Pilots Mental Skills Here’s how to Calculating Decent Rate Fast!
Let’s say your flying in you home built aircraft at a leisurely 90 knots and at 7000 ft altitude. You destination airport is at 0 MSL so it’s altitude above sea level is 0 for this example. Here’s how to Calculating Decent Rate Fast and you’ll never be confused again.
You want a gentle decent rate of 3 degrees so we will times out speed by X 300 in the calculation. If you wanted a 1 degree decent you use X100 in the following calculation.
- Aircraft flying Speed ÷ 60 minutes gives you how many nautical miles a minute you are flying. 90 knots ÷ 60 minutes = 1.5 nm per minute.
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2) Now we want to determine how far away from the airport we want to arrive at pattern height, 1000 ft agl. I want a 3 degree decent rate so I will
use x 300. Nm per minute X 300 = decent rate in ft/min 1.5 X 300 = 450 ft per minute.
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3) To determine how far away we start our 450 ft/min decent we calculate the time it takes at 450 ft per minute to decent 6000 ft at 90 knots. so we
are at circuit height of 1000ft agl. We don’t need to adjust speed but reduce power for the decent rate.
If an airport is 500 ft agl then you need to reduce this 500 ft from the amount to descend.
Decent rate ÷ altitude we desire so 6000 ft decent so we are 1000ft above the airport at circuit height.
6000ft ÷ 450 ft/min = 13. 3 nm
OK so if we decent at 450 ft/min we need 13.3 nm to decend to circuit altitude. Now common sense says we don’t want to decent and join right on the downwind leg of the circuit. If it’s busy pilots may not see us depending so let’s add 3 nm to arrive distance. So we will be at 1000ft about 3nm away from the airport but at circuit height and fly into join the circuit joining the downwind leg. Radio calls to be made as normal.
Simply add 3 nm to the calculated 13.3 so let’s start 13.3 + 3Nm extra = 16.3 nm and decend at our 450 knots at 90 knots.
All calculations complete. You could easily add this to a spread sheet on your ipad or phone to quickly do calculations automatically.
This is relevent for pretty much all aircraft.
I hope this helps to answer your question. If you love aviation and flight simulation come visit my BLOG to find more tips and tricks for your next flight.
Author.
Brendon McAliece (Gunnie) is has two plus years working on military jet fighters both in the military and contracting. He has a sport pilot license as well as having spent since the mid 1980s flying flight simulators and building PCs. He is a multi lingual Australian expat living in Thailand.
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