The only one that will get away with impersonating Bruce Lee is Chuck Norris  :ph34r:

The only one that will get away with impersonating Bruce Lee is Chuck Norris  :ph34r:

 

And not very well. :biggrin:

 

Nobody was like Bruce.

I can add to the previouse comments regarding the trimming, that it is really difficult to trim the airplane with A/T engaged on arrpoach. sometimes trimming the yoke button does not change anything and after several pushes the pitch is more than required. It is hard to keep an eye on the pitch trim parameters on the FCTL page on final approach to monitor this behaviour, but I will give it a try once.

 

In some Aircanada video's I've watched closely, the captain doesn't seem to re-trim at all, rather, pulling back on the yoke then relaxing it to adjust the pitch. In other videos, the FO uses pitch trim accopmpanied with pulling back the yoke.

Hi all,

 

Just to pitch in (no pun intended, really) but I've just filed a ticket regarding this exact issue.  Interesting info in this thread from well informed sources, admittedly most of it has flown (no really, I'm not doing this on purpose) right over my head.

 

Be interesting to see how PMDG react to this.  Otherwise, beautiful aircraft, just a PITA to hand fly.

 

Richard

Hi Guys,

 

I finally got a chance to do a little hand flying on the real 777.

Results are not spectacular as it behaves just like I have always said it does.

 

The 777 needs to be trimmed for speed changes just like any conventional aircraft.

Be it Cessna172 or B7373 or B777 it all feels the same.

 

Now the important thing here is...I am only talking about 777 trimming for SPEED changes.

I am not talking about automatic FBW compensation for thrust (pitch up/down moment) changes, configuration changes, bank angles, etc.

 

I am also not saying that there is not a heck of a lot more going on in the background (inside the FBW system) than what the pilot sees or feels.

 

What I am saying that BOEING has programmed the FBW system so that it simulates a conventional aircraft when it comes to SPEED changes.

 

I tried the following:

1) After take, passing 1000ft the FMC changed to CLB thrust and started acceleration.

I did not manually trim.

V2 was 166kt and we had 4.25 units of T.O. Stab trim.

We had the Flight Controls synoptics open and looked at the Stab trim setting.

As we accelerated and retracted the flaps, more and more foreward pressure on the Yoke was required to keep the nose from going up.

Even untrimmed at 250kt I could still hold the nose down with foreward pressure while trimmed for V2 (166kt) but you would not want to fly like this for 10 minutes.

The Stabiliser trim indicator on the Flight Controls synoptics page did not change.

It stayed at 4.25 Units.

I did not trim untill around 3000ft and fwd pressure was considerable, but not as much I thought it would end up to be.

This is just like a conventional aircraft would behave.

 

We had to level off at 5000ft which is why I started trimming out the forces around 3000ft (would not want to bust my altitude now would we!)

I leveled off manually.

 

2) When we cleared for further climb (11000ft), still in manual flight (but AT engaged) we used Vnav again to continue the climb. So AT mode went to Thrust Ref.

This time I let go of the yoke.

So the FD went up, but I did not follow it.

The aircraft stayed in 5000ft initially but then as airspeed increased it started a gradual climb. Of course it was looking for its in trim speed of 250kt.

It took only 5kt or so (so increase to 255kt) and the airplane started a 100ft/min climb or so....airspeed still increased and by the time we had 260kt, vertical speed had increased considerably (about 1000ft/min)

 

This is also just like a conventional aircraft behaves.

 

3) We were cleared to 17000ft I think it was and upon passing 10.000ft we accelerated from 250kt to 310kt.

Still in manual flight and again I did not use manual trim.

Again the aircraft required more and more nose down yoke force and again the Stab trim units did not change.

This time I let go of the yoke slowly and instantly the nose went up as the airplane wanted to return to its in trim speed of 250kt.

I could not just let go of the yoke instantly as that would have resulted in quite an uncomfortable aggrassive pitch up moment that I did not want to put the pax through.

But even slowly relaxing my nose down yoke force just a bit, we had +3000ft/min climb rate in no time!

Again I had to start trimming nose down because 17000ft was approaching fast!

 

So conclusion is that speed trim is just as for a conventional aircraft.

 

Now I know that even though my Stab Trim indication on the synoptics page did not change AUTOMATICALLY at all.....that does not mean the FBW is not doing all kinds of things in the background.

