I don't know if I am doing something wrong, but when I am lined up to take off on the runway, I have both FD's on, auto-throttle is on and I bring the thrust levers to about 60%. Once at that setting, I press the clickspot on the autopilot panel that is the screw to make the computer take over and accelerate the aircraft. This works fine until a few seconds after reaching the correct thrust setting, the throttles reduce back down to idle without any input from me. What is going on? Can someone help me?

THR HOLD

Did you set up the FMS? with V speeds?

Once again.... THR HOLD..... Check what your throttle lever position is when the A/T servo disconnects, same deal with descent in FLCH when thrust reverts to HOLD

 

V speeds have nothing to do with it, would be crazy to think any aircraft would be designed like that.

 

Regards

 

 

This works fine until a few seconds after reaching the correct thrust setting, the throttles reduce back down to idle without any input from me. What is going on? Can someone help me?

 

Hi, Matthew,

 

Are you sure you are not touching your manual throttle after clicking on TOGA?  If it is not full on and you touch it even slightly it will produce the engine rollback you are experiencing.  You can set your manual throttle to never override the autothrottle, on the CDU in PMDG Setup>Options>Simulation.  Even if you don't want to keep this setting, you can try it temporarily and see if the autothrottle then performs normally and maintains takeoff thrust.  If it does, you may be inadvertently touching your manual throttle or it might be sending its own command for some reason.

 

Another thing to try is maxing out your manual throttle at the same moment you click TOGA.

 

Mike

I have mine setup so that the Thrust Levers override the AT in HOLD mode only.

And like FLEX1978 said, this means I have to follow the AT Thrust Target with my TL (in real life the AT servo motor would do this).

 

I only have to do this if I expect the AT to go into HOLD.(like for Take Off and Flight Level Change descends)

Not when the AT is in THR REF or SPD mode. In that case HOLD will never show and thus the position of my TL is not important.

 

It takes a little system knowledge and monitoring this way so if you are new I would recommend setting the FMC to "TL overrides AT - never"

THR HOLD

 

What's that?

What's that?

 

Search the intro manual for "override" and look for a hit where it's talking about the autothrottle override options.

 

THR HOLD is a thrust mode, where the A/T drops command to the physical throttle position.  In the real aircraft, I believe this is so that the crew can reposition the throttles as required.  In the real aircraft, though, the physical throttles move with the A/T commanded position via servos.  After hitting TO/GA, the physical throttles advance (in the real plane) because the servos drive them forward to match the A/T commanded thrust.  Your hardware does not do this, so you need to do it yourself (another option, mentioned in the intro manual, would assist you in this endeavor) when using the option you are currently using (found in the FMC options menus).

 

You can either match the throttle position manually using your current option; or

You can set the option to override your hardware throttle at all times.

 

The intro manual was specifically written to answer questions like these.  Please read it.

What's that?

It's all in the docs, aren't you a 744 fan? It has a HOLD function too. The MD11 also the same thing but it's called CLAMP

 

Guys it been a while and my head is pretty much still in Airbus philosophy. Do you not have to announce each FMA change, how can you not notice HOLD on the FMA? It's part of the take off procedure. Every take off on the bus you announce each FMA change to keep yourself in the loop - TOGA | SRS | RWY. Etc etc

 

Boeing

 

THR REF | LNAV | VNAV

 

HOLD | LNAV | VNAV

 

THR REF | LNAV | VNAV SPD

 

SPD | LNAV | VNAV PATH etc etc

Well yes you announce mode changes to your Copilot (we are all captains here right :-) )

But it is company procedure really.

 

We did not have this procedure untill about two or three years ago.

Now we call out mode changes (if AP is on) or we order them (AP off).

We dont call out mode changes the aircraft does by itself.

 

So the AT going from HOLD to IDLE would not be called out.

Or when LNAV is armed and then captures, (mode change from HDG to LNAV) that is not called out at the moment.

