Descent formulas

[JustaNumber]

I know what you're saying, and technically you are of course correct. But in reality you aren't just going to make one mental calc and call it good for 60 miles, and just hope for the best. Things change along the way... Winds, TAS, temp, etc. So of course a good pilot is going to monitor his/her progress along the way for the crossing restriction. In the example given by the OP I would use 3:1 and begin my descent 60 miles out from the restriction. At about 30, I'd check again. I should roughly be around FL210. If no, then I'd adjust my descent rate accordingly. I'd check again at 20, and again at around 10.

VNAV or not, it's a very good idea to keep yourself in the loop. I used to fly an airplane that had more VNAV bells and whistles than most airliners out there (ProLine 21), but we still kept a check on things with a little bit of mental math.

It's pretty easy to get complacent with this stuff using VNAV and ATC in the USA, but even then there are times that you'll be left WAY too high if you're not used to quickly calculating a TOD in your head. Outside of the US, you'll definitely need to do this.

Definitely great points, and you're right about how easy it is to get complacent.

 

[BoilerUP]

The mental math for cross-check starts getting a bit complicated when you're flying into a 50kt headwind and program a 4.5° descent to a crossing restriction...but in the end, 3:1 combined with a little of that "pilot sh!t" will get ya where you need to be.

...just don't forget 1 mile for every 10kts you need to slow...

 

[platinum750]

you guys r so gay

 

[CRJ puppy]

My eyes are bleeding...

 

[Flyer1015]

How much wood could a woodchuck chuck if a woodchuck could chuck wood?

 

[JustaNumber]

Dividing numbers that large in my head makes me feel ill.

Last Transmission was broken and stupid.

you guys r so gay

My eyes are bleeding...

How much wood could a woodchuck chuck if a woodchuck could chuck wood?

It's kind of sad that when you want to talk about pilot stuff on a pilot forum, particularly math formulas in a thread clearly titled, "Descent FORMULAS", that some of the responses would be so imbecilic. Whatever happened to intellectual curiosity, or even a vague desire to learn something new or become a more-informed pilot? Trying to be cool about being a moron surprises me most, though.

 

[CX880]

It's gone

 

[wrxpilot]

It's kind of sad that when you want to talk about pilot stuff on a pilot forum, particularly math formulas in a thread clearly titled, "Descent FORMULAS", that some of the responses would be so imbecilic. Whatever happened to intellectual curiosity, or even a vague desire to learn something new or become a more-informed pilot? Trying to be cool about being a moron surprises me most, though.

I was going to refrain from further comment on this thread, but you're absolutely right. I was actually fairly impressed that we were having such a discussion on Flightinfo, then the B-team had to pipe in. It's sad really, but not surprising I guess.

Those that have been proclaiming the downfall of our profession certainly do have all the proof they need here... Aviation is starting to attract some real winners.

 

[RJLoser]

It's kind of sad that when you want to talk about pilot stuff on a pilot forum, particularly math formulas in a thread clearly titled, "Descent FORMULAS", that some of the responses would be so imbecilic. Whatever happened to intellectual curiosity, or even a vague desire to learn something new or become a more-informed pilot? Trying to be cool about being a moron surprises me most, though.

Well..... my first post was correct (not moron) and on topic. The poking took place after you came to the table with smarty pants info. Relax a little.

 

[Clutch]

It's kind of sad that when you want to talk about pilot stuff on a pilot forum, particularly math formulas in a thread clearly titled, "Descent FORMULAS", that some of the responses would be so imbecilic. Whatever happened to intellectual curiosity, or even a vague desire to learn something new or become a more-informed pilot? Trying to be cool about being a moron surprises me most, though.

Personally, this stuff is private pilot garbage. But dont let me stop you guys from impressing yourselves.

 

[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'