I use them for the T/O trim setting (ranging also +/-2 units above/below FMC value) after engine start, prior to taxi. This way I now the backup is working...

 

(BTW: after setting the T/O trim with the ALTNTRIM lever I briefly use the manual rudder trim knob in both directions, check the indicator and use the reset button to test its functionality, too. After that I test elevator    EDIT: aileron trim switches and yoke indication. Last thing is checking all controls by monitoring the FCTL page. Finally contacting ground: "request taxi").

^^^^ you have to do something for all the money ^^^^ B)

 

Is that SOP at your airline Claus, or is it just a personal initiative? Just out of curiosity as I don't think I've ever seen exercising the ALTN trim in the manuals before. 

Testing the ALTN TRM lever on ground makes also sense because you would not do so in flight. Noticing the alternative is not working while in the air would be a bit "late".

 

As I said, if it doesn't work it means the cable is broken. You aren't really testing anything else (as using the trim switches also uses the hydraulics). I don't know about the T7 but I haven't seen anyone use the altn trim levers preflighting a 747. still if you feel better having done it then fair enough.

 

While on ground the trim switches on the yoke would also directly move the "THS", in flight though they would first move the stabilizer's elevator (shortterm yoke input support) followed by a computed (longterm) stabilizer position adjustment (leaving the elevator in neutral position).

 

Using Airbus terminology like THS only confuses things. In flight the altn levers move the stab trim directly (they are directly connected to the trim actuators), it doesn't move the elevator first. In normal mode once the levers are released the trim is once again under the control of the FBW and will move in response to speed changes and elevator position.

 

That said, neither should the normal pitch trim with AP engaged!

 

Ah, but those switches are inhibited in AP so it wouldn't matter. The altn levers would move the stab and give the AP a hard time.

They make trimming be smoother, more precise.

I use a lever for trim and it is not smooth at all, very slow in fact.

Is that SOP at your airline Claus, or is it just a personal initiative? Just out of curiosity as I don't think I've ever seen exercising the ALTN trim in the manuals before.

So it's my personal SOP B)  And it "scares" me that "exercising the ALTN trim" functionality seems not to be daily business. 

 

I have some of those private SOPs. E.g. I only arm the T7 A/T (L/R) switches when runway alignment on line up is almost completed. It confuses me somehow that they are actually never turned off in RW, even when the A/C is C&D on the ramp... Those two GE90 are both a "hell of a gun" and keeping it always armed only interrupted by the two TO/GA switches on the thrust levers "make me somewhat nervous"...

 

 

 

As I said, if it doesn't work it means the cable is broken. You aren't really testing anything else (as using the trim switches also uses the hydraulics). I don't know about the T7 but I haven't seen anyone use the altn trim levers preflighting a 747. still if you feel better having done it then fair enough.

 

Knowing if cables from the ALTN TRM lever to the stabilizer are broken is perfectly good to me, because ALTN TRM directly moves the stabilizer on ground and in the air.

The trim switches move the stabilizer only on ground while in the air they "simply" select trim reference speed (Normal mode).  E.g. in case of a (partial) FBW/PFC failure/"excursion" you might not know what the trim switches actually do (move elevator shortterm or stabilizer longterm?). At this moment it would be fine to know that ALTN TRM lever works as a "direct stabilizer trim position master control".  (If only Airbus could incorporate this into its core design... <sorry, could not resist) 

And BTW: yes, I do feel better knowing ALTN TRM "is there" although I prefer flying without it!

 

 

 

In flight the altn levers move the stab trim directly (they are directly connected to the trim actuators), it doesn't move the elevator first. In normal mode once the levers are released the trim is once again under the control of the FBW and will move in response to speed changes and elevator position.

I never said that the ALTN TRM moves the elevator first. In normal mode the trim switches move the elevator first, followed by the repositioning of the stabilizer. And I would never suggest to use the ALTN TRM in normal (mode) flight conditions. It's more for rare conflicting moments of not knowing what the FBW/PFC is doing and than it could be a life insurance.

 

 

 

The altn levers would move the stab and give the AP a hard time.

I have no idea but I think at Boeing the engineers are not the "dual input causes CFIT" type of guys.

Human input always should override computer (AI) input, and especially, AI should never counteract human action in a deceivingly manner. So if the crew uses the ALTN TRM lever (typically in an abnormal situation) the A/P should keep its hands out of the pitch controls of any kind. (I would expect: A/P CMD degradation to roll modes only or complete A/P off.)

