Pilots Mental Skills Here’s how to Calculating Descent 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 Descent 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 descent 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 1000 ft 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 descend 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 descend at our 450 knots at 90 knots.
All calculations complete. You could easily add this to a spreads on your ipad or phone to quickly do calculations automatically.
This is relevant 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 (Aka Gunnie) is a military veteran with 23 years working on Jet Fighters, their weapons systems and ejection seat/module systems as well as munitions and R&D. Involved with flight simulation since the 1980s, he has flown all the major flight simulators over the years.
He is an Australian expat who has lived in Malaysia, UK, Saudi Arabia and more recently Thailand. He is a multilingual blogger who loves to share his life experiences here on LetsFlyVFR.com and DreamingGuitar.com, with his lifestyle and Travel experiences Blog plus his Dreaming Coffee website.
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