Loss of separation when CL65 is descended from FL260 and conflicts with a DC9 at FL250.

Attributes
ACN	399632
Time
Date	199804
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : eri
State Reference	PA
Altitude	msl bound lower : 25000 msl bound upper : 25500
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob tower : pns
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Challenger CL604
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar pilot : student
Experience	controller radar : 5 flight time total : 10
ASRS Report	399632
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 24000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was handed off 3 yyz inbound stacked on top of each other -- FL270, FL260, FL250. Aircraft at FL270 requested lower due to turbulence. To free up an altitude for him, I descended aircraft #2 to FL200. Unfortunately, I only had 4 mi with aircraft #1. I stopped the descent of aircraft #2 and descended aircraft #1, but by this time, altitude separation was only 500 ft. Contributing factors were data block overlap (5 data blocks on top of each other) and the descent to accommodate an aircraft in chop sooner than I should have.

Original NASA ASRS Text

Title: LOSS OF SEPARATION WHEN CL65 IS DSNDED FROM FL260 AND CONFLICTS WITH A DC9 AT FL250.

Narrative: I WAS HANDED OFF 3 YYZ INBOUND STACKED ON TOP OF EACH OTHER -- FL270, FL260, FL250. ACFT AT FL270 REQUESTED LOWER DUE TO TURB. TO FREE UP AN ALT FOR HIM, I DSNDED ACFT #2 TO FL200. UNFORTUNATELY, I ONLY HAD 4 MI WITH ACFT #1. I STOPPED THE DSCNT OF ACFT #2 AND DSNDED ACFT #1, BUT BY THIS TIME, ALT SEPARATION WAS ONLY 500 FT. CONTRIBUTING FACTORS WERE DATA BLOCK OVERLAP (5 DATA BLOCKS ON TOP OF EACH OTHER) AND THE DSCNT TO ACCOMMODATE AN ACFT IN CHOP SOONER THAN I SHOULD HAVE.

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.