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A/T 4-lever-servo-mechanism re engine out

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Moderator
Registered: May 2009
Posts: 5140
Hi,

I just learned that the A/T servo shouldn't move a lever that is set to idle and that is linked with a shutdown engine. The details, however, are still unclear.

Maybe one of the maintenance experts can help?

Is there a mechanical link from the fuel control switch that keeps the lever away from the servo clutch when the lever is at idle?

I can't imagine that it should be a system based on thrust lever angle differences. How could this be achieved mechanically by one common servo? Also, if that was true, how should the servo know which of the four levers is supposed to be "parked" when the servo moves all levers to idle and then out of idle?


Cheers,

|-|ardy
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I can only imagine a mechanical lock that is either linked mechanically to the fuel control switch or engaged by a solenoid. Even the idea that the clutch pack can be partially disengaged goes too far for my imagination.

First time I hear this at all, anyway.


Hoppie
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Registered: May 2009
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Location: LLBG
Quote
I just learned that the A/T servo shouldn't move a lever that is set to idle and that is linked with a shutdown engine.


Where did you read it?
For the purpose of the TO/GA switch Ground Test the Fuel Control Switches should be at CUTOFF (naturally, it is a ground test) and the throttles should be at IDLE. During the test the throttles move forward and backward.

In many cases GND TESTS switch should be in ENABLE to execute the test. The TO/GA test is NOT one of these cases (all you need is FMC, FCCs and A/T switch at ARM).

Cheers,
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Avi Adin
LLBG
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Got it from Peter :-)


|-|
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He should run the TO/GA switch ground test on the real aircraft :D
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Avi Adin
LLBG
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But that test probably doesn't test the disconnection of a single lever?
Member
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No.
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Avi Adin
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Location: Camberley, UK
I have never tried this with a live engine but if you have an engine failed the first action as per Boeing procedures is to close the Thrust Lever. That is a mandatory Boeing procedure for a FIRE or SEVERE FAILURE and is done in case the pilot is trying to shut down the wrong engine.
It then stays there. There is no question of it opening up again by itself.

It will then stay at idle or the pilot can then elect to align it with the other three levers so as to have a symmetrical grip on all 4 levers, even though one is not working.

If the autothrottle is engaged the lever will then follow the motion of the other levers. If they all move back to idle I am pretty certain that it will then open up with the others when they do...

If you think about it, if you have a surge/engine limit or stall condition, the procedure is to progressively reduce the thrust on the engine until the problem goes away - at which point you leave the thrust lever at its (probably semi-retarded) position. It stays there, with the autothrottle engaged - which is what you would want, or the surge condition would recur if the lever moved forward again - (and bear in mind this is without an engine failure EICAS or a Control Switch moved to cutoff).

There must be a thrust lever angle which excludes movement, I guess, but I don't know what it is - my guess would be "not much!".

I have seen the engine servo test - I don't think it is related to this..

Peter
« Last edit by Britjet on Sat, 06 Jun 2015 21:15:38 +0000. »
Member
Registered: May 2010
Posts: 843
Seriously, there are no mechanisms to keep the lever at idle or stationary. There is only one servo and one mechanically resistive clutchpack.

Perhaps most of the procedures you practised in the sim involved the progressive reduction of thrust for the remainder of the flight/session (after the failure procedures had been carried out) and very little increase in thrust after the dead engine lever had been brought back to the stop?

Quote
If the autothrottle is engaged the lever will then follow the motion of the other levers. If they all move back to idle I am pretty certain that it will then open up with the others when they do...


Agreed, but doesn't this disprove the locking theory?

The CMC FMC test involves the engagement of the A/T with all fuel levers in OFF. This certainly doesn't inhibit the movement of the levers. They are initially driven fully aft (if not already fully aft) and then forward.


