Descent Calculations?

[Julian101 0]

I would like to verify if these formulas are correct with regards to calculating the descent.

Let's say, for instance, I'm flying a 777-300ER from London to Paris (2 hours flight).

1) Intitial Descent
T/D = Current Altitude * 3 - Airport Elevation = ANS / 1,000 = FINAL ANS

2) Descent Rate
TAS / 2 = ANS * 10 = ft. per min

3) Initial Descent (Alternate method)
*Constantly check V/S every 10,000ft

- Altitude
Current Altitude(3 digits only) / 3 = NM (ex. 27,0(00)/3 = 90 NM)

- Speed
*1 NM per 10 kts speed reduction
Current Speed - Average Speed(200 kts) / 10 (ex. 320 - 120 = 200 / 10 = 12 NM)

- Wind
*For every 10kts of wind, add 1NM (ex. 20kts(tailwind) / 10kts = 2NM)

Alt 90NM
Speed 12NM
Wind 2NM
= T/D 104NM

4) Descent Rate (Alternate method)
GS * 5 = feet/min (V/S)

Sources:

 

 

Edited March 21, 2019 by Jim Young
Embedded the video. Videos must be embedded.

[skelsey 765]

Yes, seems about right.

Don't overcomplicate it:

FL/3 = distance to run

Distance × 3 = FL you should be at

1nm/10 kt is a decent additive for speed reduction.

For wind an alternative option is to divide the average head/tailwind component by 3 (as you will typically spend roughly 20 min's = 1/3 of an hour in the descent).

2) and 4) are the same sum ;).

[Julian101 0]

9 hours ago, skelsey said:

Yes, seems about right.

Don't overcomplicate it:

FL/3 = distance to run

Distance × 3 = FL you should be at

1nm/10 kt is a decent additive for speed reduction.

For wind an alternative option is to divide the average head/tailwind component by 3 (as you will typically spend roughly 20 min's = 1/3 of an hour in the descent).

2) and 4) are the same sum ;).

Alright, thanks!

[Julian101 0]

9 hours ago, skelsey said:

Yes, seems about right.

Don't overcomplicate it:

FL/3 = distance to run

Distance × 3 = FL you should be at

1nm/10 kt is a decent additive for speed reduction.

For wind an alternative option is to divide the average head/tailwind component by 3 (as you will typically spend roughly 20 min's = 1/3 of an hour in the descent).

2) and 4) are the same sum ;).

Wait a moment.

Should I be using nautical miles as a measurement for the distance? (Distance x 3)

[skelsey 765]

Correct.

[Julian101 0]

On 3/22/2019 at 4:47 PM, skelsey said:

Correct.

Got it.

I tried the distance from RPLL Philippines to VHHH Hong Kong which is 720NM+ and the result is 2,160. How is that? Sorry for my lack of knowledge

[skelsey 765]

44 minutes ago, Julian101 said:

Got it.

I tried the distance from RPLL Philippines to VHHH Hong Kong which is 720NM+ and the result is 2,160. How is that? Sorry for my lack of knowledge

That would be the height you would be at on a (roughly) 3 degree profile 720nm away!

Sorry, I should have been clearer -- when I referred to distance to run × 3 = FL, I meant as a way of monitoring the descent. For instance, at 50nm to run 50 × 3 = FL150, 75 nm to run then 75 × 3 = FL225, 10nm to run then 10 × 3 = 3000ft etc etc etc.

Obviously these are heights above the airfield elevation so take that in to account if it is significant but it's a good quick crosscheck.

Remember speed is a factor as well - so if you are at FL100 35 NM out at 350kt then you are high because you will need more track miles to slow down! 

Finally, and somewhat obviously, if you are using the FMC for your track mileage for these calculations it is important to ensure that the FMC route accurately reflects the route you are actually going to fly!

[taneb 2]

I assume you are flying a freeware plane, or a small prop plane with a simplified GPS? If so then all of this makes sense. There are many heuristics and rule of thumbs you can use.

The real aircraft (along with most pay ware add on aircraft) will calculate your top of descent for you, but I’m sure already know this.