Even if you fly the most sophisticated aircraft in the world, knowing the distance from the airport to the top of descent is useful.
A good pilot always knows how far out from the airport they should descend. Doing this quick calculation will help a pilot understand if the controller is leaving them high or if they can take their time.
Before we get into how to calculate the distance, I need to clarify something.
This top of descent calculation will give you a 3-degree path. But, the rate at which you descend (how fast), depends on your ground speed.
The faster you go the faster you need to descent to maintain this descent path.
So, a jet going 250 knots over the ground will need to descent at 1250 feet per minute to maintain a steady 3-degree path (250 x 5 = 1250 fpm). But a small aircraft going 90 knots only needs to descend at 450 fpm to maintain this path.
Note: multiply your ground speed by 5 to get your descent rate. Or, divide your groundspeed by half and add a zero.
Now that we’ve cleared that up, here is how you calculate the top of descent distance.
Step 1. Take your current altitude. Let’s say 24,000 feet.
Step 2. Subtract the airport elevation or traffic pattern altitude.
I’ll use the airport elevation in this example which is about 4,000 feet
(KLMT is 4,095′ to be exact, but who cares, it’s close enough)
Step 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:
Step 1. Take your current altitude: 5500 feet.
Step 2. Subtract the traffic pattern altitude of 1000 feet.
Step 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 a perfect rate every time (I.e. 500 feet per minute or 1000 feet per minute)?
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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