4, 3.5, or 3 degree descent - Page 2

rjacobs · 02-25-2008, 08:19 PM

3000 fpm is a good starting point. Like has already been said you can play with the angle to get your descent rate on progress page two. If you want to see how high or low you are on the calculated path you can go nav, page 2, maintenance, next, setup, sensors, vn data(I think those are the button pushes). The purpose of using this page is to not descend at idle and 3500 fpm and then hit your altitude 20 miles before the fix, then have to push the power up, negating the fuel savings of an idle descent, it helps you get it a lot closer. With no speed restriction I usually try to keep about 1000 feet low so that I can transition from my 3500 fpm descent and not slam the pax in their seats. With a speed reduction I will usually keep 1500, maybe a bit more so that I can transition and make the speed restriction.

TheBills · 02-26-2008, 12:28 PM

Still though even with all of the input so far, I am curious in all the different descents we get in a day how much fuel are we actually saving by changing are descent by a few hundred feet per minute. Also too, im pretty sure im correct on this, but if you have to use spoilers to get down you stayed up to long and did not save anymore fuel by staying high.

randomguy · 02-28-2008, 07:30 AM

Altitude to lose / 3 = how many minutes it will take you at a 3000 fpm descent.

GS/60 = miles per minute.

Multiply the two numbers, and that's how far out you should start down for an idle descent.

It sounds like a pain in the butt, but after you get used to it, you won't need the FMS and it'll usually get you within 5 miles or less of the fix with time to level off. You have a little extra time because your TAS and therefore GS decreases as altitude does.

I know- I think too much, but it does work and you don't have to worry about wind.

Fats Schindee · 03-04-2008, 10:43 AM

Quote:

Originally Posted by TheBills

Also too, im pretty sure im correct on this, but if you have to use spoilers to get down you stayed up to long and did not save anymore fuel by staying high.

Not sure I follow... you're not adding thrust to overcome the drag from the spoilers, are you? If not, how are you negating the fuel savings by using the spoilers to help you get down? Or, asked another way, where is the extra burn coming from?

Tacts · 03-04-2008, 11:22 AM

Descending with spoilers means you spend more time at cruise power (burning more fuel) waiting to descend.

Said another way: An idle descent without spoilers uses less fuel... not because of burn during that descent with spoilers, but because of the excess burn cruising further to the late "top of descent".

Yardley · 03-04-2008, 03:52 PM

The best way to save gas is while still at cruise altitude, decelerate to M.56 or so, then start an idle descent and gain speed back to about 300 knots.

Fats Schindee · 03-05-2008, 07:17 AM

Quote:

Originally Posted by Tacts

Descending with spoilers means you spend more time at cruise power (burning more fuel) waiting to descend.

Said another way: An idle descent without spoilers uses less fuel... not because of burn during that descent with spoilers, but because of the excess burn cruising further to the late "top of descent".

Ah, that clears up my confusion... thanks!

Crapplepants · 03-09-2008, 10:10 PM

There is absolutely no reason to come down at 3500-4000 feet per minute. Maybe if you or ATC screwed up and waited to long but normal descents should not be that high. The shuttle comes down 12 degree path at 8000-10,000 fpm with a delta wind and no engines. Pretty sure that we dont need even half of that descent rate on a normal well planned descent.

Its uncomfortable for the passengers not because the pressurization system cant keep up but because they are literally bracing themselves on the seat track in front of them. It just shows that you have no regard for their comfort...AND since you are running right at red line (if you dont have the boards out, which I see frequently) you are already 70-120kts over maneuvering speed depending on your altitude and you are running the very real risk of a high G pull up from the autopilot overspeed protection.

None of the other airlines do this as a standard procedure and there is no reason for us to do it here.

TheBills · 03-10-2008, 01:46 AM

Quote:

Originally Posted by Crapplepants

There is absolutely no reason to come down at 3500-4000 feet per minute. Maybe if you or ATC screwed up and waited to long but normal descents should not be that high. The shuttle comes down 12 degree path at 8000-10,000 fpm with a delta wind and no engines. Pretty sure that we dont need even half of that descent rate on a normal well planned descent.

Its uncomfortable for the passengers not because the pressurization system cant keep up but because they are literally bracing themselves on the seat track in front of them. It just shows that you have no regard for their comfort...AND since you are running right at red line (if you dont have the boards out, which I see frequently) you are already 70-120kts over maneuvering speed depending on your altitude and you are running the very real risk of a high G pull up from the autopilot overspeed protection.

None of the other airlines do this as a standard procedure and there is no reason for us to do it here.

I agree with you. Actually the other day into ontario, with a constant steep descent like that the pressurization controller speeds up the cabin descent sometimes to compensate for the constant steep descent (1000-1500 fpm is what I saw, which hurts the ears). That being said with all the level offs in between FL370-10,000 I dont think were saving anymore fuel by doing a 4000 fpm descent.

MKFlying7 · 03-10-2008, 03:50 AM

Quote:

Originally Posted by Crapplepants

None of the other airlines do this as a standard procedure and there is no reason for us to do it here.

But many do a thrust idle descent...