A ZAU controller used the vacated altitude rule and had a loss of separation due to the descent rate...

Attributes
ACN	1202126
Time
Date	201409
Local Time Of Day	0001-0600
Place
Locale Reference	ZAU.ARTCC
State Reference	IL
Environment
Light	Night
Aircraft 1
Make Model Name	Medium Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Transport Low Wing 2 Turboprop Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Developmental
Events
Anomaly	Conflict Airborne Conflict Deviation - Procedural Published Material / Policy

Narrative:

I was coming out of the hold and aircraft X and aircraft Y were the last two I was clearing back to kord. Aircraft Y was already direct fiyer (west-southwest bound) and the remainder of the WYNDE5 arrival and had vacated 130. Aircraft X was on a 090 heading at 140. When aircraft Y vacated 130 under a descent clearance (not crossing or discretionary) I assigned aircraft X 130. Even though both aircraft had similar performance characteristics; aircraft Y descended very slowly while aircraft X descended at a higher rate. When aircraft X; now level at 130; came within 5 miles of aircraft Y; aircraft Y was still at 122 and descending. Jets have a large performance envelope and can climb/descend at anywhere from 500 to several thousand feet per minute. When stepping down one aircraft on top of another one; I should have paid attention to the rate as derived from the mode C or just have waited until the lower aircraft had vacated the next lower altitude before descending the aircraft that was on top. That would have ensured 1000 feet of separation.

Original NASA ASRS Text

Title: A ZAU Controller used the vacated altitude rule and had a loss of separation due to the descent rate of one of the aircraft being different than the other.

Narrative: I was coming out of the hold and Aircraft X and Aircraft Y were the last two I was clearing back to KORD. Aircraft Y was already direct FIYER (West-southwest bound) and the remainder of the WYNDE5 arrival and had vacated 130. Aircraft X was on a 090 heading at 140. When Aircraft Y vacated 130 under a descent clearance (not crossing or discretionary) I assigned Aircraft X 130. Even though both aircraft had similar performance characteristics; Aircraft Y descended very slowly while Aircraft X descended at a higher rate. When Aircraft X; now level at 130; came within 5 miles of Aircraft Y; Aircraft Y was still at 122 and descending. Jets have a large performance envelope and can climb/descend at anywhere from 500 to several thousand feet per minute. When stepping down one aircraft on top of another one; I should have paid attention to the rate as derived from the MODE C or just have waited until the lower aircraft had vacated the next lower altitude before descending the aircraft that was on top. That would have ensured 1000 feet of separation.

Data retrieved from NASA's ASRS site 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.

A ZAU controller used the vacated altitude rule and had a loss of separation due to the descent rate of one of the aircraft being different than the other.