Even if you fly the most sophisticated aircraft in the world, the top of descent calculation is useful.
A good pilot always knows when they should descend so they can anticipate their descent rate.
I’ve watched the top of descent pass me by only to find out the controller forgot about me. When you know if the controller is leaving you high, you can plan for it.
Here’s how to calculate the top of descent quickly:
1. Take your current altitude. Let’s say 24,000 feet.
2. Subtract the airport elevation or traffic pattern altitude.
I’ll use the airport elevation in this case which was about 4,000 feet
(KLMT is 4,095′ to be exact, but who cares, it’s close enough)
3. Multiply that number by 3 to get the distance from the airport. So….
24,000 feet – 4,000 feet = 20,000 feet
20,000 feet x 3 = 60,000 feet/ 1000 = 60 NM
Note: make sure to divide by 1000 or you will begin your descent thousands of miles from the airport. You can drop the zeros at the beginning or end of the calculation.
This also works for lower altitudes:
1. Take your current altitude: 5500 feet.
2. Subtract the traffic pattern altitude of 1000 feet.
3. Multiply it by 3
5500 – 1000 = (4500 x 3) / 1000 = 13.5 NM
The 13.5 is the distance you should start your descent from the airport.
This is only a rough number. Someone might read this and think: “she has it all wrong, you are supposed to add 10% to the 60NM to make it 66NM!”
But, why? Do you descend at 1500 feet per minute perfectly every time?
Adding 10% defeats the purpose of “quickly” calculating TOD.
It also doesn’t matter in the grand scheme of things. You need a ballpark figure for two reasons.
First, you can adjust your rate of descent to account for any errors.
Second, this is only good for the initial descent.
Most of the time controllers will start you down but then level you off at different altitudes. Traffic or other factors will determine what happens once you start down.
It is rare to have a perfect 3-degree angle of descent all the way from cruise altitude to the airport. So, don’t worry if you are off by a couple of NM on the calculation.
If you want to get really good at this calculation, practice it every time you descend to your destination airport.
Over time you’ll start to develop rules of thumb and avoid a lot of stress when a controller forgets about you!
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