'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 ≅ 𝝁
30000 - 100 = 29900 ⇒ 29900 x 3 = 89700 ⇒ 89700/1000 = 89.7
Thus, your TD ≅ 90NM (Your destination ICAO is ± 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: ‘Brandon is doing this wrong!’ But I promise you, no one descends at a perfect 3° 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.
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 3° 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 (𝑵𝑴) = ( 𝑨𝒍𝒕𝒊𝒕𝒖𝒅𝒆 𝒄𝒓𝒖𝒊𝒔𝒆 − 𝑨𝒍𝒕𝒊𝒕𝒖𝒅𝒆𝒇𝒊𝒙 ) / 𝟏𝟎𝟎 / 𝑫𝒆𝒔𝒄𝒆𝒏𝒕 𝒂𝒏𝒈𝒍𝒆
𝑫 (𝑵𝑴) = ( 𝑭𝑳 𝒄𝒓𝒖𝒊𝒔𝒆 − 𝑭𝑳𝒇𝒊𝒙) / 𝑫𝒆𝒔𝒄𝒆𝒏𝒕 𝒂𝒏𝒈𝒍𝒆
- 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 (°)
You are at FL330 and need to descend to the next point at 12000 ft at a 3° descent angle.
D (NM) = (330-120) / 3° ⇒ 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)
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) ⇒ 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:
M is 0.80 and your descent angle is 3°
VS = (3 x 0.80) x 1000 ⇒ 2.4 x 1000 = 2400
Thus, your VS is 2400 fpm
CONVENTIONAL APPROACH (CA)
CONTINUOUS DESCENT APPROACH (CDA)
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.
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.
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.
A special thank you to @Hazza68 for the inspiration to write this essay. One maths geek to another 😉