Precision Simulator update 10.173 (24 February 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

Engine trimming

Started by Hardy Heinlin, Fri, 21 Oct 2011 02:31

Hardy Heinlin


does anybody happen to know what the trimming range is for the PW4000?

We know the values for the other models:

Below 19500 ft: +/- 1.5% N1
Above 19500 ft: +/- 2.5% N1

Below 19500 ft: +/- 0.03 EPR
Above 19500 ft: +/- 0.05 EPR

(This is a new feature, it wasn't modelled in PS1.)



Jeroen Hoppenbrouwers

Is the trimming range the authority the EECs have on top of what the A/T wants, to try to align all engines in terms of N1 or EPR? The flight engineer leaning forward and thumb-twiddling the throttles to read four times the same thrust?


Hardy Heinlin

Yes, it equalizes the thrust to a reference value if it's within the allowed trimming range and if it's 1° away from the idle and max stop (smooth transitions). I use a small quicksort algorithm running at 5 Hz to determine the second highest throttle angle of all four levers. This throttle's N1 or EPR is then the current reference.

A/T engagement is not necessary, it also works if the A/T is just armed (and if a few other conditions are valid).

This feature is one reason why I introduced the (mechanical) "throttle humanizer" option; it may demonstrate the electronic trimming.



(Screenshot from summer 2010. In the meantime the humanizer now works for USB and keyboard as well, not just for the mouse.)

Hardy Heinlin

There was the question whether the trim reference is the highest or second highest throttle. The books mention the one, sometimes the other. In my current model, the question resolved itself.

As we know, the trimmer works electronically. It works when the A/T is armed (it doesn't matter if it's engaged or not, it just needs to be armed).

The trimmer's reference is the second highest throttle lever angle (TLA).

When the A/T is engaged, the A/T servo is an additional player in the game. If the THR REF mode is engaged, the servo moves so that none of the engine exceeds the THR REF.

The A/T's reference is the highest thrust.


Here's how the "contradiction" in the books melt together and harmonize to a common logic. Example:

Say, all four throttles are in a mid range and the A/T is disengaged. At the moment, engine 3 has the highest thrust and engine 4 has the second highest TLA. The trimmer equalizes all four thrust commands to that of engine 4.

Engage the A/T and get the THR REF mode active. The A/T servo moves and stops when the highest thrust of all four is on the THR REF line. In this example it's the thrust of engine 3. Engine 3 has the highest thrust, regardless what it's TLA is; the A/T wants to protect the engine, not the lever.

The other three engines are below the THR REF line.

The trimmer keeps working. It reduces the highest thrust to that of engine 4. Why? Engine 4 has the second highest TLA. (The thrust of engine 1 and 2 is trimmed as well, it's trimmed up.)

All four engines are now equal -- and below the THR REF line.

The A/T notices this and moves the servo forward. It checks that the engine with the highest thrust will not exceed the THR REF line. In this case all four engines are equal.

All four are the highest reference now. No more servo motion required. All trimmed and set.

But, further on and all the time, the A/T refers to the highest thrust, while the electronic trimmer refers to the second highest TLA.



Richard McDonald Woods

Is it true to say that the levers are 'showing' the commanded thrust, and that the actual thrust set for each engine remains under the control of the EECs? So does the trimmer work on the lever settings only?
Cheers, Richard

Hardy Heinlin

Vice versa. The levers just show themselves: the lever angle (TLA). They are not moved by the electronic trimmer. They can only be moved by the A/T servo or by homo sapiens. The TLA may be slightly higher or lower than the commanded thrust. It depends on the individual quality of each engine system.

The TLA goes  --> to the EEC (electronic engine control) which adjusts --> the fuel flow which sets --> the final thrust.

Dollar --> Yen --> Euro --> Drachma

(Metaphor in random order.)

The four EECs talk with each other and try to make all their four thrust settings equal -- provided they are within the allowed trim range. They can't move the TLA, they adjust the fuel system directly.




English suggestion:

Replace "Slight random misalignments when moving with the mouse four levers at once"

With "Slight random misalignments when moving four levers at once with the mouse"

Disclaimer #1: I realize that I only speak American English, so I'm happy to be recall my suggestion if this is a standard British or Aussie or Kiwi or Canadian form.

