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B777 pitch control

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B777 pitch control

Old 2nd Dec 2011, 06:39
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B777 pitch control

Hi, I'm an airbus driver but I have been looking at B777 flight controls system, just out of curiosity.

It seems a very nice system, but there is something I don't understand. In normal law, how is the stick input interpreted? pitch rate? pitch target? angle of attack target?

Thanks
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Old 2nd Dec 2011, 07:44
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Hi MB2002,

This article http://www.davi.ws/avionics/TheAvion...ook_Cap_11.pdf para 11.8.1. has a description.

It sounds beautifully instinctive.
"The unique 777 implementation of maneuver demand and speed stability in the pitch control laws means that:
An established flight path remains unchanged unless the pilot changes it through a control column input, or if the airspeed changes and the speed stability function takes effect.
Trimming is required only for airspeed changes and not for airplane configuration changes."

When stick free, it sounds like the aircraft will return to the last trimmed speed.
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Old 2nd Dec 2011, 14:40
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Thanks RRR
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Old 2nd Dec 2011, 14:49
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Very interesting indeed.

I wonder what the C*U demand exactly is.

And for the Airbus Vs Boeing eternal arguments, the 777 is also flown by computers. Only it has a much better direct law and a way to revert to that law at any time. Oh, and a B777 pilot can play out of the envelope it he wants (which can make sense depending on the scenario).

I would really like to lay my hands on a 777...
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Old 2nd Dec 2011, 16:18
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It's very simple and intuitive to operate. The protections are much less complicated than the Bus for the driver.

-Overspeed. You can if you insist, but you can't trim out the control forces once you're in overspeed.

-Underspeed. Same as above and the autothrottle will wake up to save you unless you've turned off completely. Usual aural warnings.

-over bank protection. It takes increased pressure on the controls to overbank. If you ignore that, you can do it. It will talk to you though. This feature can be turned off on the MCP if you so desire. (never done in normal ops.)
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Old 2nd Dec 2011, 16:54
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C* and C*U explained

C* is a computed parameter that combines pitch rate and incremental normal load factor. It was originally developed as a handling qualities metric and then incorporated as the core of maneuver demand control laws.

C* = q + K_C* x delta_Nz

q = pitch rate
delta_Nz = incremental load factor
K_C* = C* crossover gain - chosen to balance q and delta_Nz contributions

At low speed, pitch maneuvers (and thus the C* parameter) are dominated by pitch rate - lots of q with relatively little Nz. At high speed, pitch maneuvers are dominated by incremental load factor - lots of Nz with relatively little q.

An interesting feature of C* is that the variation in C* response between different airplane speeds is much less than the variation in either q or Nz responses. Much less gain scheduling is needed when designing a control law base on C* in comparison with a control law based on q or Nz.

For steady level flight, C* is zero. Pull the 777 column and you are commanding positive C* - the airplane responds with a nose up maneuver such that C* equals the C* command generated by the pilot's pull. The opposite happens for a 777 column push that generates a negative C* command.

Now the transition from C* to C*U.

C*U = C* + K_U x (Speed_Reference - Current_Speed)

Speed_Reference = Trim speed that the airplane will return to in the absence of column input

The 777 with a steady column pull will settle at a speed lower than the reference speed such that C*U is positive matching the C*U command generated via the column. Similarly a steady push will result in flight faster than the reference speed. Note that with C*U for both a column push and a column pull C* itself will be zero in steady state.

When designing the 777, Boeing chose to include positive speed stability as a characteristic of the pitch axis airplane response. This was accomplished by using C*U as the core feature of the augmented longitudinal control law in Normal Mode. The U portion of C*U was developed to be compliant with existing FARs concerning positive speed stability.
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Old 3rd Dec 2011, 13:21
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Thank you, now I start to see how it works

So it is a blend of pitch rate and change of load factor, proportional to stick deflection right?