Quote
If you think about it, if you have a surge/engine limit or stall condition, the procedure is to progressively reduce the thrust on the engine until the problem goes away - at which point you leave the thrust lever at its (probably semi-retarded) position. It stays there, with the autothrottle engaged - which is what you would want, or the surge condition would recur if the lever moved forward again -


The surge/stall is most likely to occur at high thrust levels. If you pull back the lever where it doesn't do that, the other engines will be trying to compensate for the loss of thrust (i.e. very far forward). For the rest of the flight, perhaps the faulty engine won't even reach the point where it misbehaved. If the surge/stall occured at low thrust levels, then you would probably shut down the engine. In which case, it wouldn't matter where the lever was.

Rgds
JHW.
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Registered: May 2010
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P.S. In the surge stall scenario, if the lever didn't move from the semi-retarded position you set it at, what would happen when the other levers came to idle during flare? Would it still be stuck at the last position? That's something you definitely wouldn't want.
Moderator
Registered: May 2009
Posts: 5140
Perhaps we have a misunderstanding here. Perhaps Peter is thinking about the following two possibilities:

Example: When 3 levers are at half thrust and one lever is at idle, and THR REF engages, what happens?

All four levers move forward and this entire 4-pack movement ...

1. ... will stop when any engine reaches THR REF, leaving the formerly idle lever at half thrust (it will never reach THR REF).

2. ... will stop when the thrust sum of all four engines reaches THR REF x 4 which can be reached only when the servo keeps rotating until all four levers reach the mechanical max angle, whereby the EEC translates the upper angle section electronically (with a "knee") to the respective THR REF value.

The question is: Which method is correct?


Cheers,

|-|ardy
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Registered: May 2010
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Quote
Example: When 3 levers are at half thrust and one lever is at idle, and THR REF engages, what happens?


I seem to recall a similar debate in the past. Isn't it stated in the manuals that if the levers are staggered, the servo stops when the second lever reaches the target thrust? (At least in the case of THR REF).
« Last edit by John H Watson on Tue, 09 Jun 2015 13:36:42 +0000. »
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Hardy Heinlin wrote
The question is: Which method is correct?

I think #1 is correct, and that's also modelled in PSX. So my suggestion above doesn't really help in this discussion as #1 is closer to Peter's expectation anyway :-)


John H Watson wrote
I seem to recall a similar debate in the past. Isn't it stated in the manuals that if the levers are staggered, the servo stops when the second lever reaches the target thrust? (At least in the case of THR REF).

Perhaps you're thinking of the EEC's thrust equalization logic? In that system the second highest TLA sets the target thrust (there are some numbers on page 503 in the Aerowinx manual).

As for the A/T: I'm pretty sure that the servo stops the forward motion when the highest thrust reaches THR REF. None of the engines is allowed to exceed the limit.


Cheers,

|-|ardy
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Registered: May 2010
Posts: 843
Quote
Perhaps you're thinking of the EEC's thrust equalization logic?


Probably :mrgreen:

Quote
As for the A/T: I'm pretty sure that the servo stops the forward motion when the highest thrust reaches THR REF. None of the engines is allowed to exceed the limit.


I thought there might be some allowance to go higher if the engine limit has not been exceeded (e.g. during derated takeoff). I guess, however, that that might introduce some lateral control issues if an outboard engine was at a significantly higher thrust than the others.

Rgds
JHW
Member
Registered: Aug 2014
Posts: 24
Just to say, I concur with John, there's a single Autothrottle Servo Motor and a 4 lever clutch pack, all of the throttles will move together, until they start hitting the stops. Now if it was a B777, you can individually disarm the A/T and the FMA area of the PFD will then annunciate an individual throttle mode, i.e. L THR REF or R THR REF. Now if the B747 only operated 3 levers, shouldn't there be MODE annunciations and EICAS messages?

Kev
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Registered: Aug 2014
Posts: 375
Location: Camberley, UK
Hardy,

You might well be right about (1). That would certainly tie in. The only time that I have used/seen this scenario would have been when the engines were at THR REF I guess...

In which case the "stagger" (idle or otherwise) would be maintained provided that one lever was at it's limit..

