Enroute controller described a loss of separation event when he/she failed to note the fact that the...

Narrative:

A C550 was descending north east bound from FL210 down to FL180 because I had a B350 level at 170 north east bound as well. The B350 had pilot discretion descent to 110 issued but wanted to remain at 170. As the C550 was descending through FL194; I realized I had low altimeters all around me (I believe 29.86) and I instructed the C550 to amend altitude and maintain FL190. He advised he had just went though FL190 and asked should he go back up. I told him it wasn't necessary since he seemed to have leveled off. I then decided to descend and turn the B350 to avoid any closer conflict. I descended the C550 to 160 and turned him 20 degrees left. I am not sure if I ever broke altitude separation but lateral separation was around 4 miles. I should have had some visual indicator to remind myself FL180 was unusable and non-assignable.

Original NASA ASRS Text

Title: Enroute Controller described a loss of separation event when he/she failed to note the fact that the surrounding low altimeter settings precluded the use of certain flight levels.

Narrative: A C550 was descending north east bound from FL210 down to FL180 because I had a B350 level at 170 north east bound as well. The B350 had pilot discretion descent to 110 issued but wanted to remain at 170. As the C550 was descending through FL194; I realized I had low altimeters all around me (I believe 29.86) and I instructed the C550 to amend altitude and maintain FL190. He advised he had just went though FL190 and asked should he go back up. I told him it wasn't necessary since he seemed to have leveled off. I then decided to descend and turn the B350 to avoid any closer conflict. I descended the C550 to 160 and turned him 20 degrees left. I am not sure if I ever broke altitude separation but lateral separation was around 4 miles. I should have had some visual indicator to remind myself FL180 was unusable and non-assignable.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	998743
Time
Date	201203
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	Citation II S2/Bravo (C550)
Operating Under FAR Part	Part 91
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Super King Air 350
Operating Under FAR Part	Part 91
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

Enroute controller described a loss of separation event when he/she failed to note the fact that the surrounding low altimeter settings precluded the use of certain flight levels.