# 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?

edit
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.

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Was it the xplane descent calculator?

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What descent calculator did you use? I use this one (Precision Descent Profile Online Calculator) and it works pretty well.

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

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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

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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!

Example

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

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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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