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Calculating the distance when to start descending.

Spinrecovery

Topic Author

Spinrecovery created the topic: Calculating the distance when to start descending.

Please correct me if i am wrong.
Lets say I am flying a cessna 172 doing 90 knots and I am on and outbound from station at 46 DME and I want to descend at 500 fpm from 5500 to 2500 which is 6 minutes.
So I use this formula: 90 knots * 6 minutes = 540, divide 540 by 60 9 NM. So I start start descending when I am 37 DME outbound.
Are there any quicker, easier, or ball park figures I can use to mentally finding out the distance I need to start descending?
Just curiosity, is it possible to use 1 in 60 rule to do this?
Cheers

Spinrecovery replied the topic: Calculating the distance when to start descending.

can I use this method?
groundspeed is 120 knots which is 2 NM a minute and I want to lose 5000 of altitude at 500 FPM which it will take 10 minutes, so take 2 *10 = 20 NM
I should start descending 20 NM before reaching my destination.

bobtait replied the topic: Calculating the distance when to start descending.

There's nothing wrong with your second method. Many pilots use a rule of thumb that goes like this. At 120 knots and 500 feet/minute rate of descent the time to descend will be twice the thousands of feet you want to lose. The distance will be twice the time to descend [2nkm per minute]. So the distance for top of descent will always be four times the thousands of feet you want to lose. To lose 3000 feet, start descent at 12nm.

At 180 knots [3 miles per minute] and 500 feet/minute descent rate, use six times the thousands of feet you want to lose. To lose 3000 feet, start descent at 18nm. If you remember those two cases, it's not hard to estimate a top of descent for any other speed. In practice you don't have to be super accurate. Most people start descent a little earlier than indicated by these calculations to allow the last mile or so of the approach to the airfield at circuit-joining height in level flight.