Pratice Descent problems

[Atccfi]

Anyone know of a good site where you can go to do practice descent problems? Where they give you all the info and then you compute it and then you can check your answer? Such as you are 68dme from fix at 300kts and have to lose 18000 ft. What descent rate do you need? Any help would be appreciated! JP

 

[AlwaysLearning]

Anyone know of a good site where you can go to do practice descent problems? Where they give you all the info and then you compute it and then you can check your answer? Such as you are 68dme from fix at 300kts and have to lose 18000 ft. What descent rate do you need? Any help would be appreciated! JP

A very good rule of thumb I use on a daily basis is the 36 rule. 3 times the altitude to loose (3 * 18 = 54 miles) 6 times the groundspeed (6 * 300 = 1800 fpm) so with that at 54 miles you need to start a 1800 fpm decent to make the restriction. You can turn it around also, 108 miles at a 900 fpm descent. Then you simply adjust your descent rate as your groundspeed changes on the way down.

You could also divide your groundspeed by 60 (300/60, or 30/6, = 5 miles per minute). 54 miles at 5 miles per minute = 11 minutes. 18000 feet in 11 minutes is 1600 fpm.

Both ways are pretty close and can be done quickly in your head.

Hope that helps.

AL

 

[SWA GUY]

Here is another way, it uses altitude to lose at a constant indicated airspeed.

"Jetliner XXX cross XYZ VOR at and maintain 10,000."-ABC Center

Jetliner XXX is at FL280.

28 minus 10 is 18,000 ft to lose. In a no wind situation, and an idle descent at 280kts, figure 2 and 1/2 miles per thousand ft.

So, multiply 18 by 2 1/2 and that gives you the DME where to start down.

18 x 2.5 = 45

If you want to stay higher and come down faster use 320 kts indicated, which is 2 x altitude to lose.

18 x 2 = 36

* For each 10 kts of headwind subtract a mile, of each 10 knts of tailwind add a mile.

 

[Atccfi]

Thanks alot for the replies. Both replies help tremendously.

 

[MarineGrunt]

If you lower your nose 5 degrees from level flight, that will give you a 500' per nautical mile descent. If you have 10,000' to lose multiply altitudes to lose by 2 and get you miles for descent so thats 20 miles...

Even easier, if you lower the nose 10 degrees, your altitude in thousands to lose will equal NM.

One degree of pitch change = 100' per nautical mile.

This works in any aircraft as long as airspeed permits. Some aircraft may not be able to descend more than 5-10 degrees without gaining too much airspeed. Research the 60:1 rule. There's a million things to use it for.

 

[PropsForward]

If you lower your nose 5 degrees from level flight, that will give you a 500' per nautical mile descent. If you have 10,000' to lose multiply altitudes to lose by 2 and get you miles for descent so thats 20 miles...

Even easier, if you lower the nose 10 degrees, your altitude in thousands to lose will equal NM.

One degree of pitch change = 100' per nautical mile.

This works in any aircraft as long as airspeed permits. Some aircraft may not be able to descend more than 5-10 degrees without gaining too much airspeed. Research the 60:1 rule. There's a million things to use it for.

Sorry this is flawed. 5 degrees down does not give you a 500' per nautical mile descent except at a single very specific speed that is not reasonable. (And does not consider the increase in airspeed as the result of the descent)

The 500fpm you get on the VSI is per minute, not per mile. So the only time this would also be per mile is if your airspeed was 60kts. If your at 180kts, then you are descending 500fpm and traveling 3 miles per minutes.

Hope this helps...

 

[MarineGrunt]

Sorry this is flawed. 5 degrees down does not give you a 500' per nautical mile descent except at a single very specific speed that is not reasonable. (And does not consider the increase in airspeed as the result of the descent)

I guess the Air Force has been teaching it wrong for years then.

The 500fpm you get on the VSI is per minute, not per mile. So the only time this would also be per mile is if your airspeed was 60kts. If your at 180kts, then you are descending 500fpm and traveling 3 miles per minutes.

Hope this helps...

I didn't say anything about ft. per minute. This concept is all based on the 60:1 rule.

I don't have time to post much more, but if you want to see for yourself, run a google search for AFI 11-217 Vol. II. Skip ahead to paragraph 6.4.2.

Straight from the pubs:

"Because 1 NM = 6,076 feet, or approximately 6,000 feet, we can therefore say: 1 degree = 6,000 feet at 60 miles.This relationship is not only true in the horizontal plane, but also the vertical plane. If we weree to make a 1 degree dive, then we would have to descend 6,000 feet (1 NM) after traveling 60 NM. Through the magic of algebra, we can break this down to 100 feet per nautical mile for a 1 degree dive in pitch change."

Now, if you want to figure out your VVI from here, which you really don't need to, you just multiply your descent in feet per nautical mile by you nautical miles per minute. So if you are descending 500 pNM, and have a 180kt GS (3NM/min) your VVI is 1500fpm. If you are doing 240kts, your VVI is 2000fpm.

Seriously, read up on the 60:1 stuff. It's golden. I don't know why they don't teach it in the civillian world...

 

[PropsForward]

Sorry, my mistake. I read 500 feet per mile and instantly thought of the VSI which is per minute. My error. I understand what you are talking about now.

 

[Lead Sled]

As far as descent planning goes, nowadays we simply program the FMS and it flys the descent for us. In the old days, we used a simple formula to give us the descent point and descent rate: 3 X Desired Altitude Loss = Descent Point and 6 X Ground Speed = Descent Rate. This resulted in an approximate 3 degree descent path.

'Sled