question about when to descend

taseal

taseal

Well-Known Member
Is there a quick little formula that you can use that will help with figuring out when to descend?

lets say you are on airway V3, and you need to cross some intersection at 3,000. you are currently at 11,000 and you are doing about 120 knots.

at what DME from that intersection will I need to start my descent, and what should my rate be?
 
Well, your required rate of descent is dependent on the DME at which you start your descent, but here is a formula to use.

[SIZE=-1]groundspeed / 60 * (feet to descend / rate of descent) = how many miles out you should start your descent

you can rearrange that formula to solve for required rate of descent
[/SIZE]
 
alright...

100gs/60 x (8,000/1,000)

6x 8 = 48 miles out?

so I need to descend 48 miles out at 1,000 fpm to descend 8,000 feet? wow, sounds excessive
 
taseal said:
alright...

100gs/60 x (8,000/1,000)

6x 8 = 48 miles out?

so I need to descend 48 miles out at 1,000 fpm to descend 8,000 feet? wow, sounds excessive
Click to expand...

Naw, I ain't no math whiz, but roun' last time I done a checker on 100 divided by 60, it sure weren't 6.
 
taseal said:
Is there a quick little formula that you can use that will help with figuring out when to descend?

lets say you are on airway V3, and you need to cross some intersection at 3,000. you are currently at 11,000 and you are doing about 120 knots.

at what DME from that intersection will I need to start my descent, and what should my rate be?
Click to expand...

taseal said:
alright...

100gs/60 x (8,000/1,000)

6x 8 = 48 miles out?

so I need to descend 48 miles out at 1,000 fpm to descend 8,000 feet? wow, sounds excessive
Click to expand...

it's too late for me, but I don't think you are even using the numbers you provided in the example.....and could be missing a number, but since 1000 came up

120(gs)/60 * (8000/1000)=
2 * 8
A=16 miles out

if you went with a 1500fpm descent

120/60 * (8000/1500)
2 * 5.3
A=10.66 miles out


alright, 120 was your airspeed so maybe:

100/60 * (8000/1000) assuming 1000 was your fpm rate change that and the answer changes.
1.7 * 8
A=13.6 miles out


I could be way off though. have been flying, so I haven't been planning.... :(
 
frog_flyer said:
Naw, I ain't no math whiz, but roun' last time I done a checker on 100 divided by 60, it sure weren't 6.
Click to expand...


yeah, I had to read it a few times to make sure I wasn't going loco either...:crazy::p
 
taseal said:
Is there a quick little formula that you can use that will help with figuring out when to descend?

lets say you are on airway V3, and you need to cross some intersection at 3,000. you are currently at 11,000 and you are doing about 120 knots.

at what DME from that intersection will I need to start my descent, and what should my rate be?
Click to expand...



found this little ditty courtesy of google, fwiw

1. Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent. 2. Now take half your ground speed. This is your rate of descent in hundreds of feet. Ex. If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
Click to expand...

Using that with your numbers from above:

you are flying at 11000ft at a ground speed of 100kts and you need to descend to 3000ft, the difference is 8000ft. Multiply 8*3=24 miles out you must begin your descent. Half your ground speed is 50, add a zero, and 500 ft per minute is your rate of descent.
 
3 to 1 rule for when you need to descend. Need to come down 5,000'? Start 15 miles out. Gotta come down 10,000'? Start 30 miles out.

How fast? GS/2*10. Moving at 400 knots? Come down at 2,000 FPM. Moving at 500 knots? Come down at 2,500 FPM. Need to meet a speed restriction? Add 10 miles.

Works in my plane at least :)
 
JEP said:
found this little ditty courtesy of google, fwiw



Using that with your numbers from above:

you are flying at 11000ft at a ground speed of 100kts and you need to descend to 3000ft, the difference is 8000ft. Multiply 8*3=24 miles out you must begin your descent. Half your ground speed is 50, add a zero, and 500 ft per minute is your rate of descent.
Click to expand...

jtrain609 said:
3 to 1 rule for when you need to descend. Need to come down 5,000'? Start 15 miles out. Gotta come down 10,000'? Start 30 miles out.

How fast? GS/2*10. Moving at 400 knots? Come down at 2,000 FPM. Moving at 500 knots? Come down at 2,500 FPM. Need to meet a speed restriction? Add 10 miles.

Works in my plane at least :)
Click to expand...



ahhh.... interesting

I knew about the 3 to 1 rule, but I thought it was 'engines at idle' and 1,000 descend rate. I was confused because every plane will descend at a diff speed lol

in a 172, that would be a pretty shallow descend

120/2 = 60x10 = 600

but I guess it works eh :p
 
To add a small complication, when carrying passengers in a non-pressurized airplane, do you really want to descend at 1500fpm? I'd recommend sticking to 500fpm or less for passenger comfort. :)
 
jtrain609 said:
3 to 1 rule for when you need to descend. Need to come down 5,000'? Start 15 miles out. Gotta come down 10,000'? Start 30 miles out.

How fast? GS/2*10. Moving at 400 knots? Come down at 2,000 FPM. Moving at 500 knots? Come down at 2,500 FPM. Need to meet a speed restriction? Add 10 miles.

Works in my plane at least :)
Click to expand...



exactly! Dont need to make it harder on yourself than it is. Up here flying around in NYC airspace the dumb pilot math above always works and above all is a fast way. Talking to some bros from my squadron that fly for the airlines they use this method as well. Other than that we just put it in the box and let it do the math for us ;)
 
To make my life easier I just plan for a 500fpm descent. That keeps me from shock cooling and lets me keep my speed up.

In the 310 I was flying here is how it worked:
9,000/500 = 18
18 min x 180 = 54 NM

Of course, that Gamin 530 made the second part of that calculation easy! :D
 
I have managed to hit crossing restrictions within 100 feet of assigned altitude as we passed of the 0.1 DME distance from our point to cross using the 3 to 1 rule. It works... and it works well! I use it all the time, rather than any of the VNAV glideslope guidance we can program into the FMS.

Altitude to lose x 3 = DME to start down

For instance, you are at 11,000 feet and need to cross XYZ VOR at 3,000 feet. That is 8,000 feet to lose. Multiplied by three, 8 x 3 = 24. You start down at 24.0 DME to cross the VOR at 3000 feet.

If you were flying 120 knots, divide your groundspeed by three, and that is your descent rate. If you had a 40-knot tailwind, your groundspeed is 180 knots. Divided by two, add a zero, and your descent rate should be 800 feet per minute. Like someone said, in an unpressurized airplane, that is a comfortable descent rate. Anything over 1,000 FPM is pushing it with passengers.
 
Ugh that's all way too much math.

The CF keep it simple, stupid, method:

Distance away from point (in minutes) = D
Altitude to lose = A

A/D = fpm descent required.

Reverse engineer it around if need be.
 
I sometimes begin my decent 2 miles out at 7,000ft in my trusty old turbine suburban. It's funny when the tower asks if you will make it.
 
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