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In Jacksonville the bulk of our traffic was generally north/south, particularly on the East side, where I worked. In order to keep airplanes from banging into each other we followed the NOSE rule—North Odd, South Even. That is north (east) bound flights flew at odd altitudes (FL 250, FL 270, etc.) and south (west) bound flights flew at even altitudes (FL 240, FL 260, etc.).
The etc. in those parentheticals concerns the altitudes above FL 290. We used 1000 feet for altitude separation at FL290 and below but used 2000 feet for altitude separation above FL290. Since FL290 was odd, the next altitude, FL310, was even for ATC purposes despite the fact that numerically, it was odd. FL310, FL 350, and FL390 were all even altitudes, and FL330, FL370, and FL410 were all odd altitudes. It got so ingrained in your mind that you just automatically thought that way—instantaneously you would react to a northbound at FL310 as being wrong altitude for direction of flight.
One evening shift we had a team briefing (toward the end of the shift when traffic had died down, sectors were combined, and a large number of controllers could be released from the areas for such training) in which a scramble procedure in a new letter of agreement with FANG was being described. The procedure prescribed that the jets would launch off JAX, proceed eastbound out over the ocean on the 090 radial of the JAX TACAN to the 50 mile fix and then turn alternately north and south holding patterns. The flights going to the north were to hold at odd altitudes and we were told would start at FL290 at 2000 foot intervals, thus FL290, FL310…rrrrrrrrrttttt! What?!? FL310 is an even altitude! It took several seconds for me to realize they were talking numerical odd and even, as the south fixes were FL300, FL320, FL340, etc. which were altitudes we didn’t even use in day-to-day ATC.
It struck me as an interesting exercise in what one can mold their mind to accept as standard. I don’t know what controllers do nowadays with the Reduced Vertical Separation Minimum program which allows 1000 foot separation above FL290. I'm not sure I'll ever get used to reading about flights at FL340 or Fl380.
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