Date: 201304
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: Turbo Commander 690 Series
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Second aircraft: EMB ERJ 145 ER/LR
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was working the RADAR position at Sector X. No RADAR Associate or Tracker was plugged in; traffic was relatively light; low complexity. Several Traffic Management procedures were in effect. The sector altimeter was 2984. There was scattered moderate to heavy precipitation around. I had Turbo Commander at 170; southeast bound; with clearance to deviate around some precipitation that was dissipating around [a Class E airport]. I had an E145 inbound to [a Class C] airport; northwest bound; out of FL220; initially descending to 110. I looked and recognized the two would be close; and stopped the E145 at FL180. Obviously this was the main mistake; as with low altimeters; FL180 is not usable for separation. Still; the aircraft were approximately 15-20 miles apart; and not likely to merge. I was focusing more of my attention on an overhead chart; looking for ideal fixes to give to two other aircraft for routing that would be around any precipitation as well as complying with other special routing's. I found the fixes I wanted to use; and looked back down to the scope. The E145 was nearing the Turbo Commander at this time; and I called traffic to both aircraft; when they were approximately 8-9 miles apart. The E145 advised me he had the Turbo Commander in sight. I had utterly forgotten about the low altimeters as far as my altitude assignment to the E145; until I had finished calling traffic. At that point; besides mentally kicking myself for making such a rookie mistake; I turned the E145 to the Left; and descended the Turbo Commander to 160 and gave him a turn to the left as well. Of course it was too late to preserve separation at this point. Reviewing my bone head move in FALCON; even if I had not stopped the E145 at FL190; I could STILL have preserved separation by using Visual to descend the E145 through the Turbo Commander's altitude. The E145 was well behind the Turbo Commander on their courses. Partially in self disgust; and in the name of full disclosure; I advised the Controller in Charge that I had lost separation due to low altimeters. No conflict alert went off; of course; as the aircraft were shown at FL180 and 170. I continued the Turbo Commander's descent down to 150 and turned him back on course; as well as continuing the E145's descent to 110 and sending him on towards TLH. Normally I draw on the scope; or make some other reminder for myself to not use FL180; but I had been in a rush when I first took over the position earlier; issuing routing around the precipitation that had been at [the Class E airport] earlier (I was at Sector X maybe 25-30 minutes prior to this point.) On top of that; was I on the trailing end of that initial busy period; and was probably more relaxed than I should have been. Prior to the point of the incident; I had been doing a lot of coordination with two adjacent approach controls on a handful of issues; as well as vectoring aircraft. The most obvious; at least for my personal benefit; is to make a greater note of low altimeters. For basically a decade I have been writing something on the scope; or in some way signifying to myself for added emphasis that FL180 is unusable. Today I did not do that; and of course it ended up in a pointless loss of separation. Other controllers may or may not enjoy more scope clutter; but in this case it was definitely warranted for me; and I recommend it to other controllers who may be in the habit of forgetting the altimeter settings in the heat of the moment. The other recommendation would have been to use the other; more elegant option; and to just issue a Visual Separation clearance to the E145; since the pilot had already called the Turbo Commander in sight. That second mistake bugs me a great deal more; as it was the obvious; less jarring turn/descent option. In hindsight I should have done this immediately; and of course only thought about it after I had turned the aircraft; and in review of the incident.




Synopsis: Enroute Controller failed to remember the very low altimeter settings and non availability of FL180; resulting in a loss of separation.