Autothrottle Disarm?

[AzN1337c0d3r 3]

Has anyone else noticed that the autothrottle servos will limit the thrust to whatever is set on the FMC THRUST LIM page even with the Autothrottle Arm switches turned off?

Is this expected behavior? As I understand it, turning off those switches is supposed to cut power to the autothrottle servos and advancing the throttles to their maximum position is supposed to give redline engine thrust.

[adamant365 28]

I'm pretty sure it's the EEC (FADEC) that limits the thrust, not the A/T. The A/T is merely an "autopilot for speed." It will command necessary thrust up to what the EEC says is the maximum. Even with A/T off you are limited by what EEC deems is the maximum.

 

Have a read through Chapter 7 Section 20 of the FCOM v2. Specifically pages 7.20.7 - 7.20.10 (525 - 528 of the PDF).

[scandinavian13 5,192]

Is this expected behavior?

 

Yes.  As Adam mentioned, A/T being on or off does not affect the FADEC in terms of its limiting.

 

Ways to get other rated thrust are mentioned in the FCOM.

 

...and one of them is actually right in front of your face. You've probably looked at it at least once per flight and never noticed it.

[AzN1337c0d3r 3]

I'm pretty sure it's the EEC (FADEC) that limits the thrust, not the A/T. The A/T is merely an "autopilot for speed." It will command necessary thrust up to what the EEC says is the maximum. Even with A/T off you are limited by what EEC deems is the maximum.

 

Have a read through Chapter 7 Section 20 of the FCOM v2. Specifically pages 7.20.7 - 7.20.10 (525 - 528 of the PDF).

No, it's definitely the FMC selected mode limiting the N1.

 

For example, if I were in CRZ mode and disarm the A/T and push the throttles on my joystick forward, it will move the throttles in the VC forward until I get to the CRZ limit. At this point there is still additional travel available in my throttles.

 

If I repeat the same with CON selected, I can move the throttles a little bit further before they hit the CON limit still with travel remaining.

 

Actually if you hold F4 to actually command the maximum rated thrust and then let go and watch the throttles in the VC you will actually watch them move back to the FMC calculated limits even though A/T servos are disarmed!

 

 

[AzN1337c0d3r 3]

One more thing:

The 777 FCOMv2 is very vague about the operation of the EEC. Is maximum rated thrust available at throttle full stop? Or is only the maximum selected thrust?

I would suspect it is the former, because otherwise there would be no way to get maximum-rated thrust in an emergency in the real plane.
 

In the 737NGX, which also uses a EEC, if you disarm A/T and then move joystick throttle to full forward position, it will ignore the FMC thrust limit and actually command the EEC maximum N1 allowing you to redline to all the engine indications. This is not possible on the PMDG777 besides holding down F4 continuously to override the servos.

[AzN1337c0d3r 3]

Here is a short video of the problem:

You can see at the beginning I move joystick throttle to full maximum position but then the commanded N1 gets moved back to FMC selected thrust limit.

At 0:22 I hold down F4 to get EEC maximum rated thrust. When I let go it goes back to the FMC selected thrust limit. Doesn't matter if I'm in TOGA/CON/CRZ/CLB, all the same behavior.

[777simmer 117]

I'm pretty sure it's the EEC (FADEC) that limits the thrust, not the A/T. The A/T is merely an "autopilot for speed." It will command necessary thrust up to what the EEC says is the maximum. Even with A/T off you are limited by what EEC deems is the maximum.

 

Have a read through Chapter 7 Section 20 of the FCOM v2. Specifically pages 7.20.7 - 7.20.10 (525 - 528 of the PDF).

Thats pretty much correct.

 

Cool Topic, engine limitations.

 

Because the whole Engine Limits subject is not quite as simple as it seems.

 

I hope the information below interests you guys as much as it interests me!

 

I am going to take the GE90-90B as an example cause I am familiar with them, but it should be no different on the 110s

 

First there is the maximum rotational limit the engine is allowed to operate at.

(Max RPM). This is the red line on the N1 dial.