 

But I heard they are working on changing that as well, and then we will have to call out everything that happens on the FMA.

THR HOLD is a thrust mode, where the A/T drops command to the physical throttle position.  In the real aircraft, I believe this is so that the crew can reposition the throttles as required.

Yes exactly.

 

I am sure you know this already Kyle, so for others;

HOLD is kind of like an AT pause mode. It sits there and does nothing.

But it is not locked into that position, you can move the thrust levers as you want.

 

For instance when you initiate the descend from cruise level a bit earlier (try up to 50nm before Top Off Descend) then the AT will retard a little. You are now initially descending below the VNAV calculated descend path. The AT retards only a little bit (not to idle) to set an N1 that gives you something like a 1250fpm descend. After the AT is happy it goes into HOLD mode. Now, you will see that this shallow descend will very often become shallower or steeper as ambient conditions fluctuate. So you can adjust the AT (a little more N1 or a little less) to adapt your rate of descend.

Untill the VNAV path is captured because then the AT goes to IDLE.

 

Why HOLD, why does the AT not just keep speed and a pitch mode keeps -1250fpm?

I dont know.

Its just the way it is.

Matthew and everyone,

 

I have my manual throttle set to ALWAYS override the AT.  I believe this is the most realistic, except for the fact that the manual throttles do not follow the commands of the autothrottle as Kyle said.  However my experience is that, on takeoff, once the AT goes into HOLD, the throttles stay at the proper thrust setting so long as I don't touch the manual throttle, even if the manual throttle is still in the 55%-60% range. 

 

Rob, I would guess that the HOLD mode kicks in on the takeoff roll so that if the pilot sets a different thrust -- full takeoff power or idle or even reverse in a rejected takeoff, the AT won't "fight" his command.  The PMDG T7 certainly does "fight" manual settings when in other modes -- if I decrease the manual throttle in, say, Clb-1, when I stop moving it the AT will go back to Clb-1 thrust (which I believe is accurate behavior). 

 

Mike

Rob, I would guess that the HOLD mode kicks in on the takeoff roll so that if the pilot sets a different thrust -- full takeoff power or idle or even reverse in a rejected takeoff, the AT won't "fight" his command.

Mike

Yes that is correct. Windshear during take off roll for instance.

 

If you move the TL while the AT is engaged in any other mode than HOLD you are overriding the AT. And you feel resistance, you feel the AT fighting you (in the real thing that is). Not heavy like fighting the AP (you can override the AP too but it requires quite a lot of force and then it completely disengages) but you definately feel it fighting you. When you let go, the TL move to where the AT system wants them again.

 

You override the AT quite often on final approach because the AT tends to react slower to speed changes than the pilot might want it to. It does a great job and if you do not intervene you will be ok as well, but you know....I guess you want to stay awake and not get into the habit of not paying attention to it anymore.

Or during sudden speed changes in cruise flight (fluctuating winds). They can move you right into overspeed because the AT is even slower up at cruise FL than during approach.

 

 

Why do you want to intervene with the AT during climb?

The most economic way to fly is to get up to higher altitude asap.

Thats why VNAV climb gives you THR REF (full throttle up to the current thrust limit like CLB or CLB-1 or CLB-2)

 

Of course I can imagine things going a bit too fast when you just got the 777.

In that case just use V/S mode :-)

Once above 10.000ft things will go slower and slower (less thrust available) and you can engage VNAV again.

I don't know if I am doing something wrong, but when I am lined up to take off on the runway, I have both FD's on, auto-throttle is on and I bring the thrust levers to about 60%. Once at that setting, I press the clickspot on the autopilot panel that is the screw to make the computer take over and accelerate the aircraft. This works fine until a few seconds after reaching the correct thrust setting, the throttles reduce back down to idle without any input from me. What is going on? Can someone help me?

 

http://forum.avsim.net/topic/422085-autothrottle-combatting-manual-throttle-at-takeoff/