B737 flight crew had a TCASII RA with unknown aircraft in class a airspace.

Attributes
ACN	419117
Time
Date	199811
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : eed
State Reference	CA
Altitude	msl bound lower : 27500 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	other other : other pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 170 flight time total : 21500 flight time type : 10000
ASRS Report	419117
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : nmac
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	horizontal : 0 vertical : 500
Supplementary
Primary Problem	ATC Human Performance

Narrative:

On nov/xa/98 at approximately XA27Z and approximately 40 NM east of the eed VOR at our assigned altitude of FL280, we got a TCASII RA. The RA commanded a 2000 FPM descent, which I did. We got a visual on the intruder and passed under it by approximately 500 ft. We notified ATC immediately and when the intruder was clear, we climbed back to our assigned altitude of FL280. ATC did not know why this intruder aircraft was at our altitude. We were in contact with ZLA.

Original NASA ASRS Text

Title: B737 FLC HAD A TCASII RA WITH UNKNOWN ACFT IN CLASS A AIRSPACE.

Narrative: ON NOV/XA/98 AT APPROX XA27Z AND APPROX 40 NM E OF THE EED VOR AT OUR ASSIGNED ALT OF FL280, WE GOT A TCASII RA. THE RA COMMANDED A 2000 FPM DSCNT, WHICH I DID. WE GOT A VISUAL ON THE INTRUDER AND PASSED UNDER IT BY APPROX 500 FT. WE NOTIFIED ATC IMMEDIATELY AND WHEN THE INTRUDER WAS CLR, WE CLBED BACK TO OUR ASSIGNED ALT OF FL280. ATC DID NOT KNOW WHY THIS INTRUDER ACFT WAS AT OUR ALT. WE WERE IN CONTACT WITH ZLA.

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.