'The Brandon Method'...Amongst Others: A guide to finding your preferred method

'THE BRANDON METHOD'

By definition, the top of descent or TD is the point at which an aircraft initiates a descent to a lower level for arrival at its destination airport. A good pilot should always know when s/he should start to descend and adjust the descent rate accordingly. The ability to quickly calculate your TD is a very useful skill to have both in Infinite Flight and real-world aviation.

Here is my preferred method of how to quickly calculate your TD in three easy steps:

  • Your current ALT (𝛼 )
  • Subtract destination airport’s elevation (𝜷 )
  • Multiply that sum by 3.
  • The given product is (𝛄 )
  • Divide (𝛄 ) by 1000.
  • Et voila, your 𝝁!

TD = 𝛼 - 𝜷 (3)
TD = 𝛄 /1000
TD β‰… 𝝁

Example:
30000 - 100 = 29900 β‡’ 29900 x 3 = 89700 β‡’ 89700/1000 = 89.7

Thus, your TD β‰… 90NM (Your destination ICAO is Β± 9NM, just enough time for your final checks

Note: 90NM is a rough estimation of your TD. Your destination airport would only be ~9NM away, thus providing you enough time to do your final checks. And be certain to divide by 1000 or you could accidentally begin your descent by thousands of miles.


Some of you might read this and think: β€˜Brandon is doing this wrong!’ But I promise you, no one descends at a perfect 3Β° angle of descent all the way from your cruising altitude without with the use of A/P. Adding the customary 10% defeats the purpose of quickly calculating your TD. Under most circumstances, you should only need a rough estimate for these two reasons:

1.) This formula is only meant for the initial descent, and 2.) You can always adjust your ROD.

For those of you who are math geeks like me, and may prefer to know about different formulas, you have plenty from which to choose:

THE RULE OF 3 (Also known as 'The 3:1 RULE')

The most popular and widely used in real-world aviation. It is based on a 3Β° descent rate formula to ensure a slow, steady and comfortable descent for passengers.

This rule implies that 3 NM of travel should be allowed for every 1000 ft (300 m) of descent.



THE START OF DESCENT DISTANCE FORMULA

The start of descent distance is calculated using this formula:
𝑫 (𝑡𝑴) = ( π‘¨π’π’•π’Šπ’•π’–π’…π’† π’„π’“π’–π’Šπ’”π’† βˆ’ π‘¨π’π’•π’Šπ’•π’–π’…π’†π’‡π’Šπ’™ ) / 𝟏𝟎𝟎 / 𝑫𝒆𝒔𝒄𝒆𝒏𝒕 π’‚π’π’ˆπ’π’†
𝑫 (𝑡𝑴) = ( 𝑭𝑳 π’„π’“π’–π’Šπ’”π’† βˆ’ π‘­π‘³π’‡π’Šπ’™) / 𝑫𝒆𝒔𝒄𝒆𝒏𝒕 π’‚π’π’ˆπ’π’†

  • D = Start of descent distance before expected approach fix point (NM)
  • Altitude Cruise = Your cruising altitude (ft)
  • Altitude Fix = Your expected altitude at approach fix point you’ve chosen (ft)
  • FL Cruise = Your cruising flight level
  • FL Fix = Your expected flight level when you’ve reached your desired fix point
  • Descent Angle = The descent angle you’ve chosen (Β°)

Example:
You are at FL330 and need to descend to the next point at 12000 ft at a 3Β° descent angle.
D (NM) = (330-120) / 3Β° β‡’ 210/3 = 70

Thus, 70NM is when you should start your descent before reaching your approach point.


DESCENT RATE USING GS CALCULATION

Descent rate calculation using Ground Speed (GS) is, like others, an approximation formula:

VS (fpm) = Descent rate (%) x GS (knots)

Example:
GS is 110kt, and your descent rate is 5%.
VS = 5 x 110 = 550

Thus, your VS is 550fpm

NOTE: This formula has some flaws/restrictions:

  • GS can change during descent if it is not maintained constant.

  • Conversely, GS can be replaced by TAS (if known, or at least an approximation of TAS):

  • If TAS can be maintained constant by using a constant vertical speed, you will achieve and maintain the same rate during your descent.

  • If TAS cannot be maintained constant, but you want to remain at the same descent rate, you must adjust your descent rate. As TAS decreases when the altitude decreases, vertical speed (VS) decreases using this same formula.

For higher altitudes and speeds above 250kt, you can make approximations of TAS by using any one of these formulas:

TAS = IAS + FL/2 VS (fpm)

TAS = Descent rate (%) x GS (kt)

TAS = Descent rate (%) x (IAS + FL/2)

NOTE: The FL taken can be at the middle between the cruise FL and the expected final FL in order to compensate for the decrease of TAS.

