Thanks. I always wondered what the math behind this would be.

 

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

 

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.

 

I used to know these party tricks until I got a plane with decent VNAV. Now, typing is more useful than mental, public math.

GF

 

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.

 

Sorry, I didn't proof carefully enough. The 500 FPM is overstated, the 800 FPM is slightly understated.

 

why is it when calculating distance to descend, folks multiply: ALT * 3, but when calculating VDP, we divide: HAT / 3?

Should the same formula work for both?

 

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.

 

6000 fpm, surely you jest. Once clocked the descent at over 10k fpm. Somewhere around 380 indicated with the boards down, therefore capable of a lot more (410 was VMO, if I remember correctly). How did that go? '27 is a safe airplane, climbs like a safe, descends like a safe. Fun airplane. Miss it.

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