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cmbaviator

how to hand fly with C*U law aircraft

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Hi

 

I'm more an airbus fan than Boeing but my favorite plan is the B777, so few months ago I started to read B777 FCOM and purchase the B777 PMDG.

 

I must admit that I'm more familiar with the C* law but I have a hard time to figure out how the C*U works and how to handfly in order to maintain the flightpath.

 

i know that the B777 is speed stable, so when you trim, you're not directly moving the THS but the elevators will first move in order to reach the target Trim speed and then the THS will move accordingly to " free " the elevator so that the pilot will have full elevator deflection.

 

Also I haven't fly a GA aircraft, thus making me  a hard time understanding how to trim an aircraft to maintain the FP. In a airbus it is very easy but it's not 100% FP stable, only little correction need, but nowaircraft such as the SSJ is 99.99% FP stable it's like having CWS R&P always on.

 

Any for example, let's assume that we are flying at 3000 fts level ( AP off, AT off ) at 220 kts ( trim speed : 220)

 

GS alive, getting ready to capture the GS :

 

i will :

 

*reduce thrust

*start pitching down  ( at this stage if i release the yoke, guess that the A/C will tend to pitch down right ? , as the A/C wants to get back to the last trim speed value which is 220 kts

* When I reach the target speed, i set the correct thrust, and trim nose up until the plane is FP stable ( trim speed close to Vapp ).

 

I'm quite confused, in GA you trim to release the force on the yoke, but on a B777, you trim to maintain the trim speed regardless of the flight path.

 

So my question is how to trim the B777 in order to get your target speed and flight path stable, ie how do i have to trim the B777 in order to maintain Vapp and -3° of slope appraoch (ils standard) ?

 

As you can see it's quite messy in my head, i'd appreciate a nice explanation :).

 

That's why I mostly prefer the C* over C*U but just that my opinion, I'm not saying that C* is better though, each have pros and con.

 

C* is better for take off and landing ( approach) as it's flight path stable but C*U is better during the climb and descent, indeed after the Thrust Reduction altitude, you just have to set 250kts for trim speed and the aircraft will automatically maintain that speed by pitching up or down.

 

Thanks in advance

 

CMB

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Probably the best you can do is watching the video on this other thread:

 

https://www.youtube.com/watch?v=kae_wDCGex0

 

Besides this, I must say that, now that I understand it, I really don't find the Boeing C*U very wise.... I will probably be smashed by saying this, but, give me the good "old" Airbus C* law and I'll be just fine... 

 

In trying to do it more intuitive, I think Boeing has turned it into something awkward.

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Probably the best you can do is watching the video on this other thread:

 

Besides this, I must say that, now that I understand it, I really don't find the Boeing C*U very wise.... I will probably be smashed by saying this, but, give me the good "old" Airbus C* law and I'll be just fine... 

 

In trying to do it more intuitive, I think Boeing has turned it into something awkward.

 

thanks for the video

 

I think like you. But I think, that they have implemented the C*U for certification purpose, in order to get a certication, the aircraft needs to be speed stable which is not the case with a C* aircraft, as it will maintain the Flight path or must i say the C* value regardless of the airspeed. But as Airbus demonstrated that with all their protection . the airbus can't go over speed or either stall, maybe that's why they succeed getting their certification even if it's not speed stable but I can be totally wrong...

 

CMB

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Trim it like a Cessna.

 

Done.

 

Sure, there's all kinds of computational cool stuff happening to make things easier, but people are getting too wrapped up on that fact.  Ignore that there's a whole bunch of cool FBW stuff happening and fly the plane.  It'll fly just like every other plane (but will try to help you out in the process at certain points).

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If we just have reliable and cheap FFB system, that would make it a lot easier. Not just for 777 but for any other aircraft in FSX (minus airbus).

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Trim it like a Cessna.

 

Done.

 

Sure, there's all kinds of computational cool stuff happening to make things easier, but people are getting too wrapped up on that fact.  Ignore that there's a whole bunch of cool FBW stuff happening and fly the plane.  It'll fly just like every other plane (but will try to help you out in the process at certain points).

 

the truth is I've never flown a cessna  

 

CMB

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the truth is I've never flown a cessna  

 

CMB

 

haha - well dang.  Sorry - I was just going for the idea of "trim it like your most basic aircraft."

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haha - well dang.  Sorry - I was just going for the idea of "trim it like your most basic aircraft."

