Descent Planning
#31
Line Holder
Joined APC: Nov 2010
Posts: 68
For 3 degree descent, 5 x GS is a good approximation. The formula is derived from Tan (3°) = V/S ÷ GS.
Tan (3°) = .0524
.0524 = V/S ÷ GS ⇒ (.0524)GS = V/S
GS units are nm/hr, need to convert to fpm.
(.0524)GS nm/hr x hr/60min x 6000 ft/nm = (.0524)(100)GS = V/S
5.24 x GS = V/S(fpm) to be precise.
Tan (3°) = .0524
.0524 = V/S ÷ GS ⇒ (.0524)GS = V/S
GS units are nm/hr, need to convert to fpm.
(.0524)GS nm/hr x hr/60min x 6000 ft/nm = (.0524)(100)GS = V/S
5.24 x GS = V/S(fpm) to be precise.
#32
In Your Head
I'll have another pint of Guinness please.
G'Day Mates
#33
Gets Weekends Off
Joined APC: Nov 2012
Position: 1900D CA
Posts: 3,392
There are obviously a lot of different ways to do this. I prefer this simple mental math method:
Altitude to loose X 3 = Distance to start descent
Ground Speed X 6 = Foot/min descent
Example.
@ FL 250 you need to descend to 15,000. Altitude to loose = 10. 10X3= 30. Start the descent 30 miles out.
Assume grounspeed is 300kts. 300X6= 1800 fpm descent.
Altitude to loose X 3 = Distance to start descent
Ground Speed X 6 = Foot/min descent
Example.
@ FL 250 you need to descend to 15,000. Altitude to loose = 10. 10X3= 30. Start the descent 30 miles out.
Assume grounspeed is 300kts. 300X6= 1800 fpm descent.
#35
Gets Weekends Off
Joined APC: Dec 2012
Position: B756 captain
Posts: 102
As has been said, 3 miles per 1000 feet. This is true for almost all jets. If you want to sound smart, use 3 miles/1000 feet, add a couple for TW, and subtract a couple for HW.
BTW, if you look at the standard ILS approach, it is the same. Look NPAs, and you will see that the HAT for the FAF is approximately 3 miles/1000 feet of descent. Look at circling approaches, if you assume that you are about mile from the end of the runway, then the base leg is 1/2 mile, the final is almost a mile, the TDZ markers are .2 miles, and the distance traveled vs the descent from MDA to TDZ is about 3 miles/1000 feet.
These are all 3 degree descents, and the rate may be calculated by GS/2 + 5 * 10. So, say you are approaching at 90 kts, then 90/2 is 45 +5 is 50, *10 is 500 FPM. This is slight overstated. On the other hand, look at a B757-300 approaching at 150 KTS, 150/2 is 75, +5 is 80, *10 is 800. This is slightly overstated.
Of course in aviation, we measure with calipers, mark with a #2 lead pencil, and chop with an axe. So really, just divide by 2, * 10, and you are close enough.
BTW, if you look at the standard ILS approach, it is the same. Look NPAs, and you will see that the HAT for the FAF is approximately 3 miles/1000 feet of descent. Look at circling approaches, if you assume that you are about mile from the end of the runway, then the base leg is 1/2 mile, the final is almost a mile, the TDZ markers are .2 miles, and the distance traveled vs the descent from MDA to TDZ is about 3 miles/1000 feet.
These are all 3 degree descents, and the rate may be calculated by GS/2 + 5 * 10. So, say you are approaching at 90 kts, then 90/2 is 45 +5 is 50, *10 is 500 FPM. This is slight overstated. On the other hand, look at a B757-300 approaching at 150 KTS, 150/2 is 75, +5 is 80, *10 is 800. This is slightly overstated.
Of course in aviation, we measure with calipers, mark with a #2 lead pencil, and chop with an axe. So really, just divide by 2, * 10, and you are close enough.
#38
Gets Weekends Off
Joined APC: Jul 2016
Posts: 463
More or less the same. To be precise; when “dividing by 3” you are essentially taking altitude / 300. When you “multiply by 3” you are essentially taking altitude * 3/1000. So let’s say A= altitude. A/300 ~ 3A/1000 (close enough for pilot math.). That being said, “multiplying by 3” gives you a 300 ft/nm descent and “dividing by 3” gives you 333 ft/nm.
#39
Gets Weekends Off
Joined APC: May 2012
Position: Guppy Capt
Posts: 151
The answer is 18. You start the decent 18 miles prior to the point you want to be at FL180. If you're flying a "three holer" (727 to you young guys), even 18 is a huge variable with an aircraft that will come down at 6,000 fpm.
I'll have another pint of Guinness please.
G'Day Mates
I'll have another pint of Guinness please.
G'Day Mates
To beat a dead horse: 3:1 for normal descents most jets (280 IAS), 1 mi for each 10 kts HW/TW, and 1 mi for every 10 kts of speed you want to loose. Most airplanes cant quite make 2:1 at barber pole. Keep doing the math on the way down and you'll nail your crossing restriction. Still do it with VNAV to cross check fat fingers and wintertime winds aloft. For interview, just do 3:1.
Cheers
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