## When should i descend?

Controlling your aircraft, using the autopilot etc.

### When should i descend?

I have been flying some long flights recently on a 777-300er at around 38000 feet and 0.84mach. I have had problems with descent: I go past the airport or I end up doing a really steep dive. I have tried the descent calculator but it has proved useless. Could someone please provide some sort of a procedure they follow with altitudes and speed. All help is appreciated.
seb2222

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### Re: When should i descend?

The most common rule of thumb for descent planning is to divide your flight level by three to get the distance. So at FL380, you would start your descent at 127nm out. At FL360, you'd start at 120nm and so on. You might want to make up a little table for quick reference. Something like this, extended up to FL390:

FL150 50nm
FL120 40nm
FL090 30nm
FL060 20nm

For vertical speed, take the first two digits of your groundspeed, divide by two and put two zeroes on the end for your descent rate in feet/min. So if you look at the flight display and see you are travelling at 400kt groundspeed, you'd descend at 2000fpm. If you were descending at 360kt, your descent rate would be 1800fpm. You'll need to keep adjusting this as you descend (because your groundspeed decreases if you maintain a constant airspeed) but it will give you a reasonable descent rate.

Switching to mach is also something you need to consider. Mach speeds are normally used above about FL280-FL300 and you'd switch back to IAS during your descent. If you descend at constant mach, your airspeed will increase as you get lower. I usually start my descent at cruise mach and when the indicated airspeed reaches the speed I want to descend, say 280-300kts, I'll switch to IAS and keep that airspeed in descent.

Some aircraft give you more help than others. These are the basic rules I use for airliners.
sanhozay

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### Re: When should i descend?

That is a perfect writeup Sanhozay.

I personally make it a bit simpler and use a fixed desc rate of 2000 together with the 3x(alt/1000) rule. So for me top of desc from fl 380 is at 3x38=114. For straight in i put on some 20 extra miles so fl 380 would be about 134 miles.

During desc i keep watching the development, because front wind versus back wind can make quite a difference. I have some fixed check numbers in mind for that. Going through fl 300 i should be at about 100 mi, through fl 200 at about 70 mi, through fl 150 at about 50 mi and through fl 100 at about 30 mi. If i find myself too high or too low at any of these distances, i adjust desc rate a bit up 2500 or down 1500.

@seb2222
With a bit experience and monitoring the desc, you'll find a way that fits you best. A good starting point for fl 380 is to begin with a desc of 2000 ft/min from just under 140 mi. Then note where you are at several distances (i.e. 100, 70, 50, 30 mi) and if you are not satisfied, start desc a bit different next time i.e from 130 or 150 mi.

A very important note of Sanhozay is not to forget to switch back to IAS when it gets above 300 kts. Personally i use 310 kts below fl ~300.

Kind regards, Vincent
KL-666

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### Re: When should i descend?

Thanks I will try all these things. Just one question. What speeds do you recommend throughout the descent process
seb2222

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### Re: When should i descend?

I like as fast as possible, so my calculations are based on that. For safety against overspeed in sudden wind changes i am always just below as fast as possible:

above fl ~300: mach 0.83
below fl ~300: 310 kts
below fl 100: 250 kts

It is a safety rule to fly not faster than 250 kts below fl 100, to match more the speeds of slower props down there.

Kind regard, Vincent
KL-666

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### Re: When should i descend?

Thanks
seb2222

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Joined: Sat Aug 16, 2014 4:31 pm

### Re: When should i descend?

I guess the other thing to mention is that you might need speedbrakes in the mix somewhere, especially for that reduction down to 250kts. Some aircraft have more drag than others and the need for speedbrakes differs. Now I think about it, my preference for adjusting the VS throughout the descent probably comes from flying aircraft with low drag. By the time the 250 limit comes, the descent rate is below 1500fpm and it's easier to slow the aircraft.
sanhozay

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### Re: When should i descend?

That is indeed correct Sanhozay. i.e. The 777 has quite little drag. There are 3 techniques to slow down:

- speed brakes
- level off for a while to slowdown (i.e. desc 500ft/min for a while)
- start slowdown in time

I prefer the last. With the 777 i have to start slowdown at fl 140 and go to 1500 ft/min to be at 250 kts at fl 100. Then stay at 1500 ft/min as not to speed up again. The change in desc rate does not mess up my rough calculations, because i also go slower.

Kind regards, Vincent
KL-666

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### Re: When should i descend?

I know this is an old thread but I would just like to know if I am getting the hang of descent as I can't seem to get it right every time. I tend to use the following:
Speed 90 knots -3000ft -450fpm-10nms . Or 120 knots-3000ft-300fpm-10nms. Of course I adjust for increases in speed etc but there are times when I have descended to far and have to recover. Are my figures correct.
Robertfm

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### Re: When should i descend?

i just ran this search https://www.google.com/search?q=flying+average+descent+formula and it gave me this right at the top...
If you multiply your descent angle (1 degree) by your miles-per-minute, then add two zeros to the end (x 100), you'll have your FPM descent rate. So in this example, if you're flying at 120 knots, you're traveling 2 miles-per-minute (MPM) (120/60=2). Feb 24, 2018

How To Calculate Your Descent Rate To MDA | Boldmethod

maybe it helps? maybe not since it is only for figuring out your descent feet/minute rate...

or maybe this one is better? https://en.wikipedia.org/wiki/Rule_of_three_(aeronautics)

but yeah, your craft, speed, and distance (plus any intervening terrain) are all in play...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."

wkitty42

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### Re: When should i descend?

Thanks, I had read both what neither does is say at what distance you should start descent. The second one intimates you double the time required for descent at 120knots as the distance to start. In their example 64nm. So given that example 3000ft divided by 300fpm = 10minutes of descent @ 90knots that should be 15nm BUT
Robertfm

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### Re: When should i descend?

I use this and it works well.
http://aerotexas.com/desplan/desplan.php

legoboyvdlp

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### Re: When should i descend?

Thanks for that. It is giving me the same answers I derived manually. Heaven only knows where I'm going wrong.
Robertfm

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### Re: When should i descend?

yeah, i didn't spend much time looking... i see it as a simple (?) right triangle situation... the vertical line of the triangle is your altitude... the horizontal line is your distance from the touchdown point... the slope line is what you are going to follow and calculate... how to bring them together i'm not totally sure... that's why i generally fly around in the UFO

but yeah, if you set the angle of the slope line to 3 degrees for a ""normal"" descent, then you can work back for the distance at the various altitudes...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."

wkitty42

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Location: central NC, USA
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### Re: When should i descend?

I use the pitch function in AP and set it to the calculated FPM but the plane invariably gets to runway too soon or is too high on final. I take over about 3nm out but it can get messy.
Robertfm

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