Need help calculating the basic things.

Hello, I have been playing IF for about two months now, flying from one point to another with moderate success.
But, I want to improve, and I need the help of someone, because what´s been posted on the Internet isn´t very clear (to me).

I would like someone to help me with:

  1. Ascent rate.
  2. When and where to start my descent.
  3. Descent rate.
  4. Descent time.

I´ve been flying mostly the B737-70, if that´s important.

Have a good day/night.


How to decend

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Ascent rate is generally from 2000-4000 fpm depending on the plane’s weight load and performance, weather, and how much of a hurry you’re in ;)


You can calculate it in three steps:

As an example I use some random values.

  • Descent speed 300kts: This means 300NM/h or 5NM/min
  • ROD -2000ft/m
  • Descent from FL120 to FL50

Step 1: Altitude to lose

12000ft - 5000ft = 7000ft

Step 2: Time needed for the 7000ft to lose

7000ft / 2000ft/m = 3.5min

Step 3: s = v * t

5NM/min * 3.5min = 17.5NM

So you are travelling a distance of 17.5NM until you are from FL120 to FL50 with these parameters.


First, you will need to use waypoints so you can see time and distance to the next waypoint and final destination. See my instrument dock below.

After that it is really easy.

Let’s use 20,000 feet as an example. 20 minutes from destination, start descending at - 1000 VS. Or 10 minutes before destination you can start descending at - 2000 VS. Because ground speed slows as you descend you will always come in early and will require to level off now and then, which is what you want.

You don’t want to exceed more than 2 degrees pitch down, because the cabin crew will be working until about 10 minutes before landing.

You can use the same system between waypoints.

I fly real world flight plans, and this system has never failed me. I can recreate an actual real world Alaskan Airlines flight between Portland and Seattle right to the minute.

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Here are two real world routes which are flown by Alaska Airlines on a daily basis. I applied the STARs to what I think is an easier to read format.

The blue on the right side is the KSEA to KPDX route. Climb to 20,000ft at BUWZO. After a couple minutes start descending to the indicated waypoints altitude and speeds using the time method above. The indicated waypoints are in IF, and I am pointing to where you will find them.

The route on the left side is heading north to KSEA 16R. Reach 20,000ft by COUGA. At KRIEG start descending at - 1000 VS and again use the time method between waypoints.

I think you will find this to be a really fun way to use the Sim :)


If I want to match the plane with the red rectangles at 3000ft, and I´m flying at 30000ft, I should do:

So I should start my descent at a 81 miles from the airport and be able to match the plane with the rectangles, right?

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You make sure you start the decent 81 nm on the end of the ILS! You don’t want to descend 81 Nm from the runway!

I did that yesterday, ended up flying the rest of the NM at 3000ft. Maybe starting descent some miles before, eg: 82-85NM?

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A real ILS is usually 3 nm…

3NM from what? The runway?

Yes correct the runway

Thomas you should post more of these if you have them. I’d love to fly some real routes.


So, If I get it right:

  1. Ascent rate: 2000-4000fpm depending on weight, performance, weather and if you are in a hurry or not.

  2. Where to start my descent: EG: from 30000ft to 3000 to match the red rectangles:
    27000 divided by 1000=27
    27x3= 81 (miles to start descending).

  3. Descent rate: no less than 2000fpm with a speed of 280kts or lower.

  4. Descent time: EG: substract 30000 (actual altitude)-3000 (target altitude)=27000ft.
    Divide the result with the descent rate: 27000 divided by 2000= 13,5 (minutes until 3000ft).

Thanks to everyone!

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Once you reach 10,000 feet reduce to 250 knots

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And after reaching cruising altitude I accelerate again to, lets say, 400knt?

That depends on you!

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  1. Ascent rate:
    The initial climb of the 737 is 15-18 degrees, until about 1,000-1500 feet above the runway. At that time you gradually lower your pitch to 10 degrees, retract flaps and allow your plane to increase speed to 250kts or less while below 10,000 feet. You ascent rate is whatever it is, generally between 2800-3200 VS. I should add, that as you gain speed your pitch will start decreasing as you continue to climb at 2800-3200 VS.

  2. Where to start my descent:
    Above 10.000 feet, altitude x 4 = distance needed.
    Below 10.000 feet, altitude x 3 + 10 = distance needed.

  3. Descent rate: should never be greater than - 2000 and it has absolutely nothing to do with air speed. Easiest way is to calculate it is to take your GROUND SPEED and divide it in a half. For example; GS 300kts divided by 2 = -1500VS

  4. Descent time: Time to destination, is whatever time it takes to get there.

Or you can just do what I suggested and you will get the same results.


Max cruise speed for a 737 is M 0.78.

Since the regions in IF are small I rarely get faster than M 0.62. I generally fly at the same speed at the actual flights.

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So for the descent I should take the Ground Speed (300), divide it by 2 (equals 150), and multiplicate it by 10 (equals 1500), right?

Is OK what the others said about the place and descent rate?

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