Descent formulas

[FO 4 Life]

Okay, rather than arguing, "you're simply wrong" back and forth, let's do the math and put this question to bed. I had always been told that you descend 300' per nautical mile in a 3 degree descent. If this is true, then my way would be the most precise, and your way would be "close enough." But let's see.

So let's break out the trigonometry: we're looking at a right triangle with the base being 1 nautical mile, and the height being the altitude to lose (we'll call it "x"). We would use:

tangent (angle)=opposite/adjacent
tan 3=x/(1 NM)
tan 3=x/6000'
x=6000(tan 3)
x=314.4
Therefore, in a true 3 degree descent, you descend 314.4 feet per nautical mile.

Given your example of a descent of 18,000',
your method = start down 54 miles out (18x3)
my method = start down 60 miles out (18/3)
actual answer = start down 57.25 miles out (18,000/314.4)

You're under by 3.25 miles; I'm over by 2.75 miles. Therefore, my method is a tiny bit more precise (and a little more conservative for those who don't want to bust an altitude), but it's not "simply wrong." Bottom line, use whichever method you want; there's nothing wrong with discussing why each method works, though. I'd suggest that doing the mental math for multiplying by 3 is often just as "making stuff way too hard" as dividing by 3.

Also, the poster who mentioned the need to recalculate descent rate as you descend due to changing winds made an excellent point. This is sometimes a factor, but the change in groundspeed due to the slowing of true airspeed as you descend is always a factor.

"Beware the lessons of a fighter pilot who would rather fly a slide rule than kick your ass!"​

— Commander Ron 'Mugs' McKeown, USN, Commander of the U.S. Navy Fighter Weapons School.​

 

[COOPERVANE]

Thats an easy answer. An RJ cant get up to FL310.

 

[BentOver]

Must be a real fun trip flying with someone who is going to critique you on how you started your decent too early or a bit too late..

BTW...I take the altitude to lose x 2 and then double that...20,000 ft I would start down 80 miles out...gives me plenty of wiggle room to adjust if needed.

In a normal 310kt a/c that would require about a 2000'/min rate....

Before anyone breaks out the calculator,... a 30 year major captain taught me that easy method.... works great... Except in a .92 a/c you'd need about 2500'/min......

Moral of the story for all the "my way is the only way" guys.... There's more than one way to skin a cat.

 

[JustaNumber]

Must be a real fun trip flying with someone who is going to critique you on how you started your decent too early or a bit too late..

You're making a giant logic leap here, that just because we're discussing math details here, that I would be an a$$ in the cockpit. How did you make that connection?

If you're flying, I'll do my mental math on my own, and as long as you don't wait for a descent rate that exceeds the aircraft's capability, it's all good. Like I said, do what works for you; this thread was supposed to be about discussing different ways of doing it, because apparently the OP was confused.

To those who say I'm making it way too complicated, I dare you to explain in detail exactly how you multiply 18,000 x 3 in your head. Since no one memorizes their 18 times tables, I guarantee you're doing some mental long multiplication or addition in your head. If you were capable of describing exactly how you did it, your description would look just as complicated as anything I've written, and then other immature posters who couldn't understand it would make fun of you.

Beam me up, Scotty, there's no intelligent life left on this thread.

 

[wmuflyguy]

• 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 )

If your 5 back from the runway at 2000'. I'd expect the GS to be 500' below me. 3 to 1 rule. 5*300=1500'

 

[zasca]

Or just take your miles per min in GS, your standard descent rate in FPM and voilà.... That's how far out you start.

 

[WWEfan]

Or just descend immediately at 4,000 fpm every time and you will always make that crossing restriction.

 

[BentOver]

Or just descend immediately at 4,000 fpm every time and you will always make that crossing restriction.

Along with that.... I like to wait until center asks me if I'm gonna make the restriction...."yup"... "down 4000!"..."ey ey Captain down 4000!" I'll be in my quarters if anyone needs me........XO has the comm."

 

[Hikoushi]

To those who say I'm making it way too complicated, I dare you to explain in detail exactly how you multiply 18,000 x 3 in your head. Since no one memorizes their 18 times tables, I guarantee you're doing some mental long multiplication or addition in your head. If you were capable of describing exactly how you did it, your description would look just as complicated as anything I've written, and then other immature posters who couldn't understand it would make fun of you.

Beam me up, Scotty, there's no intelligent life left on this thread.

Simple grade-school times tables work just fine. No fancy degrees required, no complicated procedures necessary.

3:1 off of 18,000:

Drop all the zeroes. You don't need them. Work with 18. Break it down to 8 and 10.

8 x 3 = 24.
10 x 3 = 30.
30 + 24 = 54.

No long anything required.

Greatest respect to those with exceptional skills in complicated mathematics. I am not one of them. K.I.S.S.

 

[Stifler's Mom]

4 pages on a 3:1 descent formula????

I came up with this invention that is a circular disk arranged to revolve on an axis, and can be used in transportation. I need help coming up with a name for it. Wheel is already taken though.