Question: Is there a formula to calculate a desired altitude at a specific waypoint during descent?

That’s a good question

Use https://fpltoif.com/ it should give you the descent profile and the altitude at specific waypoints 👍

Thanks. I use it regularly. But I would like to know if there is a math formula to calculate it. For example: how to be at 3,000 ft. in waypoint X if I am at FL, let’s say 33,000. This is my question. Thanks again for your kind reply.

So are you trying to figure out descent?

In that situation

[Altitude to lose * GS] / [V/S * 60]

Generally I use this to 11k or 10k depending on direction of travel.

Thanks very much. I will try it.

I don’t follow any equations for descent. Many descents IRL aren’t using one VS from start to finish. Many use step descents.

Here’s an example I use that follows somewhat closely to realistic descents.

Say you’re cruising at FL380, and you need to be at FL100 by X point. Initial descent at 800/min from FL380 down to FL300, takes 10 minutes. Then, second step descent at 1500/min from FL300 down to FL100, takes 14 minutes (round to nearest). Total time is 24 minutes. However, as you get lower you will lose a lot of speed, approximately 3 minutes. So take 24-3 = 21 minutes.

So at 21 minutes from your target waypoint, you will start your descent (~160nm away from destination). Adjust your descent speed when necessary, and especially if you start gaining speed (usually happens through FL320-FL350).

Thanks. Very clear explanation.

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