Descent formulas

CLARKGRSWOLD

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  • If you are 200 miles out @ 310, ATC wants 110 @ 40 DME, when do you descent ( I WAS THINKING 80 MILES BEFORE THE 40 DME, ANY INSIGHT ? )
  • At 2000 feet on localizer, 5 miles back, where do you expect glideslope to be (This one is confusing 5 miles back from where ? Typical 3:1 here however )
 

RJLoser

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Thousands of feet to descend: 20
Times 3
=60 miles out from fix
Ground speed times 5
GS:500kts
=60 miles out at 2,500fpm
 

Stifler's Mom

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Start down at 100 DME (20K feet to lose) 20 x 3 = 60 miles. 5 x's your Ground Speed for the decent rate.
 

Flyer1015

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  • If you are 200 miles out @ 310, ATC wants 110 @ 40 DME, when do you descent ( I WAS THINKING 80 MILES BEFORE THE 40 DME, ANY INSIGHT ? )
  • At 2000 feet on localizer, 5 miles back, where do you expect glideslope to be (This one is confusing 5 miles back from where ? Typical 3:1 here however )
You live and die by the gouges. AA isn't hiring, but will soon. Don't be a 100% Gouge guy.
 

WWEfan

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Just use the banana bar. Sheesh.
 

blesko

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  • If you are 200 miles out @ 310, ATC wants 110 @ 40 DME, when do you descent ( I WAS THINKING 80 MILES BEFORE THE 40 DME, ANY INSIGHT ? )
  • At 2000 feet on localizer, 5 miles back, where do you expect glideslope to be (This one is confusing 5 miles back from where ? Typical 3:1 here however )
[/QUOT
Glide slope will be above you as you must intercept below the GS due to a possible side lobe of the 90/150 Hz amplitude phases.
 

ChickenSled

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  • If you are 200 miles out @ 310, ATC wants 110 @ 40 DME, when do you descent ( I WAS THINKING 80 MILES BEFORE THE 40 DME, ANY INSIGHT ? )
  • At 2000 feet on localizer, 5 miles back, where do you expect glideslope to be (This one is confusing 5 miles back from where ? Typical 3:1 here however )
[/QUOT
Glide slope will be above you as you must intercept below the GS due to a possible side lobe of the 90/150 Hz amplitude phases.
What? What college are you pretending you went to?
 

Salukipilot4590

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Point nose down, go down.
 

JustaNumber

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Thousands of feet to descend: 20
Times 3
=60 miles out from fix
Ground speed times 5
GS:500kts
=60 miles out at 2,500fpm
Your math is easy and works well. However, if you want to be even more precise (and be able to explain why the math works), divide the altitude you want to lose by 300. (In the example, you get 20,000/300=67 miles out). This is because if you are descending on a 3 degree glidepath, you are descending 300 feet down for every 1 NM you go forward. Hence, the altitude to lose divided by 300 feet gives you the miles it would take to lose that altitude.
 

JustaNumber

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Also for the descent rate, it can be a little easier to take half the ground speed and add a zero. It's the same thing as multiplying by 5.
 

BoilerUP

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Somewhere, a 727 captain is rolling in his grave at the loss of the mental 3:1 descent planning calculation...
 

Abernathy

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Somewhere, a 727 captain is rolling in his grave at the loss of the mental 3:1 descent planning calculation...
Blame overly-enthusiastic engineers. Like we really need an airplane to tell us when to go to Idle...and they found a great way of having it annunciate it to you. And we still have people running the thing off the runway and careening into buildings because they didn't go to idle. The damned thing tells you!
 
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RJLoser

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Your math is easy and works well. However, if you want to be even more precise (and be able to explain why the math works), divide the altitude you want to lose by 300. (In the example, you get 20,000/300=67 miles out). This is because if you are descending on a 3 degree glidepath, you are descending 300 feet down for every 1 NM you go forward. Hence, the altitude to lose divided by 300 feet gives you the miles it would take to lose that altitude.
Dividing numbers that large in my head makes me feel ill.
 

veneratio

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VNAV Baby :) But yah, take the number of feet to lose times it by 3

At FL Level 350 / Cross X Fit At 15,000 feet = 20,000 feet to descent (20 x 3 = 60), Also, you might want to add 10 miles for the initial pitch down and level off delay. Than, take the GS and divide it by 2. So 500 KIAS / 2 = 250 (add a 0) = 2500 FPM.
 

WWEfan

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VNAV Baby :) But yah, take the number of feet to lose times it by 3

At FL Level 350 / Cross X Fit At 15,000 feet = 20,000 feet to descent (20 x 3 = 60), Also, you might want to add 10 miles for the initial pitch down and level off delay. Than, take the GS and divide it by 2. So 500 KIAS / 2 = 250 (add a 0) = 2500 FPM.
Wow. What airplane are you flying that does 500 knots indicated? ;)
 
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