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View Full Version : ATC speeds and spacing

02-18-2014, 12:07 AM
I have a question for any ATC guys and gals out there.

I was wondering when you are spacing aircraft from different arrivals or when you have to re-sequence an aircraft due to a missed approach, etc. if you use any quick rules of thumb for figuring out the necessary spacing? For instance, you need 1 more mile separation between aircraft A and aircraft B, you have 20 miles to get the separation. What does the speed difference need to be between the aircraft for their current ground speeds?

I love math but I couldn't do those calculations in my head for each separation event that takes place inside New York or any other busy sector without raining metal from the sky.

Can anyone offer any insight how you personally approach it?

Thanks! And my hat goes off to you, you are all amazing at what you do.

02-18-2014, 03:13 AM
Kind of tough to compress this into a short post...

In my world--Approach/Departure control, nominally 17,000 and below--spacing is achieved with usually vectors, then maintained by assigning matching speeds to the airplanes involved. Another technique is to keep departures from increasing past 250kts above 10,000, until the desired space from the leading aircraft is met.

All of our displays show groundspeeds, so there is little or no math done in your head, you simply look for matching numbers.

02-18-2014, 06:04 PM
Match speeds and vector for spacing. The basic interval where I work is 4 miles. There are many variables including wake turb spacing, wind, pilot reaction time, fleet mix, and so on. This will usually collapse to about 3 miles inside the Final approach fix which is basic IFR separation. If you are on a missed approach where I work, you'll end up in the downwind and handled like any other arrival. I will try to build a gap so you don't have to go to the end of the line. Therefore I vector for an 8 mile interval and fill it with your airplane. If I want to put 2 aircraft in it becomes 12 miles and so on.

02-18-2014, 06:27 PM
I've often wondered why controllers use turns instead of speed sometimes. There are times I'd rather slow down than get two 60 degree turns.

02-19-2014, 03:39 AM
I am not an ATC person, but I believe the answer to your question may be found in FAA JO7110.65U. Google it, it it the bible for ATC.

02-19-2014, 03:39 AM
I've often wondered why controllers use turns instead of speed sometimes. There are times I'd rather slow down than get two 60 degree turns.

Well, if you're 5 miles behind someone we need you 20 miles behind, it would take all day to make the difference with speed control, whereas those two big turns will do it very quickly.

02-20-2014, 01:01 AM
Thanks for the replies...

Do you do the math for the vectors to figure out how many more miles you need or do you just SWAG it?

02-21-2014, 04:53 PM
Other than counting miles for the needed interval there isn't much math. Where I work, we can get huge effects from the wind. Different effects on opposite downwinds. One base is usually faster than the other. You may have a large ground speed difference at different altitudes. It can get real fun with the downwind showing say 180kts, then the base increases to 250 with the final moving at 140. Sometimes you just have to use altitude and trust what you know.

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