Disclaimer #2: I also realize that English is a living language, and you (Hardy) own it as much as any of us do, and are therefore free to invent usages of your own, obeying rules of logic rather than rules of common usage, so if you want to invent new forms intentionally, that's fine with me too.  But I thought you might want to know what sounds odd to a native speaker.
Will /Chicago /USA

Hardy Heinlin

I like your disclaimer #2 :-) No matter if it's about English, German or any other living language, the debates between purists and anarchists are everywhere. I, too, think that the best way here is always a compromise between traditions and new inventions. Traditions produce identity and nice memories, this is important. And new inventions add spice and revitalize traditions when they start to get boring and forgotten. That's also important. Aside from this, inventions are often just additions, not replacements. Without inventions, there would be no life at all.

I put the word "mouse" at the end because I wanted to emphasize the lever movement, not the mouse. In German, one can emphasize a word either by putting it in Italics, or by placing it at the beginning or end of a sentence. I thought  English provides exactly the same three options.

Meanwhile, the problem is solved anyway. I removed the mouse from the comment as the feature now works with USB and keyboard as well.




You're the most pure of the purists when it comes to fidelity to the 747-400... I guess the anarchic tendencies have to some out somewhere!  (Planting any subversive Easter eggs in your code?)  Anyway, strong work, carry on.  I didn't mean to hijack the thread.
Will /Chicago /USA

Hardy Heinlin

I don't mind when my threads are hijacked.

I'm still curious about those three options to emphasize a word; doesn't this work in English?


We don't have as much flexibility as in German when it comes to using word order for emphasis.  But this case, the issue isn't emphasis, rather, it's keeping the object close to the verb.  Here you have the verb-object pair separated:

"Slight random misalignments when moving with the mouse four levers at once"

The point is that the following sentence puts them together:

"Slight random misalignments when moving four levers at once with the mouse"

I can't cite a specific rule about this, so for now, I'll just say that the bolded bits in the sentence want to be together, and emphasis has to occur in a way that doesn't separate them.
Will /Chicago /USA

Hardy Heinlin

Harr :-) I must admit, my original sentence isn't even possible in German, even less flexible than English:

... beim Bewegen mit der Maus der 4 Hebel (wrong)
... beim Bewegen der 4 Hebel mit der Maus (good)

John H Watson

QuoteThe four EECs talk with each other and try to make all their four thrust settings equal -- provided they are within the allowed trim range. They can't move the TLA, they adjust the fuel system directly.

The EECs don't really talk to each other. Actual individual engine thrust data is sent to the FMC (via EIUs).

The FMC does the trimming calculations, then sends trim data to the individual  EECs.

No FMC or no EIUs = no trimming.

Hardy Heinlin

See, Richard. It's even more complex. The FMC is the bank :-)

"No FMC or no EIUs = no trimming." -- This condition is included in my model.

Shiv Mathur

Just out of interest, I think both concerns could have been met by using parentheses or dashes ...
"Slight random misalignments when moving (with the mouse) four levers at once".
"Slight random misalignments when moving - with the mouse - four levers at once".

Admittedly not as elegant as Will's construction, but it does throw the emphasis on the levers (and not the mouse).

Apropos living and evolving languages:
" ... I'll just say that the bolded bits in the sentence ..."

As far as I know, "bolded" has not yet entered English dictionaries ... though
I'm sure it soon will.
But is it really necessary, when a perfectly good word is already at hand ?
" ... I'll just say that the boldface bits in the sentence ..."

Sounds more elegant - to my ears!


Hardy Heinlin

Good idea. I would use the dashes -- or the parentheses (probably the parentheses).


Why not bold instead of bolded and boldface?


Shiv Mathur

I guess because it could be open to misinterpretation.

'Bold' bits in a sentence could mean the writer had penned some
rather daring or contentious thoughts.

I guess!!



I just looked at a few dictionaries, and "bold" is listed both as a noun (a heavier-than-standard typeface) and a verb (to place in boldface).  So Shiv, the times, they are a  changin'.
Will /Chicago /USA

Jeroen Hoppenbrouwers

Yup, for a while already we have Times New.


Quote from: Hardy Heinlinor the parentheses (probably the parentheses).
Shiv beat me to it - I had the same idea, but with commata:
...when moving, with the mouse, four levers at once.
Breaks the rhythm and thus generates attention (methinks...).

As to the adjective  bold, the corresponding verb exists already: to embolden... !