How hard is it to fly a plane? It's simple...

He also left out some steps in the process, which is neither surprising nor really wrong considering the intended audience. IMO a better way to make it as an equation is:

distance to descend (dtd) = current altitude (c-alt) - desired altitude (d-alt)  
Time to descend (ttd) = dtd / rate of descent (rod)  
Start of descent (sod) = ttd / (ground speed (gpd) / 60)

(assuming no wind conditions)

To shorthand the above, it ends up being a simple series of steps to follow and all you need to know ahead of time is your present altitude, your desired altitude and how fast you'd like to descend (which generally is going to be some constant speed) .

1) c-alt - d-alt = dtd
2) dtd / rod = ttd
3) (ttd / gpd) / 60 = start of descent distance

Or as an example:

I want to arrive at at KWVI at 1500 feet as my desired altitude and my current altitude is 4500 feet, that means my distance to descend is 3000 feet.

If I want my rate of descent to be 500 feet per minute, then the time to descend will be 6 minutes.

At a ground speed of 90 knots, and assuming zero wind, I'd be traveling 1.5 miles across the ground every minute. That would mean that in the 6 minutes it took to descend, I'd travel 9 miles.

 
Lest anyone think I just wrote this up to show off, this was actually a good mental exercise to do it all out so please let me know if I got this wrong somehow or with any questions. Also, I like to add 3 miles at the end, since it is better to be a little bit early to the desired altitude than late, which would just make the example's solution 12 miles instead of 9 miles.

/r/flying Thread Parent Link - airfactsjournal.com