http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

 

11.8 - very interesting reading.

 

Best regards,

Robin.

I know im a little bit late in this discussion, but heres something i just read from a 777 FCTM that kind of explains how it works.

 

 

 

The primary difference from a conventional airplane is in the pitch axis. A control

column input commands a pitch maneuver instead of commanding a surface to

move, so anything else that tries to change the pitch attitude or flight path will be

countered by the flight control system. Therefore, the system minimizes the

airplane pitch response to thrust changes, configuration changes, turbulence and

turns.

Thrust changes no longer need to be countered with a column input. This

minimizes pilot work load during accelerations and decelerations. However, for a

climb, as thrust is applied, the pitch attitude must be increased to begin the climb.

For a descent, as thrust is reduced to idle, the pitch attitude must be decreased to

begin the descent.

Speedbrake or flap changes no longer need to be countered with a column input.

The flight control system automatically counters the change in lift. A small pitch

change may be noticed as the flight control system changes the pitch attitude to

keep the flight path relatively constant

For turns up to 30° of bank, the pilot does not need to add column back pressure

to maintain level flight. For turns of more than 30° of bank, additional column

back pressure is required.

For a roll or yaw asymmetry such as an out of rig or failed control surface that is

not in the commanded position, the pilot must make a control input and trim as

needed. The system does not automatically counter the asymmetry.

The pilot still needs to trim for speed changes. Column forces increase when out

of trim to provide the conventional speed error cue.

After a short flight to check out the FBW on the PMDG 777, I can say that I am seriously impressed.

 

A simple flight was conducted. Hand flew up to 10,000 at 250kts, A/T commanding selected speed.

 

Brought up the FCTL page to see what was happening.

 

Wings level, pitched up to 10 degrees. Pitch holds as the engines spool up to maintain 250kts.

 

Now for the magic.  Increased selected speed to 295kts. Pitch remains the same as the speed increases. Looking at the FCTL page, watch the elevator and trim twitching to maintain pitch.

 

Same in reverse. Command a speed of 220kts and watch the speed bleed off, but the pitch remain constant.

 

Back to straight and level flight - 10,000ft 250kts.  Shut down an engine.  Watch that rudder slowly feed in to compensate for the yaw. Fly around on one engine, and hardly notice that you've lost a donkey.

 

At slower speeds, level flight at 220kts, and drop some flap. No pitch change! Nice - and as is different from the Airbus, watch the V/S increase as flaps are fed in.

 

Probably due to the constraints of FSX, there are some minor excursions from pitch at the edges of the flight envelope (mainly at higher speeds), but overall a darn good job WRT the FBW laws of the 777. Great work!!

Try bank angle protections and throttle wakeup with A/T disengaged but armed - great airplane!

 

Probably due to the constraints of FSX, there are some minor excursions from pitch at the edges of the flight envelope (mainly at higher speeds), but overall a darn good job WRT the FBW laws of the 777. Great work!!

 

The real thing is not perfect with it either actually. (and neither is Airbus - I saw it firsthand in an A320 sim I got to fly after we finished our 777 session)

 

One other deviation from what the manuals say - the real airplane actually *does* need some back pressure in turns at less than 30 degrees. We saw it in the simulator and our tech pilots confirmed it as well. It removes around 70% or so of the normal back pressure requirement but there is absolutely some still there in real life and we modeled that even though the manual says you don't need any.

So what happened if I soft disengage auto throttle during approach? the auto trim still working for speed?

Yes, the flight control computers will still pitch for speed, then flight path (for neutral control input). Once trimmed for a stable approach, on speed, use elevator and throttle to make adjustments to the flight path. Once on speed, you should avoid trimming as this moves the flight control computers' reference airspeed.

The question being asked in other threads and is still not clear to me (and not clear in the pdf either) is:

 

What is the real aircraft behaviour if you turn off the auto-throttle and set a fixed thrust in level flight thus giving fixed speed then you pull back and climb. As I see it, the aircraft was in trim with a fixed trim reference speed, but because it will now slow down and climb the new speed will not be the same as the level flight trim reference speed. Hence, you now manually have to trim the reference speed to match the climb speed to unload the column.

 

However the pdf says:

 

''the stabilizer will automatically trim, when necessary, to offload the elevator surface and allow it to return to its neutral surface when the airplane is in a trimmed condition''???

 

Later it says:

 

'As on a conventional airplane, trimming is required to reduce any column forces that are being held by the pilot.'

 

Confusing to me.

 

I haven't tried it yet but does the aircraft need to be manually trimmed with the A/T off when you pull back? I can understand no trimming being required with the A/T on as the thrust will increase to maintain the level flight trim reference speed.

 However the pdf says:

 

''the stabilizer will automatically trim, when necessary, to offload the elevator surface and allow it to return to its neutral surface when the airplane is in a trimmed condition''???

 

Later it says:

 

'As on a conventional airplane, trimming is required to reduce any column forces that are being held by the pilot.'

 

Confusing to me.

 .

At the moment we are having people claiming airbus like behavior.

 

Here:

 

http://forum.avsim.net/topic/419407-hand-flying-tips/

 

If this is true than we can skip all future discussions on this subject!

However I cant verify this at the moment because dont have the plane yet.

I will buy it tomorrow and let you know if the PMDG777 FBW system is realistic or not.

