Enroute Controller Question

Discussion in 'Change to my Frequency...' started by jordane93, Dec 27, 2017.

  1. jordane93

    jordane93 Final Approach PoA Supporter

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    How do you guys determine what Mach number to assign enroute if you need separation. Do you have a computer that you plug in a Mach number and it gives you XX miles of separation or do you just ballpark it? I’ve always been interested in how ATC assigns an aircraft a number.
     
  2. Timbeck2

    Timbeck2 En-Route

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    Too long to post here but its a ballpark: http://tfmlearning.faa.gov/publications/atpubs/ATC/atc0507.html

    disclaimer: Not an en route controller but I know there are lots of things one can do with a radar scope to determine speed and heading assignments. As a terminal controller, I can only scratch the surface of it's capabilities.
     
    Last edited: Dec 27, 2017
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  3. luvflyin

    luvflyin En-Route PoA Supporter

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    It's been 15 years since I worked. Then there was no 'calculator.' You just had it in your head. It wouldn't surprise me if the 'computer' can do it now. Reminds me of something funny that happened once. Told a pilot to say Mach. He comes back with "seven point four." No thang, I knew what he meant. There was about 10-15 second pause and he calls up and corrects it to "point seven four." I wondered what the conversation in the cockpit was. "dude, you better tell him, he might run an F15 up our azz."
     
    Last edited: Dec 27, 2017
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  4. luvflyin

    luvflyin En-Route PoA Supporter

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    Just noticed who asked the question, you're a high flyer if I recall right. I can't remember the numbers. You're at 370 at say .74. What's the Mach that will give you the same TAS at 330?
     
  5. Larry in TN

    Larry in TN Line Up and Wait

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    That depends on temperature.

    330 KTAS is pretty slow. 330 KTAS, TAT M45 is M0.56.

    M0.74, TAT M64 is 423 KTAS

    M0.80, TAT M59 is 465 KTAS
     
  6. luvflyin

    luvflyin En-Route PoA Supporter

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    FL370 and FL330. What are the Mach's to have the same KTAS. Assume same temperature, or whatever the typical difference is up there. Is it still roughly around 3 degrees F per thousand up there?
     
  7. Larry in TN

    Larry in TN Line Up and Wait

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    I don't understand the question. The same KTAS as what?

    Mach varies with temperature so it's not the altitude that matters, it is the temperature. KTAS varies with temperature and pressure.

    As you climb at a constant indicated airspeed, your true airspeed and mach number increases. At some point you reach either the desired, or the limiting, Mach number and transition to Mach.

    As you descend at a constant mach, indicated airspeed increases and true airspeed decreases. At some point you either your desired, or the limiting, indicated airspeed and transition to airspeed.

    Those transitions normally occur in the FL280 to FL330 range, depending on the conditions and airspeed/mach desired.
     
  8. luvflyin

    luvflyin En-Route PoA Supporter

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    I'm not talking about climbing and descending and transitioning. One airplane is at FL330 doing .74 at xxx TAS. The other airplane is at FL370. What Mach must he be at to match the FL330 planes xxx TAS. Just curious, I was a Center controller and used to do this but I can't remember if I gave the high plane a few hundredths less or more than the low guy.
     
  9. Larry in TN

    Larry in TN Line Up and Wait

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    I don't know. You can problem find some online calculators which you can use to find it by trial and error. With temperature and winds aloft being variables, I'd bet that controllers are just comparing ground speeds and adjusting the Mach assignments to get what they need.
     
  10. dmspilot

    dmspilot Pattern Altitude

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    Answer from a current ZDC controller:

    It's an art. There is no computer to tell us what Mach number. Mach numbers have to be lower at lower altitudes if we are spacing with an aircraft higher.

    e.g.
    Fl400 assigned Mach 0.80
    Fl300 assigned Mach 0.76 to keep the same speed

    But all of this depends on winds and aircraft type too.

    And that is all by chance. If a Cessna C750 is leading the pack at Mach 0.89 then this is nice for those behind. If a 737-300 is number 1 at max Mach 0.76 then this creates frustration for those behind (even for the 737-600+s that cruise between 0.78 to 0.80).

    It also depends on the miles-in-trail restrictions of the receiving facility. Sometimes it's 10 vs 25, sometimes it changes quickly from 10 to 20 or from 20 to 10, etc., or sometimes there are none.
     
    Last edited: Dec 27, 2017
  11. Kritchlow

    Kritchlow Final Approach

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    Mach is based on temperature. Not gospel, but generally temperature above FL350 is constant, thus Mach is constant. I’m guessing that’s why controllers assign, and want to know Mach above that altitude vs speed.