For the example, I am using the third formula listed:
You’re currently at FL300 and chose the initial approach fix point to be at 11000 ft. Your IAS during descent is 300KT. You decide to use FL200 for the compensation of TAS decreasing:

VS (fpm) = 5 x (300 + 200/2) β‡’ 5 x 400 = 2000

Thus, your VS is 2000fpm



THE MACH NUMBER FORMULA

Altitudes above FL280, the aircraft is flying using a constant Mach number independent of the altitude chosen by the pilot. You can initiate the first phase of descent using the constant Mach number formula:

VS (fpm) = Descent angle (Β°) x Mach x 1000

Example:
M is 0.80 and your descent angle is 3Β°
VS = (3 x 0.80) x 1000 β‡’ 2.4 x 1000 = 2400

Thus, your VS is 2400 fpm



CONVENTIONAL APPROACH (CA)
vs
CONTINUOUS DESCENT APPROACH (CDA)

If you’re still with me after all those formulas I threw at you…these next descent rate formulas are referred to as CDAs (Continuous Descent Approaches):

With these types of descent estimations:

  • You can start your descent when the time to reach the approach point is equal to the VS of 1000fpm.
  • You can start your descent rate at 1000fpm, if you are expecting to reach your final approach fix point ~20 minutes and are at FL200.
  • You can start your descent rate at 3000fpm, If you are expecting to reach your final approach fix point ~10 minutes and are at FL300.

CA vs CDAs


In conclusion, there are numerous formulas and methods to assist in timing your TD and ROD. Ultimately, it is up you to choose which formula or method works best for you. Try each one to gauge which works best for you and your flying style.

But, you should at least try my formula and let me know what you think of it afterward... or just do what I do most of the time: TLAR (all you other real-world pilots know what I mean πŸ˜‰)


As always, I greatly and humbly appreciate you taking the time to read my essay and I hope that I helped to show you something new or gave you a great refresher.

- Brandon





A special thank you to @Hazza68 for the inspiration to write this essay. One maths geek to another πŸ˜‰
35 Likes

First!

Haha haven’t seen Greek letters in a long time πŸ˜…, this will definitely help me. Thanks for making this!

1 Like

I’m smart, but I’m just gonna use VNAV. I’m smart and all, but I don’t have time to do this every flight

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I hope so as well @Stellar_G! I tried using β€˜regular/italicised’ letters, but it looked weird in the end. Plus I’m a math geek remember πŸ˜‰

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Thank you so much! This would be a great Add-on to my brains!)

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Like I said:

And it’s not about being ’smart’, it’s ultimately about what you are comfortable with choosing.

2 Likes

Brain power is the best power @Shafran

haha, yes!

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Bye bye VNAV, hello Brandon Method!

Jokes aside, this is a great tutorial and will definitely come in handy. Thanks for posting!

2 Likes

Haha thanks @Butter_Boi! But, VNAV is a great tool for every pilot and an amazing feat of engineering. I’m only trying to show other methods πŸ˜‰

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Great tutorial!

2 Likes

One question: for the Brandon method, what VS do you decend at? I know y’all are going to think I’m stupid for asking this πŸ˜‚

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Not gonna lie - same here :)

But very interesting article @Navy315!

2 Likes

There is never a stupid question πŸ˜‰ @FlightisticYT

There are a lot of factors and variables which come into play when determining your VS, it is physics and trigonometry after all.

An increase in descent speed produces a decrease for descent; consequently increasing your descent speed increases the distance required to reduce your speed to what your approach speed should be. (note: that will covered in my next tutorial)

Typically, 250kts @ ~12000 AGL (with 40nm until your destination airport), is a fairly typical altitude/speed restriction on published descent rates for commercial airliners.

So to finally answer your question πŸ˜‚ … Depending on the aircraft, altitude and distance, on average I try to maintain a VS of no more than -1500fpm.

2 Likes

Thanks for another great tutorial, @Navy315! Glad that I was somewhat helpful :D

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Thank you again @Hazza68 😊

1 Like

Very Nice Tutorial. Love the formulas.

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Thank you very @Abdullah_Zameer, I appreciate your comment. I hope the formulas help 😊

This is pretty neat

That’s true but, but if you’re a great pilot like me you can trust your intuition to know when to descend 😜
But I think I’ll jot this down just in case I’m ever doing a group flight, I’d like to be more professional then just β€œUse VNAV” ya know lol