NG FMC Descent Profile

[CarlBB]

Hi All,
Quick question on NG FMC Descent Profile - where I understand the NG FMC will command an off idle descent path.
I understand this is avoid speedbrake extension. Buy why? Is it because the aircraft can adjust the thrust, like up a bit, down a bit without asking the pilot via the message "drag required"? Or any other grounds on wanting to avoid use of speed brake i.e. wear and tear or pax comfort? Safety - preventing extension and relying on the pilot to stow it again?

On the subject of the message "drag required" - I've always been tempted to clear the message. I assume this is wrong as you need to know when to stow the s/b. Can always monitor the ND vertical deviation of course but ooi how do the real drivers do this?

Thanks

Carl

[Jeroen Hoppenbrouwers]

What I understand is that if you descend on idle (the most fuel-efficient way), and the forecasted winds do not play out exactly, and you are going to end up high, there is no room to further throttle back and you need to deploy the boards. The FMC cannot do this by itself, so it causes nuisance (work) on the flight deck.

This seemed to happen so often that Boeing/Honeywell did their calculations and adding a tiny bit of thrust did not consume that much extra fuel and prevented many times that speed brakes were needed -- which DOES always throw away whatever fuel you saved.

Then, sure, speed brakes cause vibration and noise. Better never use them.


Hoppie

[CarlBB]

Thanks Hoppie (if you recall the team in Coventry liked the message "drag required", so much so they made a video.... but I digress)
Hope all is good with you all
Carl

[Jeroen Hoppenbrouwers]

zOMG I had forgotten about that... 😅

[Will]

The speedbrakes by definition waste fuel... they are a very expensive way to turn jet fuel into turbulence.

Flying slightly above idle wastes a bit of fuel too, but theoretically would allow you to use the speedbrakes less often.

Boeing's calculation is that averaged over the activity of a whole fleet, the off-idle descents burn less fuel than periodically adding drag.

[Hardy Heinlin]

From an academic, theoretical point of view, one could even get into microscopic details and look at the hydraulic energy used for moving the speedbrakes, and compare this with the electric energy used for moving the little fuel metering valves :-) -- Both movements are used for airspeed control.

All hydraulic and electric energy comes from the rotating engine gear shafts and it reduces thrust which, therefore, needs to be increased, so hydraulic and electric operations also consume fuel. The hydraulic force in these huge speedbrake panel movements is certainly greater than the electric force in those little fuel valve movements. That might add a cent or three per minute :-)


|-|ardy

[Jeroen Hoppenbrouwers]

More importantly, the PF needs to put down his beer to reach for the speed brake lever.

[Bluestar]

I only use the speed brake in descent to meet a published speed restriction or ATC speed restriction.  Other than those two items I just let it roll down hill. 

[Takayoshi Sasano]

More importantly, the PF needs to put down his beer to reach for the speed brake lever.

Unable.

[CarlBB]

Thanks all for the input
Carl