ARTCC radar controller with 19 aircraft on frequency issued a direct intersection clearance and maintain FL390. The pilot thought the clearance was to descend to FL290 and started descent. The radar controller caught the mistake when conflict alert went off and then climbed the aircraft back up to FL390. However, loss of separation had already occurred.

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Attributes
ACN	376394
Time
Date	199708
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : geg
State Reference	WA
Altitude	msl bound lower : 37000 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse
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
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	DC-10 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : flight data
Qualification	controller : radar
Experience	controller radar : 13
ASRS Report	376394
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : required legal separation non adherence : published procedure non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	horizontal : 21000 vertical : 300
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We had a group of 4 aircraft over geg. Sector was busy (19 aircraft total). I heard my r-side giving speed restrs. I noted one of the aircraft my r-side was trying to sequence into sea was going to yvr and I advised the r-side. The r-side cleared the aircraft 'direct bacon,' maintain FL390. I did not hear the readback. I notice the aircraft at FL382 and alert had gone off, r-side verified altitude and pilot responded descending FL290 as assigned. R-side climbed aircraft back to FL390. No one from management called to ask about alert. Maybe they thought due to close proximity of a number of aircraft that we had a bad mode C or a mode C swap.

Original NASA ASRS Text

Title: ARTCC RADAR CTLR WITH 19 ACFT ON FREQ ISSUED A DIRECT INTXN CLRNC AND MAINTAIN FL390. THE PLT THOUGHT THE CLRNC WAS TO DSND TO FL290 AND STARTED DSCNT. THE RADAR CTLR CAUGHT THE MISTAKE WHEN CONFLICT ALERT WENT OFF AND THEN CLBED THE ACFT BACK UP TO FL390. HOWEVER, LOSS OF SEPARATION HAD ALREADY OCCURRED.

Narrative: WE HAD A GROUP OF 4 ACFT OVER GEG. SECTOR WAS BUSY (19 ACFT TOTAL). I HEARD MY R-SIDE GIVING SPD RESTRS. I NOTED ONE OF THE ACFT MY R-SIDE WAS TRYING TO SEQUENCE INTO SEA WAS GOING TO YVR AND I ADVISED THE R-SIDE. THE R-SIDE CLRED THE ACFT 'DIRECT BACON,' MAINTAIN FL390. I DID NOT HEAR THE READBACK. I NOTICE THE ACFT AT FL382 AND ALERT HAD GONE OFF, R-SIDE VERIFIED ALT AND PLT RESPONDED DSNDING FL290 AS ASSIGNED. R-SIDE CLBED ACFT BACK TO FL390. NO ONE FROM MGMNT CALLED TO ASK ABOUT ALERT. MAYBE THEY THOUGHT DUE TO CLOSE PROX OF A NUMBER OF ACFT THAT WE HAD A BAD MODE C OR A MODE C SWAP.

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.