Today’s lesson is in decent calculations. The long hand way. This is so everyone has a better understand of what is actually taking place, and how to better plan. I figured I’d do this by all the different responses Here. This can also be used for a stepped decent, it just needs to be calculated differently.

**Glide path**

For this all we are doing is figuring out the sides of a right triangle.

H = 30,000 feet

D = ?

GD = 90 NM

1NM = 6076 feet

30,000 feet ÷ 6076 = 4.94 NM

4.94² = 24.40

90² = 8100

8100 - 24.40 = 8075.6

D = √8075.6

D = 89.86 NM

So now how do we find the glide path angle?

So now we take out GD which is 90 ÷ that with our height which is 4.94 then *sin¹*.

So 4.94 ÷ 90 = .054

*sin¹* .054 = 3.14°

So now what if we knew our glide slope, but didn’t know how far out to start our decent?

So for this let’s say we are going into EGLC with a glide slope of 5.5° we are cruising at 30,000 feet again.

So first we need to put our 5.5° back into something we can use.

*sin* 5.5° = .096

So now we need to figure out what 4.94 ÷ ? = .096

So 4.94 ÷ 51.5 = .096

There for 51.5 is going to be our GD at the same height as before for a 5.5° decent.

Our ground distance for this would be?

4.94² + D² = 51.5²

24.4 + D = 2,652.25

2,652.25 - 24.4 = 2,627.85

√2,627.85 = 51.26

**D = 51.26 NM**

So the reason I drew this all out is because the basic height X 3 = your distance to descend isn’t always an accurate way of doing it. It works for a 3° glide slope but nothing else.

**How fast should my V/S be?**

Alright so taking what we learned from above we are also going to apply it here a bit. The question you need to ask prior to asking how fast should my feet per minute be? We need to ask **What should my feet per NM be?** this is where you will get your true answers. So in RL we figure out our ground speed by timing our known distance then figuring it out. Or if we have a GPS installed it does the work for us.

Since I don’t want to leave you guys short let’s just quickly do a ground speed equation.

So we traveled a 50NM leg in 10:00 what is our ground speed?

50 × 6 = 300

So we where traveling 300 kts per hour.

*Easy right?*

So now that we know our ground speed, and we know our distance to start decent for a particular glide slope. Now we can figure out our feet to loose per NM.

Let’s say our airport elevation is 1,000 feet. So we are cruising along at 30,000 feet which puts the distance to loose at 29,000 feet.

30,000 − 1,000 = 29,000

We are traveling at 300 Kts per hour, and our distance for the 3.14° glide slope is 90 NM.

We first figure out our feet/NM

29,000 ÷ 90 = 322.2

We need to loose 322.2 feet per NM. Now let’s figure out our FPM. (*Feet per minute*)

300 ÷ 60 = 5

We took 300 kts divided that by 60 minutes which gave us 5 NM a minute.

322.2 × 5 = 1,611

Then we take our 322.2 feet we need to loose per mile, and times that by how many miles we go in 1 minute which gives us a FPM of **1,611**.

As our altitude decreases along with our speed our FPM is going to change as the speed decreases. However our Feet per NM will always remain the same.

So if we do the short cut of GS × 5 =

We get 300 × 5 = 1500

Close but still not 100% right?

So as you can see the short cuts do help in quickly planning. I made this so you can all have an actual understanding of how to come up with the correct answer. By no means am I saying don’t use the short cut. If your going to use the short cut though you might as well know how it’s really calculated.

**Aviation shortcuts**

How to calculate ToD rule of thumb

If you guys find more shortcuts feel free to post them in the comments, and I’ll add them.