I don’t care if you fly the most sophisticated aircraft in the world, the top of descent calculation is useful.
Because a human controller dictates your descent.
Is the controller going to give you an easy descent or slam you down? What are you going to say when they ask you when you want to go down?
Or better yet, how are you going to know whether you want to head down now or later when you get a “descend pilots discretion, maintain 10,000′?”
Take today for example, the controller asked me how many miles until I needed to start my descent. I must admit, I had never heard this request from a controller before. The other pilot and I fumbled around in the FMS trying to figure it out.
We did finally.
After I relayed the information I realized I could have done it ten times faster.
1. Take your current altitude…in this case I was at 24,000 feet
2. Subtract the airport elevation or traffic pattern altitude…in this case the airport elevation 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 and you get your distance in NM. So…..
Flight Level 240-40=20 x 3= 60 NM
This also works for lower altitudes:
1. Take your current altitude: 5500′
2. Subtract 1500′ for the traffic patter altitude
3. Multiply it by 3
5500′-1500’=4 x 3= about 12 NM from the airport.
(yes, I dropped a bunch of zeros to get to the number 4 instead of 4000)
Yay! You have a rough estimate of when you need to start down at about a 3 degree rate of descent. Notice I said “rough idea,” though. I know what you are thinking! Someone may be reading this and thinking: “she has it all wrong, you are supposed to add 10% to the 60NM to make it 66NM.
Okay, but so what?
Adding steps defeats the purpose of “quickly” calculating TOD.
I could have added a step 4 to this process, but will you remember the fourth step in a year? Its more important you understand the general framework of this calculation. This request doesn’t come that often, so you had better remember it.
It also doesn’t matter in the grand scheme of things. You need a ballpark figure for two reasons.
First, if you need to descend later at a steeper angle because you were off slightly, it usually isn’t a problem.
Second, I find this calculation is usually only good for the initial descent. Most of the time controllers will start you down but then level you off at different altitudes. They know their airspace, they will usually give you the altitudes that support a reasonable descent.
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 a couple of NM on the calculation.
If you want to get really good at this calculation then practice it every time you descend to your destination airport. Or you could make a note of where your FMS calculates the TOD.
Pay attention because over time you will start to figure out when you should start descending depending on the altitude.
Then you will know you are a pro.
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