# Math help

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This is pissing me off as I can't seem to figure this out the way I think it should be done. The formula is: feet to descend * (groundspeed/(60 * miles)) = required rate of descent

So I have to descend 45,000'. Ground speed is 1,000 MPH and I'm 180 miles away. Looks like this:

(45,000 * (1000/(60*180))=

I keep getting 486,000 which is definitely not right. Google says 4166 which does seem right. I thought I knew PEMDAS, but I guess I don't. I start out multiplying 60 with 180. Then divide by 1,000. Take that answer and multiply by 45,000. What am I doing wrong?

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AH! Got it figured out like Goggle. Had to go left to right. Very confusing. Wouldn't it be (45,000 * 1000) / (60*180)= instead? My resource is here: https://www.flightsimbooks.com/foi/chapter5.php
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Your problem was dividing (60*180) BY 1000. Should be the other way around: divide 1000 by (60*180), then multiply that result by 45000.
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Is this the correct syntax or should it be different? (45,000 * (1000/(60*180))=
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I believe so, inner brackets first, then next bracket set, etc.
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So when you say inner brackets, I would multiply 60 and 180 first? I think I had to multiply 45,000 by 1,000 first then take that number and divide it by the answer of 60*180. Which just doesn't make sense to me.
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I'm not too math smart it seems, but oddly, I like math.
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And the result is 'feet per minute' of descent speed/rate. That is what you are looking for. IE: The RATE (ft/min) of descent. The speed stays the same (1000 kts). If you change your SPEED then you have to re-calculate. Best to use a table if you can find it.

Chuck B

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1. 45,000 * (1,000/(60*180))=

2. 45,000 * (1,000/10,800)=

3. 45,000 * 0.09259= 4,167

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Nicely displayed, jarmart. And CRJ, you want to remember to allow a few miles to slow down after descent, and also to remember the 250 Kt speed limit (in the U.S.) below 10,000 feet, so you'll need two calculations, perhaps.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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1. 45,000 * (1,000/(60*180))=

2. 45,000 * (1,000/10,800)=

3. 45,000 * 0.09259= 4,167

Now this is what I'm talking about. Straight and to the point for my math incompetent mind. LOL It has been 18 years since I got my GED and then high school at Job Corp. I'm so damn dumb now. Amazing how you forget things you don't use. But now that I fly a lot in the Sim, math has become a major part of it. In fact, I often calculate the time in minutes it will take me to reach my top of descent. There are other calculations I'd like to do, but I don't know how to figure them out. Like how much fuel will I burn in x minutes? I have that gauge that tells me how much fuel I'm burning, but I'd like to know how much fuel I'll approximately use before touch down. I currently mostly fly in the F22 Raptor and boy does it burn fuel at mach 2.35 and FL500. HAHA I know the real-life F22 wouldn't cruise at that speed, but I do it in the game. It's just a game after all and I can get from point A to B in no time flat. I have flown around the world three times already. Crossing the Pacific takes foreeeever. Even though I island hop between Guam, Wake Island, Midway and Hawaii. Then from Hawaii I either go to KLAX or KSAN.

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If you want to play with math in aviation, then the Aviation Formulary might interest you. There are formulas for wind triangles, great circle navigation, turns and pivotal altitudes, and much, much more.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!