 

More or less the same applies to the FBW system's reaction you describe after the use of the ALTN TRM in normal mode ("manual" flight). If the crew uses ALTN TRM (for whatever reason) I can hardly imagine that as soon as the ALTN TRM lever is released (at the desired A/C pitch angle) the FBW starts again hunting the (old) trim reference speed by elevator and stabilizer (counter!) action.

This would also be an "AI wins dual input contest" situation which I actually could not imagine by Boeing.

IMO the only thinkable action would be capturing/intercepting the "new" IAS at the current pitch angle.

 

Ah.. I think I will now fire up my T7 right of ( ) EDIT: off the runway's centerline into the skies and try some nasty stuff... :blink:

Edited September 19, 201510 yr by vr-pilot

And it "scares" me that "exercising the ALTN trim" functionality seems not to be daily business.

There are a lot of alternate or backup systems that aren't exercised during preflight. Alternate gear extension for example. These kind of things are checked during maintenance. Mechanical controls tend to keep working if they have been properly maintained and occasionally tested. Your idea of positioning the stab using the levers and so checking them isn't a bad one but I don't think it's done that way in real life.

 

Knowing if cables from the ALTN TRM lever to the stabilizer are broken is perfectly good to me, because ALTN TRM directly moves the stabilizer on ground and in the air.

 

The trim switches move the stabilizer only on ground while in the air they "simply" select trim reference speed (Normal mode). E.g. in case of a (partial) FBW/PFC failure/"excursion" you might not know what the trim switches actually do (move elevator shortterm or stabilizer longterm?). At this moment it would be fine to know that ALTN TRM lever works as a "direct stabilizer trim position master control". (If only Airbus could incorporate this into its core design...

And BTW: yes, I do feel better knowing ALTN TRM "is there" although I prefer flying without it!,>

The FBW can't fail and move the trim before the elevator, that isn't how it works. The FBW always drives the elevator. The trim only moves to keep elevator deflection to a minimum. If the elevator hasn't moved the trim has no need to move. It's analagous to how the AP drives the elevator and trim.

 

The Airbus has an equivalent system. It has a manual trim wheel, at least the smaller Airbuses do. The wheel moves when the trim is being driven automatically. In direct law the trim is manual and moved by the crew using the trim wheel. I don't have time at the moment to check about the larger Airbuses but they certainly do have alternate means to drive the stabiliser and show the crew where it is.

 

I never said that the ALTN TRM moves the elevator first. In normal mode the trim switches move the elevator first, followed by the repositioning of the stabilizer. And I would never suggest to use the ALTN TRM in normal (mode) flight conditions. It's more for rare conflicting moments of not knowing what the FBW/PFC is doing and than it could be a life insurance.

Yes, sorry Claus, you didn't say that. I misread your post.

 

I have no idea but I think at Boeing the engineers are not the "dual input causes CFIT" type of guys.

Human input always should override computer (AI) input, and especially, AI should never counteract human action in a deceivingly manner. So if the crew uses the ALTN TRM lever (typically in an abnormal situation) the A/P should keep its hands out of the pitch controls of any kind. (I would expect: A/P CMD degradation to roll modes only or complete A/P off.)

 

 

 

More or less the same applies to the FBW system's reaction you describe after the use of the ALTN TRM in normal mode ("manual" flight). If the crew uses ALTN TRM (for whatever reason) I can hardly imagine that as soon as the ALTN TRM lever is released (at the desired A/C pitch angle) the FBW starts again hunting the (old) trim reference speed by elevator and stabilizer (counter!) action.

 

This would also be an "AI wins dual input contest" situation which I actually could not imagine by Boeing.

 

IMO the only thinkable action would be capturing/intercepting the "new" IAS at the current pitch angle.

 

The guys at Boeing are no different to the guys at Airbus (who are also not the "dual input equals CFIT" type). I don't want to reopen our discussion on AF447 but dual input was not the primary cause.