(I have been flying the thing in real life for more than 10 years)

 

You can go through my (and others) previous posts in this thread in the mean time to find out how the real thing works.

Everything is discussed already.

 

As for your question;

What pdf are you talking about?

FCTM?

 

I think that the first bit, with the stab trimming automatically is not referring to trimming during manual flight.

But I would have to see the whole page to be sure.

with he AP engaged the system trims automatically for speed changes.

During manual flight it does not.

Hi Rob

 

I was referring to 11.8.1 in this pdf: http://www.davi.ws/a...book_Cap_11.pdf

 

However the pdf says:

 

''the stabilizer will automatically trim, when necessary, to offload the elevator surface and allow it to return to its neutral surface when the airplane is in a trimmed condition''???

 

Later it says:

 

'As on a conventional airplane, trimming is required to reduce any column forces that are being held by the pilot.'

 

Confusing to me.

 

It IS a bit confusing !  :smile:

 

Another extract of the same chapter:

"While IN FLIGHT, the pitch TRIM switches on the Captain’s and First Officer’s control wheels do NOT directly CONTROL the horizontal STABILIZER as they normally do on conventionally controlled airplanes.

When the trim switches are used in flight, the pilot is actually requesting a NEW referenced TRIM SPEED.

The airplane will pitch nose up or nose down, using the elevator surfaces, in response to that

reference airspeed change to achieve that new airspeed. The stabilizer will AUTOMATICALLY TRIM, when necessary, to offload the elevator surface and allow it to return to its neutral surface

when the airplane is in a trimmed condition."

 

So when you use the pitch trim switches INFLIGHT, you do not directly trim, but you set a referenced trim speed and stabilizer trimming itself is then AUTOMATIC.

 

So TRIMMING is REQUIRED (for speed changes) but it is in fact an indirect trimming.  :smile:

 

Guy

Hi Rob

 

I was referring to 11.8.1 in this pdf: http://www.davi.ws/a...book_Cap_11.pdf

Ok yes, seems pretty clear to me.

 

We are talking speed changes here, plain old fashioned pitch trim, to trim away the yoke forces a pilot would have to execersis after a speed change!!!:

We are not talking about the advanced automatic compensation features the 777 has for flap config changes, thrust setting changes, etc!!

 

The stab wont trim trim out of the blue, nor by itself, nor automatically for speed changes.

What is said is that if YOU trim, so if you give a manual trim input you are not actually changing the Stab position directly.

There is no connection from your trim switch to that Stab actuator.

In stead your trim switch input goes to the Primary Flight Computers (PFC).

Inside the PFCs you have now changed the in trim speed.(trim reference speed)

(it changes about 10kt for every one second you hold the trim switch).

In response, the PFC sends a signal that ends up changing the elevator position so the airplane pitches up or down.

And then sensors feel/see that the elevator is deflected and so another signal is given so that the stabiliser streamlines with the elevator.

The Stab streamlines automatically with the elevator, but not unless you first have given a manual input.

 

Why first elevator and then streamline the stab?

I dont remember, but I guess elevator trim will react faster than that huge, heavy stabiliser.

Why not just leave the elevator in that position?

Well at some point you would run out of elevator control and you cant pitch up (or down depending on which limit you have reached) with yoke control anymore.

Not Good!

 

 

Despite all the things happening behind the scenes, to the pilot, in regard to trimming for speed, the 777 trim feels just like Cessna trim.

 

Well, the real 777 does!

I will see tomorrow myself what the PMDG777 feels like.

.

So TRIMMING is REQUIRED (for speed changes) but it is in fact an indirect trimming. :smile:

 

Guy

Yes.

 

But you would not notice anything of that.

See my post above.

 

I keep saying this, It feels just like trimming a Cessna!

(or a PMDG737NGX if you want)

Does it really feels like trimming a Cessna or is it more like trimming the DCS black shark helicopter, if you know about that game. I've noticed it works pretty well for me especially after takeoff, I pitch for the speed I want then just toggle the trim switches for a second and then let go and watch the plane settle. So to me it feels like a normal plane, except that you don't have to hold the trim switches to move the stabilizer enough to neutral the elevator. You just flick the trim switches and let go. At least so it appears to me.

Does it really feels like trimming a Cessna or is it more like trimming the DCS black shark helicopter, if you know about that game. I've noticed it works pretty well for me especially after takeoff, I pitch for the speed I want then just toggle the trim switches for a second and then let go and watch the plane settle. So to me it feels like a normal plane, except that you don't have to hold the trim switches to move the stabilizer enough to neutral the elevator. You just flick the trim switches and let go. At least so it appears to me.

Well, a Cessna does not have trim switches....soooo......yes the idea is the same as trimming a Cessna, but you dont have a trimwheel in your hand.

 

For instance, after T.O. when you clean up and accelerate to 250kt, if you do not trim you have to hold the nose down with more and more force on the yoke as you accelerate.

This gets quite heavy when accelerating from for example 180 to 250kt.

You should need 5 or 6 trim inputs nose down, a good 2 seconds long each input, to trim this nose down force away.

In our sim at home we dont have force feedback, so you dont feel anything but you should be foreward of neutral with the yoke quite a bit if you dont trim.

Just a question 777simmer: are you a rated 777 pilot?