So I am not saying that elevators and Stabiliser dont do anything during those few test.

I could not see the Stab was doing anything but those flight control indications are pretty rudimentary too.

You can open up maintenance pages on the lower EICAS that show a vast amount of ever changing numbers. And no, I had no time to look at them close enough during manual flight to see what exactly all those numbers were doeing.

 

All I am saying is that for speed changes....you have to use manual trim.

There is no AUTORIM that trims away the force you feel when you change speed.

 

Now, PMDG is already looking at it so I am convinced they will inform us of their findings real soon.

 

In the mean time, enjoy the PMDG777 by not trimming manually or by flying in the Secondary flight control mode ;-)

 

Regards, Rob.

 

 

As we accelerated and retracted the flaps, more and more foreward pressure on the Yoke was required to keep the nose from going up.

 

Hmm, wouldn't that have been a configuration change it should have trimmed away for you while keeping the trimmed 250 knots?

 

Hmm, wouldn't that have been a configuration change it should have trimmed away for you while keeping the trimmed 250 knots?

 

HE accelerated while retracting Flaps, so the aircraft needed to be re-trimmed. If he didn't change the airspeed, only retracted the Flaps, I should have trimmed automatically.

 

 

777Simmer,

 

Thank you for this flight report! You confirmed all the suspicious we had with the real 777 FBW system! I'm Glad it works like that in the real aircraft too!! I Wouldn't like the B777 at all if it had that AutoTrim stuff making my hands on flying feels like a video game!

 

Now, if the Stabilizer is moving or not while you were holding the Yoke forward, we don't know, as you trim the aircraft, as we understand, it will first use the elevator, then Trim with the Stabilizer, but just with Yoke Input is something different. But I guess the real point here is the final behavior of the FBW system!

 

Let's wait for SP1!

 

 

All I am saying is that for speed changes....you have to use manual trim.
There is no AUTORIM that trims away the force you feel when you change speed.

 

Hi Rob.

 

I was afraid you were going to confirm this...

 

So here we have it. As the FCOM states, and many us would think after extensive testing, there is no auto-trimmig at all in the real aircraft for speed changes...

 

We'll have to wait to see PMDG conclusions and probably a fix for this in a hotfix or SP1.

 

Thank you very much Rob.

 

Regards.

Sorry if my question is answered.Must I leave the autothrottle engaged on my way to runway when handflying ?If so when can I disengage it?(Now I disengage it around 50AGL to flare)

So you ask how the system knows what it's trimming for... simply because auto trimming for config changes, and [we believe fallacious] auto speed trimming, occur at different times. If they were occurring together, we wouldn't need to fight the out of trim condition with elevator input.

Well the problem I had with understanding what was going on is that you can't separate one form of trim from another. It's either trimming or it isn't. Carlos's post about short term and long term changes confused me too.  My thought was that if a config change occurred at the same time you reached a new speed then it would trim for that at that new speed. (I say it, I mean the 777X, not the real thing). I did some more background reading about the C*U law the 777 uses and then it began to make sense. It turns out the FBW is trimming all the time. The trim switches change the trimmed speed the FBW uses as a reference.  If you are not at that speed this adds an error term to the calculation, in other words an offset to the trimmed position.  The further your speed is from the datum the more the error is, so the greater the out of trim force you feel with the column.  If you release the column the aircraft will return to the datum speed.  Also the higher your airspeed the larger the forces will be if you are not at the datum speed, just like a conventional aircraft.

 

So basically when you trim the real 777 out with the yoke switches, you are actually adjusting the trim reference speed until it matches the current airspeed, which makes the column force zero.  Now I understand the principle behind how this thing works without having to somehow separate long term and short term inputs (or somehow filtering out seed and only correcting for config) and yet still be able to simply control the pitch attitude with column as it does.  It's quite like the NGX in CWS mode only with a speed error term added to the control loop.

 

Sorry for all this waffle, but without understanding how the real aircraft trim actually works I couldn't see exactly how the 777X wasn't working.  The situation in the 777X is compounded by the almost complete lack of aerodynamic speed stability in the FDE.  Very different from the 747X, the MD11 and the NGX where significant trim changes can be seen as speed changes.  So even if the FBW in the 777X was working properly it still wouldn't feel right.