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PostPosted: Sat Sep 10, 2005 6:29 pm 
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Location: CYOP - Rainbow Lake, Alberta
I remember there was a topic about this a while ago, but dammed if I can find it, what was the formula used for figuring out when you should start to descend? Thanks



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Rick Butler

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PostPosted: Sat Sep 10, 2005 9:08 pm 
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My descent profiles for any of the "larger" a/c is actually quite simple. Multiply how many thousands' feet you need to descend by 3.5 . It is a general formula that seems to work out pretty much of the time.

20,000 to 3000' field level is 17 x 3.5 = 59.5 nm from touchdown.

This gives me a pretty constant 1800 per feet descent for most of my flights.

Hope this works for you....or someone can give you a "proper" formulation! :D

Harry



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PostPosted: Sat Sep 10, 2005 9:40 pm 
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Location: CYOP - Rainbow Lake, Alberta
That was the one I was lookin for, thanks Harry!!!



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PostPosted: Sun Sep 11, 2005 7:46 am 
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I spoke to a pilot of a real world Dash-8 and he indicated they use a basic formula:

Current Alt - Target Alt x 3 = NM's out to begin decent at a speed of about 240, decent rate of 1500 fpm.

So, example:

20000 - 5000 = 15000 * 3 = 45 NM's out to begin decent with the rate / speed indicated.

General rule that I have used on most aircraft and it seems to work as well most of the time.

Thanks!



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PostPosted: Sun Sep 11, 2005 4:19 pm 
Yup, the easiest rule of thumb (and the one I use) is to simply triple the altitude you have to lose.

But then you have to know how fast to decend....

Try taking your groundspeed, divide by 2, then add a zero.

e.g. 250kts/2 = 125, add a zero = 1250fpm descent rate.

This way, you won't be leveled off while still 30 miles from the airport and trying to figure out what went wrong ;)


Darryl: That's exactly what's done in the "real world" ... Take your groundspeed and convert that to nm/min. Then figure out a time to descend based on a known rate of descent. Multiply your G/S (in nm/min) by the time to descend and the product is your TOD.

e.g. Flying 240kts G/S = 4 miles/min. We want to descend at 1500fpm and we have to lose, say, 10,000 feet. So.... 10,000 feet at 1500fpm = 6.6666666666666666666 minutes. At 4 miles/min, that's approx 26.7 nm (6.67*4). Pull the power back at 27nm, keep the VSI pinned at 1500fpm and you'll look like a star!

To add a level of preciseness, calculate your TOD from the point at which you will be commencing the approach: Remember that it could be 20 miles on "this" side of the airport, or 20 on the "other" side.

Oops: I'm getting a little off-topic from "rules of thumb" ... Doh!

Cheers!

Gary :cool:




Edited By Gary Madore on 1126456932


  
 
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PostPosted: Sun Sep 11, 2005 5:05 pm 
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Joined: Sun Oct 19, 2003 3:38 am
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Location: CYOP - Rainbow Lake, Alberta
Excellent information guys, I appreciate it!!!

Gary: That info on decent rate is great, I always wondered about a good way to calculate that and if there in fact was one, I will definately be using that now!!!



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