B777 THR VNAV SPD mode
 

Hello everyone,

Could someone please explain me which value of thrust is computed by the AT THR mode on the triple when the aircraft is flying in VNAV SPD during descend? I understand in VNAV SPD, the aircraft controls the speed using the elevators…but why does the AT command IDLE at some given time…and THR at other? Is THR commanded when we are below the flight path to not ‘dive’ in idle and therefore not increase our profile to the calculated path?
Did not find anything in the FCOM regarding this subject.

Thanks!

The speed can only be controlled by one thing at a time, either the pitch or the thrust.

If the pitch is flying a straight line such as VNAV PTH (or ALT, or VS, or GS) then the thrust is controlling the speed, in SPD.

If, instead, the pitch is controlling the speed (VNAV SPD or MCP SPD) then the thrust can not be also controlling the speed. It is fixed, either at idle or a climb setting.

If you engage VNAV and are not on the path (or open the speed window), you will enter VNAV SPD as you would know. Above or near the idle descent path, you will get IDLE then HOLD. Below the path (or if you commence an early descent), you will get THR then HOLD as it gives you thrust to approximate a 1250fpm descent, and you can subsequently vary the thrust to modify the actual RoD. My FCOM describes this in the FMC Cruise section.

Hey,

In VNAV SPD mode during descent, the autothrottle (AT) adjusts engine thrust to control speed. It usually goes to IDLE thrust when the plane is on or slightly above the descent path to prevent speeding up. When the plane is below the descent path or needs to slow down, it commands THR thrust to control the descent rate and match the profile.

I get your frustration about not finding info in the FCOM, but sometimes real-world experience or insights from others can fill in the gaps.

Hope this helps!
Mark Smith

No it does not. I see what you mean from the later text, but please let's be careful to be clear for those that are learning an airplane, and maybe learning English at the same time.

Speed is controlled only by the elevator, or only by the throttle; never both at the same time (I think Concorde is the only exception to this).

...fixed it