How to calculate a right descent???

[david2000]

What would be the right procedure to make a perfect path and descent at the speed and altitude desired? to a fix, airport or approach procedure.
thanks

[ricecakecm]

Take the altitude you need to lose and multiply it by 3, then drop the last three zeros. That gives you the distance at which you need to start down. Then, take your groundspeed, divide it in half, then multiply that by 10 to get the rate at which you'll need to descend at.

So, for example:

You're told to cross XYZ VOR at 5,000 ft. You're currently at 9,000 ft with a ground speed of 150 knots. You have 4,000 feet to lose, so you'll need to start down at 12 DME prior to the VOR (9000-5000 = 4000, 4000 x 3 = 12000, 12000/1000 = 12). You'll need to descend at about 800 FPM (150/2 = 75, 75 x 10 = 750FPM).

If you were told to cross 10 before XYZ, you'd need to start down at 22 DME (12 + 10).

Most people will add in 1 or 2 DME as a fudge factor to the distance at which you need to start down.

[ppragman]

Originally Posted by ricecakecm

Take the altitude you need to lose and multiply it by 3, then drop the last three zeros. That gives you the distance at which you need to start down. Then, take your groundspeed, divide it in half, then multiply that by 100 to get the rate at which you'll need to descend at.

So, for example:

You're told to cross XYZ VOR at 5,000 ft. You're currently at 9,000 ft with a ground speed of 150 knots. You have 4,000 feet to lose, so you'll need to start down at 12 DME prior to the VOR (9000-5000 = 4000, 4000 x 3 = 120000, 12000/1000 = 12). You'll need to descend at about 800 FPM (150/2 = 7.5, 7.5 x 100 = 750FPM).

If you were told to cross 10 before XYZ, you'd need to start down at 22 DME (12 + 10).

Most people will add in 1 or 2 DME as a fudge factor to the distance at which you need to start down.

It depends, but other than a few things, thats smack on.

I find that in a turbine, 3 to 1 works great, but go to fly a piston again and use 3 to 1 may end up shock cooling it. In pistons I use 4 to 1 or greater if its really cool out. Keep in mind the conditions, but this is just one more reason why turbines are waaaaaaaaaaaaaaaaaayyyyyyyyyyyyy beter than pistons. Flight idle any time you like.


Pat

[USMC-SGT]

T.L.A.R

That Looks About Right

seems to work pretty well...or vnav on the fms

[awacs94]

How do you adjust the calculation for head / tailwind?

[ctab5060X]

Originally Posted by awacs94

How do you adjust the calculation for head / tailwind?

Formula uses groundspeed to begin with. No need to adjust the calculation.

Originally Posted by ricecakecm

Take the altitude you need to lose and multiply it by 3, then drop the last three zeros. That gives you the distance at which you need to start down. Then, take your groundspeed, divide it in half, then multiply that by 10 to get the rate at which you'll need to descend at.

So, for example:

You're told to cross XYZ VOR at 5,000 ft. You're currently at 9,000 ft with a ground speed of 150 knots. You have 4,000 feet to lose, so you'll need to start down at 12 DME prior to the VOR (9000-5000 = 4000, 4000 x 3 = 12000, 12000/1000 = 12). You'll need to descend at about 800 FPM (150/2 = 75, 75 x 10 = 750FPM).

If you were told to cross 10 before XYZ, you'd need to start down at 22 DME (12 + 10).

Most people will add in 1 or 2 DME as a fudge factor to the distance at which you need to start down.

[awacs94]

Duhhhh... yeah thanks. One of those moments.