 

Well I began to start flying on a airbus ( virtually of course ) lol :=)

 

i'm truly an airbus hardcore fan for years now :)

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If we just have reliable and cheap FFB system, that would make it a lot easier. Not just for 777 but for any other aircraft in FSX (minus airbus).

 

I wish! Trimming a real plane like a Cessna is simple. Get it on the speed and pitch that you want, and trim it until the controls are neutral. Easy!

 

If there was decent force feedback available for FSX, it would make hand flying so much easier.

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Believe me.... it's a lot simpler to trim a GA, thanks God.!...

Sorry but that C*U law... stinks.....  Give me my good old 747 / NGX style.... or the pure Airbus C*

 

Poor PMDG having embraced this... and I am sure they are doing the best possible, given the access to the proprietary algorithms is certainly not possible at all! But don't need to go further apart - simply watch that video of an EVA Air captain.... The problem IS NOT in PMDG's 777 IMHO...

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Believe me.... it's a lot simpler to trim a GA, thanks God.!...

 

 

And how this is different? In my experience it almost the same:

 

Nose goes up when you leave the yoke? Trim down. Nose goes down when you leave the yoke. Trim up. 

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With the A/T engaged, it's very different than one would imagine. You expect the nose to go up as you add power, even if you're mantaining speed. It's that way in a Cessna. But in the 777 it will keep the VS you put the aircraft, and keep the airspeed. It feels very artificial in my opinion

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And how this is different? In my experience it almost the same:

 

Nose goes up when you leave the yoke? Trim down. Nose goes down when you leave the yoke. Trim up. 

 

 Problem is when you add power to the system, as Alexis pointed out.

 

Look at the Eva Air video.... You trim for a speed and then pitch down and it'll start descending and mantaining your trimmed speed. Now start a climb by pitching up, and the speed will remain constant ( of course provided you are subtle in your column interaction... ). Is this the way we do it in a Cessna?

 

BTW: see how the video ends :-)

 

Now, bring AT into the system and think about it....

 

I may be wrong, or still missing something, but honestly I think that PMDG should have designed the Boeing FBW instead - I liked it more as it was in the RTM :-/

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Look at the Eva Air video.... You trim for a speed and then pitch down and it'll start descending and mantaining your trimmed speed. Now start a climb by pitching up, and the speed will remain constant ( of course provided you are subtle in your column interaction... ). Is this the way we do it in a Cessna?

 

He is flying with AT at Eva Air video. 

 

For example, you are flying straight and level, aircraft is trimmed out: If you pitch down, speed would increase, and kick you out of trim. You need to decrease power, or to retrim the aircraft if you want to regain the balance.

 

Start climbing: airspeed would decrease and kick you out of trim. You can either increase power or trim it up if you want to regain a balance.

 

That's consistent with Cessna 172 for example.

 

With AT engaged, it would fly almost like airbus as long as AT is capable holding the speed - as you see in the video. 

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OK here are a couple of examples with AT in various modes, exhibiting the behaviour of the system (beyond the excelletn video presented above).

 

Example 1 : Assume AT is ON and you fly straight and level. The aircraft is trimmed for the current AT speed. The AT speed mode is SPD (check your FMA) and your trim reference speed is equal to the MCP speed target (enable the FBW trim speed arrow from options for the purpose of training).

 

Now assume you want to climb...Pitch up and follow the bars {if you have the flight director on). Notice how the AT adds thrust to maintain speed. Do NOT touch the trim buttons. Once AT thrust has stabilised let go of the controls. The aircraft will now climb at the prescribed speed.

 

Example 2: Continuing from Example 1 disconnect the AT. The aircraft continues to climb as before at prescribed speed. Gradually advance the thrust levers. Do NOT touch either the controls or the the trim buttons. The aircraft will increase its rate of climb.

NOTE : If you whack the throttles the system will start responding "aggressively" once current IAS is greater than 15-20 knots. Subsequently in this case your IAS will go under and over the target a few times in a diminishing fashion until stabilised. In the end the aircraft will climb at the speed of Example 1. The system features a simulation within a simulation : relatively aft CG aircraft with a relative small stabiliser (for cruise fuel economy) violently thrown out of aerodynamic equilibrium.

 

Example 3: AP and AT are ON and FMA is THR REF|HDG SEL or LNAV| VNAV SPD. The aircraft climbs at 250 (below 10000') speed is stable and plane is trimmed out. Disconnect the AP maintain THR REF. At 10000' FMC speed target now increases to 300+ knots. Do NOT touch the controls. Hold the trim down button for approximately 1 second per 10 knots required speed increase. E.g. say you want to accelerate from 250 to 320 knots. Hold the trim button down for 7-8 seconds. The aircraft will now drop the noise and eventually stabilise at a new speed of approximately 320 knots. Bleep trim to micro adjust.

