Air carrier Y had less than standard separation from air carrier X. System error.

Attributes
ACN	203730
Time
Date	199202
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mia
State Reference	FL
Altitude	msl bound lower : 10000 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mia
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other descent other ground : parked
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 11000 flight time type : 3500
ASRS Report	203730
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 10000 vertical : 400
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X 40 mi northeast of the mia airport at 10000 ft at 250 KTS. Shortly after being slowed to 250 KTS the controller issued a clearance to immediately descend to 10000 ft and turn left to 090 degrees. Already being at 10000 ft I questioned the clearance to verify that it was for our aircraft. The controller then came back and issued an immediate turn to 090 degrees and to descend to 9000 ft. The first officer then started the descent and 090 degree turn as cleared. In the meantime air carrier Y, from another carrier, was approaching us from our 4 O'clock position and gaining on our aircraft rather quickly. Air carrier Y was converging on us and descending through our altitude. The controller realizing this ordered air carrier Y to stop his descent and climb back to 11000. By making the 090 degree turn and the first officer having a visual on the target the conflict was resolved without further incident. My feeling is that the air traffic controller misjudged the closure rate of the 2 aircraft due to the differences of the speed in which we were traveling.

Original NASA ASRS Text

Title: ACR Y HAD LTSS FROM ACR X. SYS ERROR.

Narrative: ACR X 40 MI NE OF THE MIA ARPT AT 10000 FT AT 250 KTS. SHORTLY AFTER BEING SLOWED TO 250 KTS THE CTLR ISSUED A CLRNC TO IMMEDIATELY DSND TO 10000 FT AND TURN L TO 090 DEGS. ALREADY BEING AT 10000 FT I QUESTIONED THE CLRNC TO VERIFY THAT IT WAS FOR OUR ACFT. THE CTLR THEN CAME BACK AND ISSUED AN IMMEDIATE TURN TO 090 DEGS AND TO DSND TO 9000 FT. THE FO THEN STARTED THE DSCNT AND 090 DEG TURN AS CLRED. IN THE MEANTIME ACR Y, FROM ANOTHER CARRIER, WAS APCHING US FROM OUR 4 O'CLOCK POS AND GAINING ON OUR ACFT RATHER QUICKLY. ACR Y WAS CONVERGING ON US AND DSNDING THROUGH OUR ALT. THE CTLR REALIZING THIS ORDERED ACR Y TO STOP HIS DSCNT AND CLB BACK TO 11000. BY MAKING THE 090 DEG TURN AND THE FO HAVING A VISUAL ON THE TARGET THE CONFLICT WAS RESOLVED WITHOUT FURTHER INCIDENT. MY FEELING IS THAT THE AIR TFC CTLR MISJUDGED THE CLOSURE RATE OF THE 2 ACFT DUE TO THE DIFFERENCES OF THE SPD IN WHICH WE WERE TRAVELING.

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.