Wind correction for descent distance ?

[flyzhz]

The MGR Rules of Thumb say:
20) Top of Descent (Idle thrust - 3° descent path)
TOD (NM) = ∆ FL/3
FL 280 down to 2000 feet
TOD = 260 / 3 = 87 NM
25) Wind correction for descent distance
Wind Corr (NM) = 10% for each 40 kt component
Example Thumbrule 20) with 20 kts Tailwind
Add 58 to 87 = 92 NM (here it is a mistake?)

[Wizofoz]

Yes.

It should be add 5%. Just a typo.

[captjns]

I use the KISS method. Alt. * 3 + 15. Works good lasts along time.

[Wizofoz]

Not when you've got 100kts up the chuff.

[junebug172]

1 mile for every 10 knots of TW.

[dss3000]

Hi
Interesting I use 3xAlt then 1/2 gs for VSI eg FL 430 GS 540 Kts gives me
129 miles out at 2700 fpm initially then adjust VS as needed to stay on profile.

[flyzhz]

Hi
UTC+8:D

[flyzhz]

This is what I'm trying to do after finding 2 mile is too much.

[flyzhz]

ALT×3，is too hard to me,cause ALT is changing :ugh:so ,I use ALT(100m)
I think if we use GS/2,no need to correct TW.is that because TW make it impossible to get GS/2 ?

[Oceanic815Pilot]

VSI for these calculations is either GSx10/2 or straight GSx5. Use whichever you can wrap your brain around. This method corrects for the TW and HW but as stated before may not be the best option.

If you are flying an Airbus with 100 knots or more on the tail and you are descending from a very high altitude in managed descent you may get close to the MMO limit even with the protections when the winds start shifting during the descent.

With a 100 knots HW your VSI may only be 1700fpm or so and may not be the most optimum descent even though it is 3 degrees.

Now flying VIP pax I will often do the math for a more shallow constant rate descent if I'm not expecting turbulence or CB's in the descent so it's more comfortable for them.

Just my 2 cents.

Oceanic

[flyzhz]

Thank you a lot!:ok: I just unstand how dose the TW and HW affect the desced.I used to just ignore the HW,that is bad.

[Capt Fathom]

Wind Correction 1nm per 10kts ???

If your descent takes 20mins, that's a third of an hour.
Say a 60kt average wind component, for a third of an hour is a 20nm correction.

[Virtus]

What I use is change-in-altitude (in thousands) multiplied by 3 for the distance back to start the descent. You can add whatever factor you feel appropriate to that - if I have to go to the opposite side of the airport to do the approach then I might subtract from that but if I'm doing a straight in then I'll add maybe 10 miles to get slowed and configured for landing.

E.g., you're cruising at FL280 and landing at 1,000' ASL. The equation is: 28 - 1 = 27 x 3 = 81 nm.

For the descent rate don't worry too much about it - your GS will constantly change on the descent. To get an estimate of what descent rate you need use: GS x 10 / 2; or GS x 5.

Throughout the descent check your flight path by verfiying you're still on a 3 degree path. As you pass through FL200 you should be 57 nm from your destination (plus your factor). At 15,000' you should be 42 nm back. Then adjust your descent rate as appropriate.

With a little practice it becomes easy. I don't bother at all with a wind correction factor.

[junebug172]

Capt Fathom

Join Date: Jun 2001
Location: FNQ ... Still!
Posts: 2,058
Wind Correction 1nm per 10kts ???

If your descent takes 20mins, that's a third of an hour.
Say a 60kt average wind component, for a third of an hour is a 20nm correction.


----------

What?!? No, you're making it too complicated which you can't do with RoTs. If my 3:1 gives me a ToD of 100nm and I see a TW of 50kts, I'm going to add an extra 5nm to my 100nm for a new ToD of 105nm.

Don't add a time function into this.

And say I need to slow from 300 to 250 for a restriction too, I would then add another 1nm per 10kts of slowing for a grand total ToD of 110nm.

Of course, while in the descent, you're always checking your 3:1 calc and fine tuning the descent as needed. Good RoT for a required descent rate is to add a zero to your GS and divide by two.

Ex:
400 kt GS
Add 0 to get 4000
Divide by 2 to get 2000 FPM.

KISS so it can be done quickly. You won't get perfect results but you'll be well within what you need to get the job done. Don't let the slop keep you up at night and don't try to figure out more accurate numbers because you undermine the entire reason for having a RoT.

[junebug172]

Sorry virtus, didn't see your post.

[Natstrackalpha]

you meant 60 right? Not 57 . . ?

Or am I wrong again . . .?
==============================
F.A.O: junebug
Ex:
400 kt GS
Add 0 to get 4000
Divide by 2 to get 2000 FPM.

KISS

Junebug, I just halve my groundspeed. ---->KeepItEvenSimplerS
==================================

Hi
UTC+8:D
ZSPD? Singapore, HK, RCTP?==go to bed!

==============================
With changing winds (ie 98% of the time) the continual reasessment is really the only way to go
:ok::)

[junebug172]

If you just halve your GS, you only get 200FPM with a 400kt GS. You need to add that zero at some point.

And I'm in the American southwest. I guess I should change that time correction.

[Natstrackalpha]

One would feel very alert - doing 400 in a 2000 fpm descent . . .!

[AtomKraft]

The way this is done, is to do it continuosly.

Start down when you fancy it. The wind aloft is going to change as you descend, but have a stab at it.

After a couple of minutes, reasess the situation- high or low?

Make a correction, then after another minute or two, have a another wee look (I use the old track miles x 3 rule- but you might have a better system).

Really clever methods will work- but still-air only.

With changing winds (ie 98% of the time) the continual reasessment is really the only way to go.

[junebug172]

Start down when you fancy it.


Ummmmm, no.


The wind aloft is going to change as you descend,

It is. But the 3:1 works extremely well to get you started and the wind corrections make is even more accurate. The corrections you then make as you're checking your profile every few thousand feet are small.

but have a stab at it.

Again, no. There is no "stab at it". There are RoTs that get you very close to profile.

[junebug172]

One would feel very alert - doing 400 in a 2000 fpm descent . . .!


Not really. Its not that uncommon.