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.

 

[Baronman]

The official XJT method, start down once you get the restriction, no matter what and annunciate, "ahh well...I'm just gonna start down now" ....Then, once you realize you're 40 miles out with 2 thousand to go, bleed it back to a 100fpm descent to make it look like you meant to do that ;-)

 

[flythere]

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

hopefully there will soon be some 60+ 737 capt's rolling over as well!!!

Dec 16th, 2012.......

 

[Salukipilot4590]

Fine, I'll give a serious reply this time:

From 280 to 180 = 10,000'

2,000FPM (F'k the damn fuel savings....this ain't the damn space shuttle)

10K @ 2000FPM = 5 minutes

What's our f'k'n groundspeed? 405KT?

405KT is how many miles per minute....6 something right?

WRONG!

ROUND DAT ISH UP! Its now 7 miles per minute (420KT....heh 420....is 7NM per minute brah)

...so now you have SIMPLE MATH....

5 minutes times 7 miles per minute =


COME ON SUSAN



35 miles.


Since you're not going to want to throw -2000FPM into the AP controller and throw Dave's coffee into Becky's chest region you're going to add 5NM to that for a lead in and lead out....



So.....



Now you've got 40 miles...plus or minus whatever the cap'n wants to add since he's doubting your abilities as a competent pilot....you damn kids.

 

[Uncle Bunkle]

Pull thrust to idle and pop the speed brakes, works every time.

 

[Pogue Mahone]

Four pages....only on Flight Info.

 

[xjhawk]

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.

A long time ago I was told to add 10 miles to the 3:1 rule when you are going below 10,000 for the slow down. If I am not mistaken V nav does not take that into account. Nor does it take into account the short legs where it never seems to cool down in the CRJ so when full I never do idle decent and like to keep some air going back there. Below 10 I then start the APU for the slam dunk I can expect at half the places we fly. If I hear that future interviews will be a bunch of unrealistic math problems involving two trains and a collision I will be sorely disappointed. Is there truth to the rumor that Delta asks a question that goes something like this....."if on approach at speed 250 and are told there is an A/C in front of you going 210, how long before you over take it?" My answer will always be "never, I will slow down before that happens."