 

If you did decide to move the 777 ALTN TRIM with AP engaged the AP will try and fight back with elevator to control pitch. It really isn't something you would want to do. You may not think Boeing could design a system in this way, but this is how the ALTN trim works in the 777. Moving the trim with AP engaged is something you should never do. In most aircraft it causes the AP to disengage, but not the 777. So the FCOM warns specifically against doing it. You certainly couldn't have a situation where the AP simply ignored the effect an ALTN TRIM input. Anyway, it only sees trim position, not what moved it. If it did do as you suggested, when would it decide to respond in pitch again? If there is some kind of trimming problem the AP should be disengaged, because the AP needs the trim to be working. That way the AP logic is kept straightforward and the pilot is always in control.

 

ALTN TRIM is for the situation where the yoke switches don't work. That would always be a situation where the AP was off because you can't use the trim switches with AP on. If there was a trim runaway the ALTN levers can oppose that, but again the AP should be disengaged first.

 

As I said, if you use the ALTN TRIM levers it not only moves the stabiliser directly but it also changes the trim reference speed. When you release the levers it won't return to the previous trim speed. Thus the FBW continuing to trim as normal is not a problem. Again the system would become horrendously complex if it had to be designed for every possible input scenario. The FBW and AP always do what they are designed to do and it's up to the crew to monitor what is going on and act accordingly. You say what if the crew don't know precisely what the FBW is going to do. Imagine if it was programmed in the way you suggest. The number of possible behaviours would multiply dramatically and the crew would never know what to expect. It's complex enough as it is without trying to make it intelligent.

Thanks Kevin for your brilliant explanations. I really appreciate it very much B)

Your logic approach of system analysis is quite fascinating (I really mean what I say!):

 

ALTN TRIM is for the situation where the yoke switches don't work. That would always be a situation where the AP was off because you can't use the trim switches with AP on. If there was a trim runaway the ALTN levers can oppose that, but again the AP should be disengaged first.

 

 

Yes, I tried to irritate the PMDG T7 A/P by setting/holding the ALTN TRM lever to nose down and nose up positions. The A/P stayed on and counteracted as you described with "pure" elevator action (with cockpit yoke response/movement (think of Airbus Flightstick here)) against the (odd) stabilizer positions. So the A/P stayed on, in contradiction to e.g. the 737(NG) where even trim switch usage turns off CMD/CWS pitch modes immediately... (I will test what happens when using the 737 stab wheels manually, but I would also suppose there will be the A/P off aural and red light warning.)

 

 

I don't want to reopen our discussion on AF447 but dual input was not the primary cause.

 

Yes, dual input was just one of several "coffin nails" on this flight. Anyhow Capt. Sully (!)  describes the Airbus dual input as "problematic" and states that with a Boeing design that accident would have been "less likely"...

(I also don't want to reopen our AF447 discussion, but this thread about elevator trim vs. ALTN TRM lever somehow predicted it IMO.)

 

As I said, if you use the ALTN TRIM levers it not only moves the stabiliser directly but it also changes the trim reference speed. When you release the levers it won't return to the previous trim speed. Thus the FBW continuing to trim as normal is not a problem.

 

Yes, that is exactly what happend. So extremly straight forward and easy to predict.

 

Again the system would become horrendously complex if it had to be designed for every possible input scenario. The FBW and AP always do what they are designed to do and it's up to the crew to monitor what is going on and act accordingly. You say what if the crew don't know precisely what the FBW is going to do. Imagine if it was programmed in the way you suggest. The number of possible behaviours would multiply dramatically and the crew would never know what to expect. It's complex enough as it is without trying to make it intelligent.

 

Again a brilliant explanation, I really like it! B)

FBW "the Boeing style" seems to be quite predictable and keeps the pilots in control (and command!) at "any time". Airbuses are so to say 99.99999999999% reliable, maybe even more reliable or service friendly than Boeings, but "at the end of the day" IMO an almost unaffected pilot's command is really worthwhile...

https://youtu.be/8erXQGgr7VE?t=226

Interesting test part starting from 3:48.

At 4:44 the core sentence IMO:

(quote): "it provides a lot of capability. It helps them (the crew) where they can use some help.  And yet gives them the ultimate control of the aircraft in having it do what they want it to do." (quote end)

That was in 1994.

 

 

Thanks Kevin for your brilliant explanations. I really appreciate it very much B)

Your logic approach of system analysis is quite fascinating (I really mean what I say!):

Hi Klaus,

 

I'm glad I was able to help. Aircraft systems analysis is part of my day job so I get plenty of practice.

 

Cheers

 

Kevin