 

Example 4: Same as above. At 10000' push the controls down. Do NOT touch trim. Release the controls. The plane will raise the nose (pitch rate and correction action depends on difference between current IAS and target trim reference IAS which remained. at 250). If you really insist on a nose dive you 'll feel the controls becoming "heavier" even without force feedback.

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Just watched the video, it is clear now, thanks a lot for the link :)

 

but there is something that I don't understand ?

 

When he trim up or down, the plane remain level, How is that possible ??? He was barely giving input just like in a a320 where the FBW can't maintain perfectly the flight path while decelerating especially 300kts to 220 kts  ????

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When he trim up or down, the plane remain level, How is that possible ??? 

 

Because while trimming, he's applying pressure at column as long as it resists. You do not feel the force at your home yoke so it can never be so perfect.

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Because while trimming, he's applying pressure at column as long as it resists. You do not feel the force at your home yoke so it can never be so perfect.

 

yes but he was barely appying pressure, on a A320 the VS would have dropped to -500 to -600 fts/min

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I found an issue with the FBW and trim. I'm going to try and bullet point this to make it as clear to understand as possible.

 

* Aircraft is OUT OF TRIM, and "nose heavy".

* I pull back on the control column to counter the out of trim condition

* Once I stabilized the pitch, I start trimming nose up, and returning the control column to center at the same time (same as on the NGX).

 

PROBLEM: the trim reference speed changes, but the trim is NOT applied until I return the control column to dead center, at which point ALL the trim change I made is applied at once, and if I got the trim wrong, the aircraft would then aggressively pitch relative to that out of trim condition.

 

The result is that I can't trim the aircraft by feel. If I don't have the trim reference speed displayed, it is virtually impossible to trim.

 

Seperately, the trim often either doesn't move, or even moves in the opposite direction to that selected.

 

Anyone else see this?

 

Best regards,

Robin.

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yes but he was barely appying pressure, on a A320 the VS would have dropped to -500 to -600 fts/min

Never tried to fly airbus, but at some videos I watched, that stick is really sluggish.

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Don't forget that the Airbus pitch mode in Normal Law is a g demand law. It blends to a pitch rate demand law at low altitude/on the ground.

 

Best regards,

Robin.

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Don't forget that the Airbus pitch mode in Normal Law is a g demand law. It blends to a pitch rate demand law at low altitude/on the ground.

 

Best regards,

Robin.

it's more a pitch rate demand at low speed < 240 kias, on the ground pitch mode is in direct law

 

:)

 

Regards

 

CMB

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Probably the best you can do is watching the video on this other thread:

 

https://www.youtube.com/watch?v=kae_wDCGex0

 

Besides this, I must say that, now that I understand it, I really don't find the Boeing C*U very wise.... I will probably be smashed by saying this, but, give me the good "old" Airbus C* law and I'll be just fine... 

 

In trying to do it more intuitive, I think Boeing has turned it into something awkward.

 

Thanks for that video! That helped a lot.

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I found an issue with the FBW and trim. I'm going to try and bullet point this to make it as clear to understand as possible.

 

* Aircraft is OUT OF TRIM, and "nose heavy".

* I pull back on the control column to counter the out of trim condition

* Once I stabilized the pitch, I start trimming nose up, and returning the control column to center at the same time (same as on the NGX).

 

PROBLEM: the trim reference speed changes, but the trim is NOT applied until I return the control column to dead center, at which point ALL the trim change I made is applied at once, and if I got the trim wrong, the aircraft would then aggressively pitch relative to that out of trim condition.

 

The result is that I can't trim the aircraft by feel. If I don't have the trim reference speed displayed, it is virtually impossible to trim.

 

Seperately, the trim often either doesn't move, or even moves in the opposite direction to that selected.

 

Anyone else see this?

 

Best regards,

Robin.

Yep... I've noticed the trim going the wrong direction too Robin, Try to trim say up, and it suddenly reverses and goes down.

 

Not sure about the other points you mention, will have to test.

 

 

 

 

I think many of you are over thinking trim.

 

As confirmed by a real world 777 pilot on the forum, just trim the way you do with any aircraft, it shouldn't feel any different to trimming a 757 or 737.

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