Why is my decend plan inaccurate?

Hi Everyone, I have been using this formula for my descend now in infinite flight, but it seems to always fail to the point where I still have more feet to descend (by the odd -5000ft perhaps). This leads to me adapting the plan before I land which I find stressful . Any improvements I can make?

Formula for Descending:

  1. Cruise Altitude – Final Approach = Amount of Descend
  2. Amount of Descend / Rate of Descend = Descend Time
  3. Speed of Descend/ 60 min = Nautical Miles per minute
  4. Nautical Miles per Minute x Descend Time = Start of Decend

e.g.

35000ft/600ft = 58 minutes
250kts/60min = 4.17nm
4.17nm x 58 = 242nm start decend

Decend rate -700fpm (100fpm more for safe decend)

You can always use a calculator if you’re unsure about your descent.

http://aerotexas.com/desplan/desplan.php

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I have learned a thumb rule which seems to work well for the A320 family.

The topic below will explain it all. Thanks to @Clipper747PA

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Oh I see what’s happening: In step 3. you’re actually traveling much faster than you’re expecting.

If you set 250kts as your decent speed that’s your IAS. Your true airspeed will vary throughout the decent. It will be higher than 250kts until you get to 0 MSL. That’s why you’re traveling faster than you anticipated and are overshooting the airport.

I recommend just using the ETE dest and the vertical speed that coincides. Then this effect will occur in reverse and you’ll undershoot which is a lot more desirable. Let me know if this wasn’t clear! 🦅

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You can look at this for a much more simple way and it works out perfect for me.

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I like using this calculator: https://x-planetools.com/descent.html
You just put there the cruising altitude (in ft), target altitude (in ft), ground speed above 10000ft (ground speed at your cruise), ground speed below 10000ft (300kts works good), and your desired descent VS or how many miles before you want to start. It works really well!

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You need to use ground speed for this in order for it to be accurate. That will give you the actual distance you’re covering.


All of this can be placed into a single equation:

(Altitude to lose * GS) / (V/S * 60)

If you were really only wanting to descend at 700 FPM, even though that’s extremely shallow, and were really going as slow as, say 360 knots GS (to just pick a number that might be close to what it would be at 250 KIAS) it would actually work out to (I’m using 35k but you wouldn’t actually descend to the ground with it):

(35000 * 360) / (700 * 60) = 300 nm

It’s the fact that you’re using airspeed that gives you the shorted distance and why you have issues with it.


Note, it’s safe to say that if you call into Approach at FL260 and try to descend at 600 FPM, you’ll find yourself in a hold real fast. And hear “expedite altitude change” quite a bit I imagine.

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Thank you this helps a lot!

Is it ok if you can explain in a bit more detail, I’m still new to all this stuff :) thanks

Definitely- so if you set your airspeed to 250kts at FL280 you’re actually going a lot faster than 250kts over the ground (unless there’s a serious headwind). For step 3 you would need to use your ground speed, not indicated airspeed.

In this picture you can see that my ground speed is 570kts but my indicated airspeed is only 272kts. The difference between these two speeds increases with higher altitude (plus I have a tailwind). At the top of your decent the difference is very high. If you go all the way down to sea level the difference goes away. This difference means that throughout your decent you’re traveling faster than 250kts.

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Ohhh I see, thanks helps alot!

Okay that makes sense now, how do I determine the ground speed, will I have to calculate it while in the air?

Nope. Just read it straight off the HUD, it’s at the top left corner of the mini-map. (You can see in the image above a GS of 570 kts just below the IAS indicator scale.)

It’s also an item that can be added to the bottom and is on the MFD in the A320 series.

Note that your GS will decrease as your altitude decreases, although your IAS stays the same. If you did the calculation solely of your cruising GS you would Jed up descending far too early, which was a problem I had when I first started out. Just something to keep in mind.

Thank you for the support about keeping the GS (ground speed) in mind. That’s very helpful. Due to the ground speed varying between different altitudes I’m a little puzzled on how to plan my descend.

I used this formula on my recent flight from EGCC-GMAD. Just add the airport elevation to the altitude and add 20 nm to the total, and descend at VS -1500. It calculated it perfectly.

Things happen during descent that can change everything. Winds change speed and direction, temperature fluctuations and speed changes too. It is why I give about 20-40 miles of wiggle room for my descents. If I am at FL360 and I descend at 120 nm during a 80 knot tail wind I will over shoot the runway but if I descend at 180 nm during a 70 knot headwind I will be at 5,000 feet 60 miles from the airport. That will take up time and may interfere with other traffic patterns.

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I think another problem is due to the calculators using your speed above 10,000ft and below. We all know all too well we just don’t go from 320knots to 230 knots as soon as we go below 10,000ft. Likely we’re going under 250 knots before we even hit 10k. Then there is your final where you’re going much slower than the speed you calculated by. Therefore it changes the distances of when you need to descend. It’s a lot of math to get it 100% right every time but using the calculators can really give you a good idea. I wouldn’t rely on it 100% though.

Honestly once i see the runway I just got -50 degrees lol.

Captain Joe explans in his vid below. In a night shell its straight forward, take your flight level / 3 add 1nm for every knt of speed you need to lose before landing and you have the DTG when you should start your descent.

Been using this forumula for as long as i can remember and it always works for me!

Eg

@ FL350
IAS 265 kts

350 / 3 = 117nm

265 - 145 = 120/10 = 12NM

117+12nm = 129nm = Top of Descent