Climb gradients

[saxman66]

Say a runway with obstacles is given to you, and the charts say, maintain at least 16:1 slope to clear it. What does that mean exactly. Climb 16 feet per 100 feet on the ground, or 160 per 1000 feet on the ground? I always get confused when calculating these climb and descent gradients and converting it to feet per minute. Can some of you chime in? Thanks

Chris

[J-Wec]

Originally Posted by saxman66

Say a runway with obstacles is given to you, and the charts say, maintain at least 16:1 slope to clear it. What does that mean exactly. Climb 16 feet per 100 feet on the ground, or 160 per 1000 feet on the ground? I always get confused when calculating these climb and descent gradients and converting it to feet per minute. Can some of you chime in? Thanks

Chris

16 feet per 100 feet on the ground is the same rate as 160 per 1000 ft.

[fish314]

Actually you've got that backwards. If you needed a 16:1 slope to clear it, it would mean climb 1 foot for every 16 feet across the ground. In other words, the first number is the distance you travel forward, and the second number is the distance you travel UP.

Most climb gradients that I've seen, though are not given in simple ratios like that. Most of the time they are given in feet per nautical mile. When they are given in feet per nautical mile it means you climb that many feet for every nautical mile forward that you travel. (So if you are keeping track, that means that when the climb gradient is expressed as ft/NM it is expressed in the opposite order as when it is expressed as a ratio).

For example: 152'/NM means 152 feet UP for every nautical mile forward, but 40:1 means 1 foot up for every forty feet foward. Incidently, these two climb gradients are exactly the same. Try the math yourself. 6080' (1NM)/152'= 40!

Going back to the example that started this whole conversation, 16:1 is the same as a 380'/NM climb gradient.

[saxman66]

The reason I get 16:1 climb gradient is because that is what is displayed on Airnav.com's airport runway information. It'll say slope to clear the trees on the end of the runway is 16:1 or whatever. But yes, feet per nm is much easier.

[desertdog71]

Originally Posted by fish314

Actually you've got that backwards. If you needed a 16:1 slope to clear it, it would mean climb 1 foot for every 16 feet across the ground. In other words, the first number is the distance you travel forward, and the second number is the distance you travel UP.

Most climb gradients that I've seen, though are not given in simple ratios like that. Most of the time they are given in feet per nautical mile. When they are given in feet per nautical mile it means you climb that many feet for every nautical mile forward that you travel. (So if you are keeping track, that means that when the climb gradient is expressed as ft/NM it is expressed in the opposite order as when it is expressed as a ratio).

For example: 152'/NM means 152 feet UP for every nautical mile forward, but 40:1 means 1 foot up for every forty feet foward. Incidently, these two climb gradients are exactly the same. Try the math yourself. 6080' (1NM)/152'= 40!

Going back to the example that started this whole conversation, 16:1 is the same as a 380'/NM climb gradient.

I cannot expand or improve on this description and it is exactly correct.

Also goes for when you are driving and you see a grade that says for example 7% grade. That means a 7 feet change in elevation for every 100 feet traveled.

[fish314]

Originally Posted by saxman66

The reason I get 16:1 climb gradient is because that is what is displayed on Airnav.com's airport runway information. It'll say slope to clear the trees on the end of the runway is 16:1 or whatever. But yes, feet per nm is much easier.

If you are ever trying to convert it, just divide 6080 by the first number (16, for example). Why 6080? Because that's how many feet are in a nautical mile.

So 6080/16=380. Therefore 16:1=380ft/NM

And if you are trying to convert the other way (from ft/NM to climb gradient as a ratio) it's the same thing. Divide 6080 by the ft to climb per nautical mile.

So 6080/380=16. 380ft/NM=16:1

[NJA_Capt]

To expand further....
The only approach plates that I am familiar with express climb gradients as a percentage, not as a ratio. Altitude/ Distance = gradient

Ex KASE: The Lindz4 in ASE requires 460 feet/nm to 14000 MSL (a 7.6% degree gradient). rise/run = gradient.......460/6000 = 7.66%
Our AFM lists the gradients as a percent also.

To convert the gradient to VS, multiple the gradient x GS. (7.6 x 140 kts = 1064 fpm)