This line is allways the same and for the GE90-90 for example this is 109% N1

N2 also has an RPM redline (117% for the GE90-90)

 

Then there is the maximum amount of thrust the engine is allowed or can produce.

The maximum Thrust Limit, is the Amber line on the N1 Dial.

This line moves a bit along the N1 dial.

It indicates at what N1 RPM, under current ambient conditions you would reach 90.000Lbs of thrust. Or, if the current ambient conditions are such that you cant reach 90.000Lbs, then the amber line would be aligned with the N1 (max RPM) Redline.

 

Then there is a maximum EGT limit.

1030C is the absolute limit for the 90B exhaust gasses and max continuous EGT is 1015C.

You are only allowed to operate between 1015C and 1030C for 5 minutes (or 10 if so certified) during TO and GA.

During any other phase of flight, 1015C is your limit.

From 1014C or below you should be able to fly all day long (continuous EGT) without doing harm to the engine.

 

Max Take Off, max Go around thrust, max climb thrust, max cruise thrust are all limits that extend the life of the engine. So they are not actual hard limits,but they are used because if you would take off every time with thrust at the amber line, the engines would need an overhaul quicker and that is exspensive. But even so, Max TO thrust and max GA thrust can be used only for 5-10 minutes (depending on the certification the airline bought)

 

Max continuous thrust is an amount of thrust that can be set for an unlimited amount of time without any consequences to the Engine.

Max continuous thrust is not the same as max continuous EGT.

 

The EEC is continuously calculating all those limits and keeping them all within limits.

Pffff thanks god, right :-)

On top of that, the EICAS system is smart in that it knows how long you can operate at what limit and it will only give you a warning if applicable.

So you dont need to memorize all the above (as in the old days!)

 

If the EEC is working, then disconnecting the AT and setting the thrust levers fully foreward, will give you Maximum Continuous Thrust, amber line.

So for the 90B, 90.000 Lbs of thrust (or less if it cant produce 90.000Lbs under the current ambient conditions)

This is quite a nice feature because when you have Reduced Thrust set and enter windshear, or approach a terrain obstacle, etc, you can just disconnect the AT, slam the thrust levers full foreward, and you dont have to worry about the Engines blowing up in your face.

The EEC will not allow more thrust than allowed (rated)

It will not allow overboosting.

 

If the EEC fails then the FMC calculates the maximum thrust limit and then you have to watch out with MANUAL thrust!

Slamming the thrust levers full forward then, can cause an overboost (more than 90.000Lbs of thrust set) and can cuase engine damage.

The AT, when engaged, will not go beyond the FMC calculated limit, but you can with manual thrust!

 

The FMC thrust limit page should not be hard limits where you cant get passed with manual thrust. I have not tried yet, but it should be possible to exceed these settings.

Have you tried on the ground at a place where it is not +50C or so?

Also make sure the AT is disengaged, or if you have the AT engaged make sure you set the PMDG option "thrust lever overrides AT" to "allways".

(otherwise the AT keeps limiting you).

 

By the way, as you climb, the engine can produce less and less thrust and at some point Max TO, Max CLB, max CRZ thrust are all the same.

[AzN1337c0d3r 3]

Edit: Stupid me. It seems that it is correctly modeled by PMDG, although the VC throttle position is a bit misleading. It doesn't stay at its full forward position and moves back to the yellow-line position. Also it seems on all thrust limits, the FMC calculated limits are pretty close to the yellow-line maximum thrust limits so I never actually see it go over the FMC-thrust limits.

Thanks for all the great information 777simmer!

On a somewhat-related note. Do you know why at cruise attitude, CON limit is higher than GA?

[777simmer 117]

Edit: Do you know why at cruise attitude, CON limit is higher than GA?

You 're welcome.

 

No, I dont know that to be honest. Does not sound logic does it .

How these limits are calculated is a science by itself.

 

I have seen them often where all limits are the same (CRZ,CLB,GA) but I have not noticed CON being higher than GA so far. (but I dont look at them very often either during cruise).

Not that it matters at this point of your flight as you would not use GA thrust in cruise, but strange yes.