So it behaves like a conventional airplane. If I push the yoke with a force and keep it at a given position there is a load factor that curves the path down and speed of course increases. My question now is: pitch keeps going down as long as you keep the stick in that position or load factor becomes progresively 1 like in a conventional airplane. In the airbus, if you keep the sidestick deflected the airplane will give a constant load factor until protections activate. You have to release it to neutral to "trim" it.
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Old 3rd Dec 2011, 14:10
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FCeng84;

Thank you for that very clear explanation of the C* law that I never quite understood.
I take it that K_C* is scheduled as a function of something, I would guess speed ratio to a minimum speed. Could you perhaps elaborate on that aspect?
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Old 4th Dec 2011, 05:54
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C* blend of pitch rate and load factor

K_C* defines the balance between q and Nz. For a wings level (i.e., zero bank angle) pull-up there is a relationship between q and Nz that is a function of true airspeed. K_C* can be thought of as defining the true airspeed where the q and Nz contributions to C* are equal. Speed up above that speed and Nz contribution to C* is greater. Slow down and the q contribution dominates.

K_C* is chosen such that the q and Nz contributions to C* are equal for a wings level pull-up maneuver at 400 ft/sec. This parameter is constant - no flight conditions scheduling involved.
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Old 4th Dec 2011, 10:45
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FCeng84;
Thanks again. I understand it better now.
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Old 4th Dec 2011, 15:40
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Who is interested, there is the detailed patent.

US-patent C*U

franzl
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Old 5th Dec 2011, 12:23
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great!

I'm reading it, but I got surprised by this:

It is fairly well known that at low airspeeds, pilots expect movement of the control column to produce a change in pitch rate. At high airspeeds, pilots expect movement of the control column to produce a change in normal acceleration.
I had no idea, and I don't quite understand that yet. I am also surprised by the little if any mention to stick forces in these texts. I always thought that I flew the airplane by means of forces or pressures on the columnm rather than moving it. At high speed, the same force moved less the column, but had a similar effect in g force. I have always looked at conventional pitch control as one in which the pilot input is a force, rather than an elevator deflection, so that similar forces gave similar load factors, irrespective of speed.

But I'm being impatient, I'll keep reading...
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Old 5th Dec 2011, 17:05
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Conventional Pitch Response and Fs/g

Microburst2002 - The section you have quoted points to the notion that at low speeds a commercial transport pilot tends to regulate the magnitude of the pitch response based on the pitch rate achieved. At low speeds considerable pitch rate can be produced with relatively low normal acceleration. For example, pilot training ususally targets a desired pitch rate during takeoff rotation/climbout, go-around, and flare maneuvers. At high speed the cue that the pilot tends to pay most attention to is Nz as the associated pitch rates are very low.

You are spot on with your reference to stick force per g. Fs/g is a handling qualities parameter that is a focus of control law design. The aim is to keep Fs/g fairly constant throughout the flight envelope. For pitch control laws that use controller position as their input (as is the case with the 777), both the controller force/displacement characteristics and the gearing between controller position and C*U command must be considered to determine the resultant Fs/g.

On an airplane that has a direct mechanical cable connection between the pitch controller and the elevator, the force/displacement characteristics (i.e., the stiffness) of the controller are a function of impact pressure as elevator hinge moments go up with increased speed. For the 777 Boeing decided that having the column become stiffer as impact pressure increases is a situational awareness characteristic worth retaining. This is a point of intentional differentiation between Boeing's fbw philosophy and that of Airbus. The 777 stick force per degree of column travel increases by a factor of about 3 from low speed to high speed. Having the abililty within the C*U control law to vary column stiffness also allows using that same feature to provide additional pilot awareness as part of the 777 envelope protection functionality.
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Old 5th Dec 2011, 17:18
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8500+ 777 hours and you learn something new every day.

Thanks guys!
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Old 5th Dec 2011, 17:47
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777 Convensional Characteristics

Jumpjim - Your comment is exactly what Boeing hoped to hear. The 777 airplane/pilot interface was designed with the intent that pilots not have to learn a new way of flying and thus would not have to know all the details. Speed stability and pitch trim should feel natural. The balance between workload reductions (such as not having to trim for thrust/configuration changes and not having to add nose up command to maintain altitude during normal turns) and situational awareness cues (such as speed stability/trim limits and column stiffening with increased speed) was chosen with great care.

I'm very interested in how you have found the 777 from the perspective of handling qualities, intuitiveness of response, and easy of maintaining situational awareness. I would imagine that your exposure to the 777 protection functions has been limited to time in the simulator, but I wonder if you have any comments on the overspeed, underspeed/stall, and bank angle protection functions.

