Setting thrust by 60/70/80 kias

[flyin74]

Is the reason why we set thrust by 60/70/80 kias (depending on aircraft type) due to performance? Is this what our take off distances are predicated on? For example, if we set the thrust after 60/70/80 then we will require a longer takeoff distance. Or is there another reason for setting the thrust by these speeds?

[Dan Winterland]

I assume from your handle that you are a 747 pilot. If so, the answer is yes - it's a performance thing.

But also, if you are flying a 744 or a Classic with FFRAT and using the AT for take off, you should have set take off thrust by 65 Knts IAS. This is because the THR HLD funtion is activated and the thrust lever motors are declutched. If you reach 65 Knts IAS and you haven't set your TO EPR (N1 for GE engines), the AT obviously won't do it and you have to do it manually. If you haven't set it by 80 KIAS, you should really be thinking of setting GA thrust because by now your take off calculations are now compromised.

[flyin74]

Thanks for your reply! Its all coming back to me now.

[QAVION]

"This is because the THR HLD funtion is activated and the thrust lever motors are declutched."

Or, more accurately, the 744 thrust levers servomotor(singular) has it's electrical excitation removed. No physical(mechanical) declutching takes place ;)

There have been several excellent postings on setting EPR before certain speeds on the PPRuNe (Try using the search keywords "setting EPR").

e.g.

http://www.pprune.org/forums/showthread.php?s=&threadid=59626&highlight=setting+EPR

Regards.
Q.

[pancho]

It's also got to do with the difference between the dynamic and static element of the power achieved. Thats why full G/A power settings are at a slightly lower EPR or N1 ie; you are still getting full power but achieved from a much higher dynamic speed.

[Thinair]

Another consideration.

In a company where I worked before, our performance engineer told me that our performance calculations(B737) (weightbook) where considering thrust set AT brake release point. So no rolling take-off.
I don't know in how far this may be generalized to other performance tables.
So I think there ar two ways to look at your question:
from a technical point of view(systems, autothrottle etc., CRM) and from performance-side (do you meet the results of your take-off calculations if your engines are spooled up later than at brake release point).
(A rolling take-off compensates somewhat because you have already a certain speed at the theoretical brake release point)

[siddique2]

Also it has to do with 'ram recovery' . ie the increasing speed causing the pressure in the intake to increase due to dynamic pressure. I think an earlier responder alluded to this. Below approx 60kts the EPR indication can be taken 'as red' and the thrust set will be the thrust originally calculated. Above this speed it is not possible to accurately set the thrust correctly.

[Flight Detent]

Hi all,
I differ on the B747 Classic A/T ops - The FE calls manual thrust if the predicted thrust hasen't been set by 80 kias, or THR HOLD isn't indicated, if the A/T is being used.
And the FE then sets/trims to the power required.
Then, if using reduced power, the Captn decides, for whatever reason, to use maximum power, as he can, he will call 'set GA power' (NOT max power), as GA power above 80 kias IS MaxPwr below 80 kias.
Setting Max power over 80 kias will overboost the engines!
Cheers