Air carrier came in close proximity to another aircraft while in descent.

Attributes
ACN	140244
Time
Date	199003
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ywt
State Reference	ON
Altitude	msl bound lower : 20000 msl bound upper : 21000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : cyyz
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 120 flight time total : 4000 flight time type : 700
ASRS Report	140244
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 14000 flight time type : 3000
ASRS Report	140241
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance non adherence : required legal separation other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : insufficient time
Consequence	Other
Miss Distance	horizontal : 1000 vertical : 300
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The controller issued several instructions at once; i.e., depart waterloo on the 063 degree right, plan runway 24, descend to FL210. Prior to reaching FL210 I thought we were issued another clearance to descend to FL200 and I set 200 in the altitude alert. At FL200 we passed an small transport which flew over and behind us. After this, center said we were cleared only to FL210. The cause of the problem was either the controller not remembering our clearance to FL200 or our misunderstanding of a clearance which was not meant for our aircraft. To prevent a recurrence, greater diligence on both pilots and controllers on clearance issued and readbacks. Also, the controller should notify pilots of conflicting traffic and monitor transponder altitude readouts to ensure proper altitude level-offs.

Original NASA ASRS Text

Title: ACR CAME IN CLOSE PROX TO ANOTHER ACFT WHILE IN DESCENT.

Narrative: THE CTLR ISSUED SEVERAL INSTRUCTIONS AT ONCE; I.E., DEPART WATERLOO ON THE 063 DEG R, PLAN RWY 24, DSND TO FL210. PRIOR TO REACHING FL210 I THOUGHT WE WERE ISSUED ANOTHER CLRNC TO DSND TO FL200 AND I SET 200 IN THE ALT ALERT. AT FL200 WE PASSED AN SMT WHICH FLEW OVER AND BEHIND US. AFTER THIS, CENTER SAID WE WERE CLRED ONLY TO FL210. THE CAUSE OF THE PROB WAS EITHER THE CTLR NOT REMEMBERING OUR CLRNC TO FL200 OR OUR MISUNDERSTANDING OF A CLRNC WHICH WAS NOT MEANT FOR OUR ACFT. TO PREVENT A RECURRENCE, GREATER DILIGENCE ON BOTH PLTS AND CTLRS ON CLRNC ISSUED AND READBACKS. ALSO, THE CTLR SHOULD NOTIFY PLTS OF CONFLICTING TFC AND MONITOR XPONDER ALT READOUTS TO ENSURE PROPER ALT LEVEL-OFFS.

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.