Peter
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Registered: Aug 2014
Posts: 375
Location: Camberley, UK
I have just flown out a number of take-offs with 10.0.5 Beta 1 and the thrust levers work as I would expect! They all move as placed around the clutch - yet the other day I found in the same situ that the closed thrust lever opened up and slowly came up to join the others...go figure?

I guess everything is all right then? Panic over! :-)

Sorry guys - maybe I'm getting a bit old - starting to see things now...

Peter
Moderator
Registered: May 2009
Posts: 5140
If the A/T is engaged in SPD mode and the current speed and the speed trend is very low and the target is very high, the servo keeps rotating until all levers are at max. If it wouldn't do that, you would have no speed protection and you may stall as the A/T would stop at the first lever hitting the max while the others may be at half thrust. But this is only in SPD mode, not in THR REF.


|-|ardy
« Last edit by Hardy Heinlin on Sun, 07 Jun 2015 16:33:06 +0000. »
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edited after your edit..
Although this edit now comes before your earlier edit....:-)
« Last edit by Britjet on Sun, 07 Jun 2015 17:51:49 +0000. »
Moderator
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Maybe not. I just edited my post above :-)


|-|
Member
Registered: May 2010
Posts: 843
John H Watson wrote
I seem to recall a similar debate in the past. Isn't it stated in the manuals that if the levers are staggered, the servo stops when the second lever reaches the target thrust? (At least in the case of THR REF).



Hardy wrote
Perhaps you're thinking of the EEC's thrust equalization logic? In that system the second highest TLA sets the target thrust (there are some numbers on page 503 in the Aerowinx manual).



Now I'm confused again.... From the Engineering Training Manuals Book 10

"Trim commands
..... The trimming function is active when the Autothrottle is armed. Each FAFC transmits an EPR Command, EPR trim limit and TRA to the FMC...

In the Automatic Mode (Autothrottle engaged) the FMC drives the Autothrottle servomotor to position the Thrust Levers to a setting at which the second highest EPR command matches the Target (Pilot Commanded) EPR. The FMC then uses this value as the Trimmer Reference from which it calculates the EPR Trim for the other Engines."
Member
Registered: May 2009
Posts: 149
"Not my department, but ..."

The trim discussions etc. all seem to assume that the all engines are running under the autothottle logic. But, if I understand correctly, the B744 autothottle does just fine on one engine (or two?) out, but as noted the procedure is to retard the bad engine and cutoff its fuel. It doesn't seem to make sense to then have the autothrottle logic try to keep the 'dead' engine lever trimmed up with the others. Am I missing something here - shouldn't there be a modified servo function to handle only 3 engines on with the autothrottle on?

Cheers,
Torrence
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Registered: May 2009
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If I quote myself: "The allowable trim range depends on the current pressure altitude and varies between 1.6 and 2.6% N1 on GE models, respectively 0.03 and 0.05 EPR on the other models."

If the difference to an engine is greater, the respective engine will not be equalized with the second highest thrust.


Cheers,

|-|ardy
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Registered: Aug 2014
Posts: 375
Location: Camberley, UK
Incidental to this discussion, but the autothrottle will disengage with a second engine failure, and can't be re-engaged.

Peter
Member
Registered: Nov 2010
Posts: 139
Location: Vienna
Hi,

just my 2ct as I have a 747-400 control stand: the servo motor is connected to the autothrottle servomechanism/brake assembly. This is a single unit which moves all thrust levers as commanded. There is no mechanical connection to the fuel control switches.

The only mechanical connection is from the servo assembly to the thrust levers and the rods to the position resolvers.

There is an interlock mechanism for the thrust levers, but only to prevent the reverser from being opened.

Some parts of this mechanism are visible on different pics in my gallery.

Tonight I'm going to add some new pics, because I finshed my handcrafted potentiometer assembly for the thrust levers.

BR
Hans
_______________
Hans G. Schuetz
http://B747-400.net

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