CL600 flight crew is issued a descent clearance and receives TCAS alert during descent.

Attributes
ACN	699007
Time
Date	200606
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zla.artcc
State Reference	US
Altitude	msl bound lower : 27500 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla.artcc
Operator	common carrier : air carrier
Make Model Name	Challenger CL600
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zla.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 12000 flight time type : 2000
ASRS Report	699007
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe
Independent Detector	other controllera
Resolutory Action	controller : issued advisory controller : issued new clearance flight crew : took evasive action flight crew : returned to original clearance
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 800
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

While operating a scheduled flight as first officer and PF; the captain acknowledged and read back a clearance to FL240. We were at FL280. We started our descent promptly and were approximately FL275 when ATC inquired why we were descending. After verifying 'yes' he gave us an immediate left turn 30 degrees. We made the turn at the same time we got a TCAS advisory. We spotted the traffic visually as well as on the TCAS. At this time he cleared us to FL280 again. He asked us if we received descent clearance from a previous controller. We responded that it was him that cleared us lower. Soon after this incident we were told to switch frequency.

Original NASA ASRS Text

Title: CL600 FLT CREW IS ISSUED A DSCNT CLRNC AND RECEIVES TCAS ALERT DURING DSCNT.

Narrative: WHILE OPERATING A SCHEDULED FLT AS FO AND PF; THE CAPT ACKNOWLEDGED AND READ BACK A CLRNC TO FL240. WE WERE AT FL280. WE STARTED OUR DSCNT PROMPTLY AND WERE APPROX FL275 WHEN ATC INQUIRED WHY WE WERE DSNDING. AFTER VERIFYING 'YES' HE GAVE US AN IMMEDIATE L TURN 30 DEGS. WE MADE THE TURN AT THE SAME TIME WE GOT A TCAS ADVISORY. WE SPOTTED THE TFC VISUALLY AS WELL AS ON THE TCAS. AT THIS TIME HE CLRED US TO FL280 AGAIN. HE ASKED US IF WE RECEIVED DSCNT CLRNC FROM A PREVIOUS CTLR. WE RESPONDED THAT IT WAS HIM THAT CLRED US LOWER. SOON AFTER THIS INCIDENT WE WERE TOLD TO SWITCH FREQ.

Data retrieved from NASA's ASRS site as of January 2009 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.