Is this the correct usage of the descend from the top calculator?

I found this linked with the resource post, it’s under flight plan. I was flying from KSMF to KLAX and decided to use it, since I’ve been having trouble getting the descents correct. For my top altitude I had 22,000FT my target altitude was 1,000. My speed for above 10,000 was 330 knots. My speed for below 10,000 was 220 knots. My descend FT/Minute was set to 400. This gave me an answer of 248.75 miles. I didn’t do this however. I thought it was wrong? I did a bit more rapid than 400 in the end and it worked. But is that the correct way to use it and read the answer? With those inputs, I would really hit 1,000ft if started descending at 400ft/M and it would take 248.75 miles?

Problem solved. I used the speeds for above and below 10,000 incorrect. I used airspeed when I should have used ground speed.

Depends which calculator you use, but generally yes. Just make sure you input ground speed not airspeed.


Was it the xplane descent calculator?

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What descent calculator did you use? I use this one ( and it works pretty well.


400 ft/min is a very low rate of descent. You should probably plan for something like 1500 - 2000 ft/min initially.


Wait a second, so instead of putting the IAS (Indicated Airspeed) you need to put the GS (Ground Speed) on the descent calculator you are using?

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Yes, airspeed generally isn’t accurate enough


Oh alright, thanks for your clarification!

Ohhh! Thank you so much. Yes I added airspeed I had no idea. But nonetheless it would take 248.75 miles?

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No. The speed would effect it.

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Ahh I see. So using the ground speed the answer would be the correct one?

Yes it was

Suggest that you dont need to overcomplicate it. Use the Rule of 3 …, ie Take your FL, devide by 3 which gets your distance out to start your top of descent. Add 1nm for every 10kts of IAS you are over your landing speed and there you are!


38,000 ft = FL380
380/3 = 127nm

IAS = 165
LAnding Speed = 145
Difference = 20kts = 2nm to add. (dont forget to allow for head or tail winds!)

127 + 2 = 129 nm from airport = TOD.

See this vid from Capt Joe which details how proffesional pilots do the same calulations

Ref VS, as a rule of thumb i use the following as a guide:

CRUISE - FL250 = VS -2500
FL250 - FL120 = VS -2000
FL120 - 6000 AGL = VS - 1500
6000ft AGL- Landing = VS -1500 to -700 as necsssary


I appreciate the explanation. This was easiest for me and quick to! I appreciate it David. But to clarify I don’t land at that speed. I slow down heavily in the last 5nm usually.

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yes thats normal to slow down in the last few miles, the above claulation takes that into account!

Speed wise I aim to level out at FL120 in order to get below 250kts when i decsend below 10000, then between 42-24nm to go i aim to be about 220kts, 24-12nm 190kts then slow down to 150kts for entering the ILS.

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I need to learn that. I seem to find myself constantly lowering altitude and then adjusting VS up until touchdown. Basically in the last 1,000ft it would be nice to just glide right in

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