Aircraft descended to an altitude assigned to another aircraft.

Attributes
ACN	108159
Time
Date	198904
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : nuw
State Reference	WA
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	other
Make Model Name	Fighter
Flight Phase	descent other
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	108159
Person 2
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument
Events
Anomaly	non adherence : published procedure non adherence : required legal separation other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	none taken : insufficient time
Consequence	faa : investigated
Miss Distance	horizontal : 1500 vertical : 0
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was working ZSE ARTCC sector R3. This sector is seattle north to the canadian boundary. I was providing instruction to an r-side trnee. An small transport was northbound to yyj at 10000'. 2 fgts called for clearance to nuw from olympic MOA. The fgts were cleared direct to nuw at FL230. There were other flts of fgts returning from the east. Much sequencing was involved. Icing was involved in other aircraft at the same time. The fgts were given 11000' for descent to nuw. Crossing traffic was the small transport at 10000'. After leveling at 11000', the fgts requested lower because of ice. My trnee was slow in reacting, so I started to take over the sector. My first transmission was to the fgts with 'traffic 2 O'clock, 3 mi, nwbnd at 10000', expect lower when clear.' the pilot read back, 'roger, descending to 10000'.' I missed the readback. Loss of sep happened almost immediately. The targets merged. I asked the altitude of the fgts and they said level at 10000' assigned. Recommendations: I believe the pilot had lower altitude in his mind. It didn't matter what I said, I think he was going to do what he wanted to.

Original NASA ASRS Text

Title: ACFT DESCENDED TO AN ALT ASSIGNED TO ANOTHER ACFT.

Narrative: I WAS WORKING ZSE ARTCC SECTOR R3. THIS SECTOR IS SEATTLE N TO THE CANADIAN BOUNDARY. I WAS PROVIDING INSTRUCTION TO AN R-SIDE TRNEE. AN SMT WAS NBOUND TO YYJ AT 10000'. 2 FGTS CALLED FOR CLRNC TO NUW FROM OLYMPIC MOA. THE FGTS WERE CLRED DIRECT TO NUW AT FL230. THERE WERE OTHER FLTS OF FGTS RETURNING FROM THE E. MUCH SEQUENCING WAS INVOLVED. ICING WAS INVOLVED IN OTHER ACFT AT THE SAME TIME. THE FGTS WERE GIVEN 11000' FOR DSCNT TO NUW. XING TFC WAS THE SMT AT 10000'. AFTER LEVELING AT 11000', THE FGTS REQUESTED LOWER BECAUSE OF ICE. MY TRNEE WAS SLOW IN REACTING, SO I STARTED TO TAKE OVER THE SECTOR. MY FIRST XMISSION WAS TO THE FGTS WITH 'TFC 2 O'CLOCK, 3 MI, NWBND AT 10000', EXPECT LOWER WHEN CLR.' THE PLT READ BACK, 'ROGER, DSNDING TO 10000'.' I MISSED THE READBACK. LOSS OF SEP HAPPENED ALMOST IMMEDIATELY. THE TARGETS MERGED. I ASKED THE ALT OF THE FGTS AND THEY SAID LEVEL AT 10000' ASSIGNED. RECOMMENDATIONS: I BELIEVE THE PLT HAD LOWER ALT IN HIS MIND. IT DIDN'T MATTER WHAT I SAID, I THINK HE WAS GOING TO DO WHAT HE WANTED TO.

Data retrieved from NASA's ASRS site as of August 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.