Understanding Speed Control/ Calculating Miles Per Minute

user

Newcomer
Aug 15, 2014
3
0
1
Good Day StuckMic Forums,

Can someone please break down understanding speed/ computing miles per minute for me. I understand that you take ground speed drop the zero and you get the number of miles the aircraft will go in 6 min. What I need help in understanding is taking that calculation and applying it to sequencing and spacing on final.

thank you.
 

Genot

Trusted Contributor
Feb 7, 2010
534
10
18
A Dark Dark Room
That's not how a final works at all. Lets say we do this your way and you assign a speed of 180 to every aircraft on final. Not many aircraft can land at that airspeed so they WILL slow. What will they slow to? You don't know. So even if you had time to run calculations (you won't) you still wouldn't know what speed to plug in in your formula. Make sense?

Depending on wind (do we have a strong headwind close to the surface?) type aircraft (business jets tend to slow more than commercial airliners) and wake turbulence you'll aim to have a certain distance between aircraft and aircraft flying the same speeds down the final once that distance is obtained. The gap between aircraft will be one that allows the aircraft in front to slow, causing compression, but not so much that you lose separation.

How big is that gap? Depends. Two 737s down a final same speed with no strong winds, 4 miles is enough, even a little less is fine if you are aiming for 2.5. 737 following a Citation? I'm probably going to want 5 miles.

Edit: I see you posted this in the Academy section too. For OKC, 5 miles between small/small and large/large. If wake turbulence is involved add one mile for ever mile over 3 you need at the threshold.

If you have speed differences your best bet is to keep the slow guy in close and build a gap. If you have a plane doing 120 and another on final doing 170/180 add a mile for every two minutes the slow plane will be on approach.
 
Last edited:

NovemberEcho

Epic Member
Dec 8, 2010
4,388
68
48
Long Island
Are you in TSEW? They had us doing all sorts of math to figure out gaps and whatnot for sequencing. It was extremely distracting and I felt like I was doing physics while controlling. Whenever just tried doing it there wayi would fall behind but when I did it the way I had been doing it for the past 6 years I did fine. No one does that stuff in real life. Know your Sep minima and get it done.
 

lowapproach

Epic Member
Oct 29, 2010
1,316
33
48
WV
60 knots is a mile a minute, and 30 knots is a mile every 2 minutes. Let's say that you have two aircraft, A and B, that must be separated by 3 miles all the way to the runway threshold.

If B is 60 knots faster than A (e.g., gaining a mile every minute), and A is 7 miles in front of B, then you will lose separation between A and B after 4 minutes if speeds remain constant. If A lands in 4 minutes or less, then they are separated. If not, then you will need to do something like assigning a speed.
 

password

Senior Member
Oct 19, 2011
184
0
16
On of the instructors taught me this in OKC and it is one of the things I still actively use in the field. It is confusing at first but I have gotten so used to it I can almost do instantly now. Let's try and explain this....

I think he called it the 10 knot rule. Aircraft A is showing 100 knots on a 5 mile final. Drop the 0 and make it a fraction 5/10 which equals 1/2. Aircraft B is showing 180 knots. An 80 knot overtake. For every 10 knot over take add 1/2 mile plus wake turbulence plus 1 or 2 extra depending how close you want it to be. So in this case 4+3(this depends on the type aircraft so I am just using 3 for now)+1 equals 8 miles. That is how far aircraft b needs to be behind aircraft A when aircraft A is on a 5 mile final. And you will have 3 at the threshold.

Example 2. Aircraft A indicates 140 on a 7 mile final. See the theme here? Use easy fractions. 7/14 = 1/2. Aircraft B indicates 180. 40 knot overtake. So 2+4 (different type aircraft) + 1 equals 7 miles. If aircraft B is 7 or more miles behind aircraft A, you are golden.

Seems difficult and it is at first. The more you use it the easier it gets and the faster you get at it. Just my 2 cents.
 

RobertB

Senior Analyst
Aug 18, 2008
868
6
18
Rough calculations:
60 knots- 1 mpm (mile per minute)
90 knots- 1.5 mpm
120 knots- 2 mpm
150 knots- 2.5 mpm
180 knots- 3 mpm
210 knots- 3.5 mpm
240 knots- 4 mpm

So if you assign aircraft A 180 and you're trying to catch him with aircraft B assigned 210, you're gaining 0.5 miles every minute they fly. So if you needed to close up a 5 mile gap then you'd have to let them fly 4 minutes to go from 5 miles to 3 miles. Typically (especially on feeder) it takes a jet aircraft 1 mile to slow 10 knots after you assign a change in speed, but that depends on wind, altitude, etc. So if an aircraft is showing 350 at 12,000 feet and I need to slow them to 210 knots (which is showing 280 knots across the ground), then I need to plan on roughly 7 miles for that aircraft to slow to show that 70 knot difference.

