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I’ve had to do some digging on this subject, as it was already in place when I hired in, so it was a fundamental part of my job (once I started working sectors) and not really something I thought about. According to my research, PCA was established on 4 March 1965, including all airspace over the Continental U.S. from FL240–FL600.
I started flying shortly afterward, so as my training advanced, so did conversations among fellow students as to the magical and sometimes mysterious workings of what was then known (although not by us) as the National Airspace System. In fact NAS (the acronym) didn’t enter my vocabulary until I was downstairs on the floor of the control room at ZJX, and by then it had become a synonym for the computer system then being tested.
I may do a separate article on the enroute computer system, but this is one is reserved for PCA, so let’s proceed. There are two other dates which I’ve not yet tracked down, and that is when the floor was lowered to FL180 in certain parts of the country (ZAU specifically, but ZOB and ZNY were included—possibly a couple of west coast facilities, as well), and when it was lowered throughout the Continental U.S. It was certainly still FL240 and above when I was in ZJX, as I can demonstrate below. A colleague, starting in the Agency just a month after me in ZAU, has told me it was always FL180 and above during his career.
A little operations background for the non ATC readers—there are defined regions of airspace in high altitude (above FL240, and usually above FL310) which can be delegated to DoD authorities for the purpose of ACM (air combat maneuver) exercises or GCI (ground controlled intercept) practice. One such area at ZJX was called Tarheel, as a large portion of it was over North Carolina, although we had a good chunk of it, too, over northern South Carolina.
When we released it, it was under the control of NORAD (North American Air Defense Command), and they had a callsign, but I no longer remember what it was—remember, I’ve been gone for >40 years. Much of our interaction was in concert with the SCANG (South Carolina Air National Guard) operating F-102s out of MMT, near CAE. We would get a call from BasOps (Base Operations) with a flight plan for usually two, sometimes more, F-102s off MMT to operate in Tarheel. When they departed, the CAE sector climbed them to FL230, handed them off to High, who climbed them to (typically) FL350, and then hand them off to NORAD.
After the exercise was done (an hour, or less, usually) NORAD would call CAE High, handoff the Deuces (controller talk for F-102s), and switch them over, usually at or descending to FL350. When they came on frequency it was customary to ask them if they wanted an enroute descent (radar guidance all the way back to base) or if they wanted to descend out of APC (as we called it, then, hence the parenthetical in the title) and cancel IFR. So long as the MMT weather (and general weather—that is, no overcast underneath them) was decent, they more often than not, elected to cancel out of APC.
So, we’d clear them down to FL240 and aim them generally toward MMT, and ask them to report leaving FL240 and canceling IFR. Because airplanes are airplanes, whether IFR or VFR (a concept, unfortunately lost on weaker controllers), I would always point out the fast movers to Low Altitude so they’d have an idea what/who they were.
So, that’s how I know that lowering PCA from FL240 to FL180 was done partially sometime before 1968, but not nationwide before roughly 1973. 1973, because I was gone by February, and it hadn’t happened in ZJX yet. This also, pretty much concludes the story of PCA. It’s no longer called that, anyway. In the great airspace reclass in 1993, it became known as Class A or Alpha Airspace. Positive control, nonetheless—every airplane on a clearance.
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