I want to know what my fpm is based on the following

 

I'm travelling at 180KIAS

I need to drop 900'

withing 21.6 nm

 

what is my fpm and what is the formula to calculate that?

There are several ways to do it. The way that I was taught is pretty simple...

 

Descent Point = 3 X Your Required Altitude Loss (in 1,000s of feet)

 

Descent Point = 5 X Ground Speed

 

Example: If I'm at FL370 and want to descend to 3,000' the descent point would be 37 - 3 = 34 X 3 = 102 NM. The initial descent rate, assuming a 400 knot GS would be 400 X 5 or 2000 fpm.

 

There are other rules of thumb that work just as well and perhaps are even more accurate, but this works close enough and is simple enough to do in your head. Every few thousand feet just take a look at your ground speed to revise your descent rate and you'll be right on the money every time. It works well to quickly calculate crossing restrictions as well. This was all we had to use in the days prior to VNAV. Hope this helps you.

 

Or the really easy way, and more accurate, is DesPlan (Descent Planner): http://aerotexas.com/desplan/index.html

doesn't answer my question..you gave me the formula to compute my distance.. distance is a given 21.6

I want to know my fpm

the answer is obvious.

 

Time is distance divided by speed.

 

Rate of descent is height change divided by tiime.

At 180, it will take you 7 minutes and 12 seconds to cover 21.6 miles, so that means a descent rate of 125 fpm to drop 900 feet in that distance.

 

You can calculate that by dividing 180 by 60 to give you how many miles you will cover in a minute, which is of course 3. Then divide 21.6 by 3 and you get 7.2, which gives you how many minutes it would take to cover 21.6 miles. Now you divide 900 (your desired descent) by 7.2 (how many minutes you have in which to do it) and 900 divided by 7.2 gives you 125.

 

Alternatively, you can just stick it in a dive, level off 900 feet lower and report you are at the new required altitude LOL Remember to announce over the PA: the louder you scream, the faster we go.

 

Al

I want to know what my fpm is based on the following

 

I'm travelling at 180KIAS

I need to drop 900'

withing 21.6 nm

 

what is my fpm and what is the formula to calculate that?

 

The way you are asking that question implies that you want to start a descent now to reach that altitude 21.6 miles away. In which case you would do it as mgh and chock has described and descend at 125fpm. There are no handy pilot rule of thumbs for that. However, that is not the way a pilot would look at the problem. In real life, we would first decide where to begin a 3 degree slope descent to be at that point at the proper altitude. Then we would decide what rate of descent is required to maintain a 3 degree slope.

 

We use altitude to lose (in thousands) multiplied by 3 to approximate a top of descent point. In your case, it would be 0.9x3= 2.7 miles. A 3 degree slope begins approximately 2.7 miles away. As I am sure someone will be apt to point out, multiplying by 3 is not an exact calculation of a 3 degree slope, however, it works well enough when you are busy flying an airplane with the rest of your mental capacities. Along the same lines, the approximation of the rate of descent required for a 3 degree descent is to multiply your groundspeed by 5. In your example, since you only gave airspeed, and not groundspeed, assuming that you are in a no wind situaiton, 180x5=900 fpm. Begin a 900 fpm descent 2.7 miles away from the point at which you need to be 900' below.