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Author Topic: Autothrottle Operation  (Read 439 times)

jlpilot

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Autothrottle Operation
« on: Tue, 27 Oct 2020 05:54 »

Hi everyone,

I'm trying to understand how the autothrottle system on the 747 works. I've had a look through the Engine Severe Damage or Separation drill and unlike the 787 with Left & Right AT Arm switches which are turned off, the autothrottle on the 747 stays on it seems. 

If the autothrottle system is driven by one motor through friction clutch plates, does that mean that the throttle that was reduced to idle will move again or does it stay at idle?

With the autothrottle system on and all engines operating normally, if you pull one engine back, I'm assuming the AT system will bring that one engine up to match the others.

I always thought it was 4 motors driving 4 levers separately. With my very limited technical background, I'm trying to see if I can design an AT system using one motor. I know some people on here have done it already with a real TQ so just curious as to how it might work.

Does anyone know how this is achieved using one motor?
Regards,

Jermaine.

Jeroen Hoppenbrouwers

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Re: Autothrottle Operation
« Reply #1 on: Tue, 27 Oct 2020 09:00 »
If the autothrottle system is driven by one motor through friction clutch plates, does that mean that the throttle that was reduced to idle will move again or does it stay at idle?
Yes it will move if the whole package moves up. All throttles will move until they individually run into the forward or aft stop while the rest keeps moving until the A/T is happy.

Quote
With the autothrottle system on and all engines operating normally, if you pull one engine back, I'm assuming the AT system will bring that one engine up to match the others.
No, it will bring all throttles up. It cannot move just one lever, unless all other levers are already at their end stop. You have to manually keep that one throttle at idle to see the other three move up to compensate.

Quote
Does anyone know how this is achieved using one motor?
With clutch plates!

Imagine a pencil and four laundry pegs clipped onto it. Turn the pencil -- that is your single motor. You can move the pegs (throttles) individually by hand, but when you rotate the pencil, they all try to move.


Hoppie

Hardy Heinlin

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Re: Autothrottle Operation
« Reply #2 on: Tue, 27 Oct 2020 10:19 »
In addition to what Jeroen wrote ...

Just in case you wonder when you play with the thrust settings:

The 744 has another feature called thrust equalization; it can work even when the A/T is disengaged. Of all four thrust levers the system uses the highest lever position for reference, and all other thrust settings whose levers are no more than one knob width appart from that highest reference, will be increased automatically until all thrust settings are equal. But those other levers will not move; the system will directly control the respective fuel metering valves until all EPR or N1 values agree (despite some tiny random fluctuations). When you retard a lever by more than one knob width (away from the highest lever), the thrust of that engine will promptly drop out of the equalization range.


Regards,

|-|ardy

andmiz

  • Join date: Jan 2019
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Re: Autothrottle Operation
« Reply #3 on: Tue, 27 Oct 2020 12:39 »
Everything Hardy and Hoppie have said is spot on.

The way the system works is most evident when completing the engine fire memory items.  When retarding the thrust lever on the engine on fire, the thrust doesn’t decrease until it’s outside of one knob/knuckle.  Once it’s back at idle, the other engines increase their thrust to compensate (AT engaged), so unless the thrust lever is held at idle, it will follow in unison to the other thrust levers too

emerydc8

  • Join date: Jun 2015
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Re: Autothrottle Operation
« Reply #4 on: Tue, 27 Oct 2020 13:42 »
In addition to what Jeroen wrote ...

Just in case you wonder when you play with the thrust settings:

The 744 has another feature called thrust equalization; it can work even when the A/T is disengaged.

It's been a long time but if I remember correctly, the thrust equalization is armed by the A/T arm (toggle) switch.

Jon D.

Hardy Heinlin

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Re: Autothrottle Operation
« Reply #5 on: Tue, 27 Oct 2020 14:20 »
That's correct. And there are some more conditions; the details are described in the Aerowinx Operations Manual, page 514.


|-|ardy



Of all four thrust levers the system uses the highest lever position for reference, ...

Correction: It's the second highest.

jlpilot

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Re: Autothrottle Operation
« Reply #6 on: Tue, 27 Oct 2020 20:13 »

Thanks very much for the information guys! It's a lot clearer now  ;)
Regards,

Jermaine.

emerydc8

  • Join date: Jun 2015
  • Location: Tucson, AZ USA
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Re: Autothrottle Operation
« Reply #7 on: Wed, 28 Oct 2020 08:53 »
In addition to what Jeroen wrote ...

Just in case you wonder when you play with the thrust settings:

The 744 has another feature called thrust equalization; it can work even when the A/T is disengaged. Of all four thrust levers the system uses the highest lever position for reference, and all other thrust settings whose levers are no more than one knob width appart from that highest reference, will be increased automatically until all thrust settings are equal. But those other levers will not move; the system will directly control the respective fuel metering valves until all EPR or N1 values agree (despite some tiny random fluctuations). When you retard a lever by more than one knob width (away from the highest lever), the thrust of that engine will promptly drop out of the equalization range.

I tried this on the 767 during cruise. Move one thrust lever slightly aft and the EEC will increase that side back to match the other N1 setting. Retard it more than 1/2 knob width and the EEC on that engine gives up and lets the thrust reduce to match the position of the thrust lever that was pulled all the way back to idle. The remaining thrust lever increases to MCT to maintain the cruise speed. By 1/2 knob width, I mean that the front of the retarded lever is exactly even with the center of the knob of the non-retarded lever.

Jon

Hardy Heinlin

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Re: Autothrottle Operation
« Reply #8 on: Wed, 28 Oct 2020 12:44 »
Perhaps they made the tolerance 50% smaller because a single 767 lever has the power of two 747 levers :-)


||-||ardy