TheBills

It is my understanding that the steeper the descent and the longer we hold off on the descent saves that much more fuel. But really how much are we saving, is it that much of a difference as to make such an uncomfortable descent for the passengers. Im just thinking at a 4 degree descent with calm winds your pushing 3500+ FPM, thats hard on the ears! What do you guys think. I usually do 3.5, or whatever the boss wants.

WS

4 deg at 450 kts GS (calm winds) = 3000fpm.
cabin rate of descent about the same at 3.5 or even 3 deg descent from what i can tell 400-500fpm.
i don t think the average person can tell the difference of 100/150fpm.

greazytaco

i do 4.5 with calm wind/headwind or less than 30 or so knots on the tail.. 4 deg with tail wind up to 80 or so knots.. and 3.5 any other situation or if I think ATC may be giving speed restrictions.. dending on a/c weight you can get about 3500-3900 fpm at idle thrust from about 370-290.. about 3300-3500 from 290-200.. and 2800-3000 from 200 down to 10.. roughly

kidicarus5897

The CFM and dispatch paperwork both plan on idle descents. So take that for what it's worth. Maybe try a 3* and 4* descent to a crossing restriction while following the carrot and see the difference in fuel burn. Make sure you take into account the extra time spent at altitude for the steeper descent to make it realistic. Personally, I usually use 4*. Maybe 3.8* with a huge tail wind or 4.2* with a headwind, but it doesn't seem to be uncomfortable. I think what the pax hate is the acceleration downwards when transitioning from cruise to the descent. Some people I fly with slap the TR's to idle and roll in to 4000f/min in one fell swoup. I usually put my hands up in the air and scream like I'm on a roller coaster. Makes it more fun.

__________________
Hey, it's not lying if what I said would be true if the facts were different......

MKFlying7

The technique I've been using lately is to play with the angle a little bit in the flight plan, then go to Progress page 2 and see what rate of descent it's calculating. **Typically** if it is calculating around 3000 fpm you can do a thrust idle max speed descent following the VNAV path.

It's typically been ranging from around 2.5 to 4.5 depending on wind and speed restrictions.

If you're planning on using a thrust idle descent, it's not a bad idea to take a look at the winds down lower on the paperwork, so you don't work yourself into a jam as the wind picks up or drops off.

av8tr4610

I usually do 3.5.....most guys I fly with do 4, but I always worry about getting caught with my pants down, if we need to slow up early. It has happened twice since I have been here. I just like that extra room to account for early speed reductions and or wind changes.

UFOGUY

Well it's not so much the fuel that your particular flight is saving, but the total amount of fuel company wide. If everyone flew that way (steep descents at idle thrust) over the course of a year it may add up to a huge savings. Save money where you can I suppose. Personally I haven't had too much experience experimenting with different descent angles yet, but I normally use either 3 or 3.5.

TheBills

Heres the catch though, what about all the times one would do a 4 deg. descent to add thrust to level off and fly at lets say 75% for awhile. On the other hand lets say you did 3 or 3.5 deg. for a slightly slower descent at 40ish % to get lower altitudes along the way and never having to add thrust to level off. I know theres many scenarios here but think about it company wide with all the different scenarios we get in a day how can we be sure that a 4.5 deg. glide is saving the company money.

UFOGUY

I see where you are coming from. I am sure they are basing this on the thought of a perfect world and perfect flight plan where you would stay as high as long as possible and then chop and drop and configure to land. Where safety and comfort are top on the list, I agree with the gradual descent profile. But they are also thinking of even if the chop and drop saves, lets say 10 gallons of fuel per flight; and there are how many flights per month?

TexFlyer

Here is my technique that I like to use. Perf-Init page I use 4 degrees to start. Then scan the release to see what winds I can expect. Like MKF7, I use the progress page 2 to see the planned descent rate. I will use what ever descent rate will yield 3000 fpm. Obviously with a headwind I can get away with maybe 4.5 or even 5 a few times. Just pay attention when the winds die down. As the purple carrot comes up on the PFD, I will select VS 1000 fpm. As the plane starts to pitch over, start slowly increasing VS to 2000 fpm. Leave 2000 fpm set until the purple carrot starts moving down, then slowly adjust VS to 3000 fpm. If you need more because the winds are slower then you have some room to correct. I also use a 6 nm point before any crossing restriction to be level at and if need be to slow to 250 kias. Don't be afraid to use spoilers if need be. I used them probably 3 times in the last year with this descent profile. Which probably means I need to be more aggressive I then slowly start to select 500 fpm within 1500 ft of level off to make a smoother transition (I try to have 500 fpm selected by the time I have 500 ft to level off.) If ATC starts you down early descend at 1000 fpm until top of descent. Again it is just my technique, if it's your leg the only thing I truly care about is being early at a crossing restriction.