Last edited by FCeng84; 5th Dec 2011 at 17:49. Reason: correct typo
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Old 5th Dec 2011, 18:33
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I'm very interested in how you have found the 777 from the perspective of handling qualities, intuitiveness of response, and easy of maintaining situational awareness. I would imagine that your exposure to the 777 protection functions has been limited to time in the simulator, but I wonder if you have any comments on the overspeed, underspeed/stall, and bank angle protection functions.
Having been on the aircraft for 12 years now and with experience across 777-200 and -300 models I have to say I absolutely love it. I came across from the 757/767 and it was an easy conversion.

Handling qualities - A pleasure to fly! Config changes, particularly the likes of Flap 20 deployment are much easier with the lack of resultant pitch input requirements. Pitch up during turns becomes instantly intuitive and overall hand flying the aircraft is an absolute pleasure. I do a lot of hand flying around various caribbean destinations including visual approaches from 30K regularly and they are a pleasure to fly. As for intuitiveness of response there is nothing to say negatively which speaks volumes.

I am firmly a "Child of the Magenta line" and as long as you are professional and rigorous in your approach to FMC changes etc. then I personally don't feel that my SA is lower in this aircraft than any other I have flown. What you do have to be careful of is FMA mode changes. FMA awareness is fairly critical along with a broad knowledge of the traps (FLCH/Throttle hold trap in visual approaches etc.).

My only real experience of the protections was a bank angle exceedance avoiding a 737 which had decided to go for the same gap between two Cu clouds that I was heading for. All I felt whilst visually manoeuvring quite hard was a firm but insistent nudging in roll on the control column which communicated what was happening to me without any requirement to look at the PFD. Ideal solution if you ask me.

The other thing that sells the aircraft to me is the TAC (Thrust Asymmetry Compensation for non-triple drivers) which puts the rudder in automatically in the event of an engine failure, or ANY instance of asymmetric thrust. It comes in to its own flying single engine circuits. With the TAC and the flight path vector to indicate level flight on the PFD they are an absolute breeze.

I'm down for a job introducing the 787 into my company and can't wait to see what the difference is between the two.
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Old 5th Dec 2011, 19:56
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777 and 787 Response Comparison

Jumpjim - thanks for sharing your 777 experiences. I have heard many pilots speak highly of TAC. I think the major short-coming of that system on the 777 is its availability. Because it is based on engine data to compute left and right thrust estimates, it is somewhat vulnerable to an engine failure that takes out the associated sensors. Just when TAC is most needed, it may not be available. For that reason 777 pilots must go through engine out training without TAC just in case. On the 787 the equivalent of TAC is provided without such heavy reliance on engine data such that it is no longer necessary to train for an engine-out during takoff without the ITAC (inertial sensor based TAC).

As SLF it is music to my ears that with the 777 you have found an airplane that is a pleasure to fly manually and thus you do so often. The additional and ongoing exposure to manual flying that you are providing yourself has to translate into you being better prepared in the event that you are forced to get in the loop some day. When you hand fly, do you go with manual throttles as well, or do you tend to use the autothrottles while manually controlling the flight path?

We will all be interested to hear your comments comparing the 777 and 787. The designer's intent is that you will have a hard time telling the difference with regard to handling qualities (particually in pitch - roll and yaw should be more consistent on 787 vs. 777).
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Old 5th Dec 2011, 22:30
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If you have any documentation on c*U I would be very grateful. I always like to read up
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Old 5th Dec 2011, 23:33
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C*U References

The patent link provided above by RetiredF4 and the document below are good references for C*U.

http://www.davi.ws/avionics/TheAvion...ook_Cap_11.pdf
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Old 6th Dec 2011, 11:11
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FCeng84

I you don't mind I will bother you with a few questions, because I was deepening my knowledge on flight control systems in general and Airbus vs Boeing fbw concepts in particular.

I am a 320 driver myself and sadly I haven't enjoyed flying any other jet, other than 757 simulator long time ago.

The variables are kind of messing in my head: elevator angle, hinge moments, stick forces, pitch rate, load factor, speed, dynamic pressure, lift coefficients and everything about stability, of which I thought I had some knowledge.

I'll try to put some order in my brain, first

thanks
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