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.