ZSE controller lost separation with 2 descending aircraft at approximately 11000 ft.

Attributes
ACN	596464
Time
Date	200310
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : sea.vortac
State Reference	WA
Altitude	msl single value : 10800
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zse.artcc
Operator	common carrier : air taxi
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 135
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Experience	controller non radar : 20 controller radar : 2
ASRS Report	596464
Person 2
Affiliation	government : faa
Function	instruction : instructor
Qualification	controller : radar
Experience	controller radar : 13
ASRS Report	596453
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	other controllera other controllerb other flight crewb
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 28200 vertical : 600
Supplementary
Problem Areas	ATC Human Performance Aircraft
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was controller in charge, instructor and trainee were working sector 01, low and slow evening arrival push. I evaluated the sector 6 mins before error and felt a d-side was not needed. The instructor/trainee were using speed and vertical to achieve sequencing needed. Error occurred when controller assigned descent to 10000 ft and aircraft X read back 'down to 11000 ft.' then had no mode C readout for 2 mins (computer problem FAA or aircraft unknown). When mode C came back aircraft X read '108', controller then assigned '110' to overtaking aircraft as allowed by manual rules. Aircraft X lost mode C again for 1 min. When mode C came back aircraft X mode C was '109' and error occurred with overtaking aircraft air carrier Y descending to 11000 ft. Contributing factors: 1) hearback/readback 10000 ft versus 11000 ft, 2) mode C no readout air carrier X for 2 mins, then 1 min aircraft or FAA computer -- unknown where problem was and 3) poor radio on aircraft X contributed to hearback error (very scratchy readback) difficult until played tape twice.

Original NASA ASRS Text

Title: ZSE CTLR LOST SEPARATION WITH 2 DSNDING ACFT AT APPROX 11000 FT.

Narrative: I WAS CIC, INSTRUCTOR AND TRAINEE WERE WORKING SECTOR 01, LOW AND SLOW EVENING ARR PUSH. I EVALUATED THE SECTOR 6 MINS BEFORE ERROR AND FELT A D-SIDE WAS NOT NEEDED. THE INSTRUCTOR/TRAINEE WERE USING SPD AND VERT TO ACHIEVE SEQUENCING NEEDED. ERROR OCCURRED WHEN CTLR ASSIGNED DSCNT TO 10000 FT AND ACFT X READ BACK 'DOWN TO 11000 FT.' THEN HAD NO MODE C READOUT FOR 2 MINS (COMPUTER PROB FAA OR ACFT UNKNOWN). WHEN MODE C CAME BACK ACFT X READ '108', CTLR THEN ASSIGNED '110' TO OVERTAKING ACFT AS ALLOWED BY MANUAL RULES. ACFT X LOST MODE C AGAIN FOR 1 MIN. WHEN MODE C CAME BACK ACFT X MODE C WAS '109' AND ERROR OCCURRED WITH OVERTAKING ACFT ACR Y DSNDING TO 11000 FT. CONTRIBUTING FACTORS: 1) HEARBACK/READBACK 10000 FT VERSUS 11000 FT, 2) MODE C NO READOUT ACR X FOR 2 MINS, THEN 1 MIN ACFT OR FAA COMPUTER -- UNKNOWN WHERE PROB WAS AND 3) POOR RADIO ON ACFT X CONTRIBUTED TO HEARBACK ERROR (VERY SCRATCHY READBACK) DIFFICULT UNTIL PLAYED TAPE TWICE.

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.