Less than standard separation between air carrier medium large transport and an unknown aircraft.

Attributes
ACN	278677
Time
Date	199408
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : olm
State Reference	WA
Altitude	msl bound lower : 11300 msl bound upper : 11300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sea
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	arrival other enroute airway : sea
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	cruise other
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp pilot : commercial
Experience	flight time last 90 days : 95 flight time total : 10500 flight time type : 6300
ASRS Report	278677
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 1000
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Cleared to descend from 12000 ft MSL to a lower altitude, approaching 11500 ft I noticed an airplane below and to my left which appeared like it would become a conflict. A brief discussion with the non-flying pilot and we decided to climb back up to 12000 ft. At same time we started the climb, TCASII gave us a monitor vertical speed followed by a climb RA. Shortly thereafter approach control gave other aircraft a lower altitude and also gave us a lower altitude. Remainder of flight uneventful. TCASII worked as it should have.

Original NASA ASRS Text

Title: LTSS BTWN ACR MLG AND AN UNKNOWN ACFT.

Narrative: CLRED TO DSND FROM 12000 FT MSL TO A LOWER ALT, APCHING 11500 FT I NOTICED AN AIRPLANE BELOW AND TO MY L WHICH APPEARED LIKE IT WOULD BECOME A CONFLICT. A BRIEF DISCUSSION WITH THE NON-FLYING PLT AND WE DECIDED TO CLB BACK UP TO 12000 FT. AT SAME TIME WE STARTED THE CLB, TCASII GAVE US A MONITOR VERT SPD FOLLOWED BY A CLB RA. SHORTLY THEREAFTER APCH CTL GAVE OTHER ACFT A LOWER ALT AND ALSO GAVE US A LOWER ALT. REMAINDER OF FLT UNEVENTFUL. TCASII WORKED AS IT SHOULD HAVE.

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.