

Narrative: ACR X TALKING TO SOCAL APCH COMING IN TO ORANGE COUNTY, WE WERE GIVEN A HDG OF 160 DEGS AT 170 KTS, WE WERE FLYING AT 4000 FT, THEN APCH TOLD TO DSND TO 3000 FT AND 3000 FT UNTIL ESTABLISHED CLRED FOR ILS RWY 19R APCH. AT THAT POINT IN TIME NOTICED A TARGET ON OUR TCASII 6 MI SCALE. WE WERE DSNDING TO 3000 FT. I TOLD MY FO TO INCREASE THE RATE DOWN AT THE SAME TIME RECEIVED THE RA TO (DSND NOW). I LOOKED UP AND SAW ACR Y 757 PASSING OVER US APPROX 400-500 FT ABOVE, THE MENTIONED THEY HAD TO CLB TO 4300 FT TO CLR THE RA. WE JUST INCREASED OUR RATE OF DSCNT. I BELIEVE IF WE KNEW ABOUT THEM WE WOULD HAVE INCREASED OUR RATE RIGHT AWAY FROM 4000 FT TO 3000 FT. WE WERE FOLLOWING THE GS DOWN TO 3000 FT FOR THE LOC INTERCEPT. A NORMAL APCH AND LNDG WAS MADE AT ORANGE COUNTY. I WAS NOT ASKED BY ANYONE TO MAKE A CALL. I CALLED ON MY OWN TO SOCAL APCH IN SAN DIEGO TO RPT THIS, AND FOUND OUT THE ACR Y 757 CAPT DID ALSO.

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.