Simviation Forums

I saw this post a very long time ago with a math equation on when to start a decent. I was wondering if somoene knew of the post or the equation?

thx

Sounds like you mean the 'Rule of Three', Davesok.

If so, aeroplanes tend to take 3 miles distance to descend 1,000 feet. So if you're at say 30,000 feet, start your descent at (30 X 3) 90 miles out.

Lets say that your travelling at 250 kias, your around 100kts from the airport and at a height of 30,000 feet.

Firstly work out how long it is going to take you to get to the airport:

100 Divided by 250 = 0.4

To turn the answer into minutes by x it by 60:

0.4 x 60 = 24

So it will take you roughly 24 minutes to reach the airport travelling at 250 kias.

Now to work out the rate of decent:

30,000 dived by 24 = 1250

So if you decend at a rate of 1250 fpm (feet per minute) at a speed of 250kias from 30,000 feet at 100 kts from the airport you should be roughly at a low altitude when you reach the airport.

This is only a rough estimate of what rate to decend at due to the fact you might change speed during the decent, you made get vectored a long way to the airport which will increas your distance to the airport etc.....

So on your decent just keep checking how far away you are from the airport and what altitude are you at and increase or decrease your rate of decent to suit it.

... or install the descent calculator gauge from the downloads section into your panel!

Thx ppl. most informative.

Nav was close to what I rememeber, but EGNX gave the workings. tyvm

Glad it helped, Davesok.

There's no need to be too scientific about it in Flightsim - in real life ATC will sometimes tell you to come down to a certain height by the time you reach a specific point, which calls for calculations -  but that sort of thing doesn't happen on here!

The main thing is probably to start letting down earlier rather than later - getting down to approach height a little early is usually no problem, but it's a bind having to lose height in a hurry because you're too high too close in.

A couple of 'tricks':-

1. In a new aeroplane, it's a good idea to put it on autopilot, reduce the height setting, see what rate of descent the autopilot defaults to, and use that as a guide. From memory, jetliners are usually about 1,800 ft./min., Cessnas etc. about 800. You can get a quick guide to appropriate rates of climb that way as well.

2. As soon as you start your descent (assuming you're flying a jet) it's a good idea to bring your airspeed down to 250 knots indicated. Obviously you'll be going a lot faster than that in the thinner air at height, but it ensures that you'll be sensibly slowed down by the time you get close in and start organising your landing.

The BOD (Beggining Of Descent) is actualy FL/10*3+10 that is if you fly at 30000ft you are actualy in FL300 which makes:
300/10=30, 30*3=90, 90+10 =100 miles inbound. Or in more simple words:
thousands of feet * 3 + 10 to find the BOD point.

Then about the descent rate it is based on your ground speed (GS):
GS/2*10 feet per minute. For example say your ground speed is 280 knots, 280/2 = 140, 140*10 = 1400 FPM

If you follow the above at all times you will always be dead right on your approach.