As far as hitting gaps on final, most people here use 2 mile range rings and we are trying to vectoring to a quarter mile. When you're hitting a gap, you aren't sequencing aircraft A (say 12 mile final) and aircraft B (in downwind) as much as you are sequencing aircraft C (on a 18 mile final) to aircraft A. If you built an appropriate gap (6 miles in above example is fine) then you are expected to keep the gap and hit the gap. So when aircraft B is at the 10 mile range ring in the downwind, he's in the turn and most likely slowing to 180 knots (what most use here when turning base). I'm then calling traffic for C to look for B and then I'm calling traffic on B to look for A. As far as when to turn them from the base to final, you take their ground speed (say 180 is showing 180 in above example) and I'm turning them with a 20-30 degree intercept track when aircraft B is roughly 1.8 miles from the final. If he was going 210 knots, I'm turning him 2.1 miles from the final.
 

FM_Weasel

Senior Analyst
Dec 9, 2008
991
7
18
That "C to A" method is something I've had half explained to me several times and I still don't get it. Never been anywhere busy enough to need it either so I guess I'm ok for now.
 

Jax

Senior Analyst
Nov 17, 2010
869
32
28
N90-EWR
Rough calculations:
60 knots- 1 mpm (mile per minute)
90 knots- 1.5 mpm
120 knots- 2 mpm
150 knots- 2.5 mpm
180 knots- 3 mpm
210 knots- 3.5 mpm
240 knots- 4 mpm

So if you assign aircraft A 180 and you're trying to catch him with aircraft B assigned 210, you're gaining 0.5 miles every minute they fly. So if you needed to close up a 5 mile gap then you'd have to let them fly 4 minutes to go from 5 miles to 3 miles. Typically (especially on feeder) it takes a jet aircraft 1 mile to slow 10 knots after you assign a change in speed, but that depends on wind, altitude, etc. So if an aircraft is showing 350 at 12,000 feet and I need to slow them to 210 knots (which is showing 280 knots across the ground), then I need to plan on roughly 7 miles for that aircraft to slow to show that 70 knot difference.

As far as hitting gaps on final, most people here use 2 mile range rings and we are trying to vectoring to a quarter mile. When you're hitting a gap, you aren't sequencing aircraft A (say 12 mile final) and aircraft B (in downwind) as much as you are sequencing aircraft C (on a 18 mile final) to aircraft A. If you built an appropriate gap (6 miles in above example is fine) then you are expected to keep the gap and hit the gap. So when aircraft B is at the 10 mile range ring in the downwind, he's in the turn and most likely slowing to 180 knots (what most use here when turning base). I'm then calling traffic for C to look for B and then I'm calling traffic on B to look for A. As far as when to turn them from the base to final, you take their ground speed (say 180 is showing 180 in above example) and I'm turning them with a 20-30 degree intercept track when aircraft B is roughly 1.8 miles from the final. If he was going 210 knots, I'm turning him 2.1 miles from the final.

I guess this is what we all do...just never thought of it that way. I never bothered to try to learn the math behind it. Back when I was a trainee (25 years ago), we were taught to just "eyeball it". :)
 

atcbrownie

Trusted Contributor
Jun 14, 2008
661
9
18
Warrenton Va
These methods require way too much calculation and thinking about things out of your control. For example you have no idea when both aircraft involved are going to slow to approach speed. If the lead aircraft slows to approach speed at the marker while the trailing aircraft holds 170+ to 2 mile final all the calculations that you just made were of no help to you. I never calculate speed vs distance I just eyeball it. From my experience those who do all the calculating can't handle the same amount of aircraft as those that eyeball.
 

meanjoe

Trusted Member
Apr 23, 2011
490
6
18
By then tower can see them and plays Captain Save-a-ho, as mentioned above...or you get one back.
 

JStewart66

Junior Member
Nov 19, 2008
132
3
18
N90 - EWR
www.stuckmic.com
The easiest way it was explained to me was "5 miles will get you 3, 7 will get you 5, etc." That means if you have an airplane at 170KIAS followed by one at 220KIAS and you have 5 miles, slowing that back guy down at the 5 mile point will give you three. More or less, mostly more.

Also I eye-ball it. :)


Sent from my iPhone using Tapatalk
 

FM_Weasel

Senior Analyst
Dec 9, 2008
991
7
18
Maybe in a perfect no wind environment. Also who is doing 220 on a 5 mile final?
He meant 5 miles behind the preceding aircraft, not a 5 mile final.

The numbers he provided are good places to start. Of course you adjust them as needed for wind as you see how the compression plays out on any given day.
 

RobertB

Senior Analyst
Aug 18, 2008
868
6
18
Who the hell uses 220 knots?!?! (Coming from someone who was taught 250,210,180, slower for speeds in final box)