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Thread: Airbus Control Laws - LONG and technical

  1. #1

    Default Airbus Control Laws - LONG and technical

    Hi all,
    I decided to post this to help clarify some of the misperceptions regarding Airbus fly-by wire control laws. There was a post in the FS2002 forum yesterday by "Lauchit" regarding that now seemingly age old question "Airbus vs. Boeing". In looking back, I responded a bit abruptly to his post. My only intent was to prevent an endless flame war on this subject as it's been beaten to death many times. However, I feel Chit deserves a better response than I gave him yesterday so here goes.

    THIS POST ISN'T INTENDED TO START ANY FLAME WAR BETWEEN THE MERITS OF AIRBUS VS. BOEING DESIGN. EACH MANUFACTURER HAS IT'S OWN REASONS FOR THEIR DESIGNS.
    IT'S IMPORTANT TO NOTE THAT THE BASIC DESIGN PHILOSOPHY OF AIRBUS IS ONE OF INCREASED SAFETY AND PASSENGER COMFORT. THE AIRCRAFT ARE AIRLINERS AND NOT INTENDED IN ANY WAY TO BE ACROBATIC.

    Disclaimer - The Airbus fly-by-wire series of aircraft includes the A320 (319/320/321), A330, A340, and upcoming A380. While the basic control laws governing all series are most likely the same, there might be subtle differences between the various aircraft types. In addition, there are likely minor differences between Airlines (I know there are VERY minor differences between United A319/320's and Frontier's). As such, my discussion is limited to the A319/320 series as flown by United Airlines.

    The A320 series flight control is governed by various "Control Laws" through various Flight Control Computers (FCC's). These control laws govern the way the aircraft flys and prevents the pilot from stalling the aircraft through intentional means, or by pilot error. The basic Airbus philosophy was to develop an aircraft that both eases the pilot workload, and increases it's safety by not allowing the aircraft to exceed it's flight envelope. There have been numerous accidents caused by pilot error that would not have occured had they been flying an aircraft with the Airbus design philosophy.

    Note- There are numerous FCC's with mupltiple redundant failure safe systems.

    The various control laws are as follows;
    Normal Law, Alternate Law, Direct Law, and Mechanical Law. In addition, there's an "Abnormal Attitude Law" that takes effect in the highly unlikely event that an outside upset (eg., mountain wave turbulence) pushes the aircraft outside the normal flight envelope.

    NORMAL LAW (think of BYPALS);
    B - Bank Angle; Limited to a max of 67 degrees by flight control computers. At bank angles up to 33 degrees, the aircraft auto-trims (there's no trim switch on the side-stick controllers). Beyond 33 degrees requires increasing back stick pressure to maintain altitude as auto-trim is inhibited. There's also a 45 degree limit imposed through either high AoA or high speed protection features (meaning bank angle is limited to 45 degrees by Alpha Protection for AoA or High Speed as governed by high speed protection). Roll rate limited to 15 degrees/sec for pax comfort.

    Y - Yaw; Full Yaw Damping and Turn Coordination are provided.

    P - Pitch; 30 degrees nose up (flaps 0-3) 25 at low speed, 25 degrees nose up (flpas full) 20 at low speed, 15 degrees nose down in all configurations. note - Pitch law is where UAL and Frontier's control laws are slightly different.

    A - AoA; Alpha Protection - Bank angle limited to 45 degrees (as mentioned above), Speed Brakes automatically retract in Alpha Prot, auto pilot disconnects, and pitch up trim is inhibited.
    Alpha Floor - Initiates TOGA thrust. This function occurs at a "pre-determined" AoA between Alpha Prot and Alpha Max.
    Alpha Max - Highest AoA the aircraft can achieve before the FCC's won't allow you to go any further.

    All Alpha functions occur at the bottom of the speed envelope. The specfic speed at which they are triggered varies depending on aircraft configuration, weight, and load factor.

    L - Load Factor; +2.5 to -1.0 G (flaps up), +2.0 to 0.0 (with any flap extension).

    S - Speed; Vmo+6 kts / Mmo + 0.01. If you jam the stick forward and hold it the aircraft can initially go to Vmo + 30 / Mmo +0.07 but will stabilize at Vmo to Vmo + 16 or Mmo to Mmo +0.04.


    ALTERNATE LAW - The aircraft control law will "degrade" out of normal law through multiple failures (or through intentionally disabling various FCC's - not a good idea).

    B - Roll direct (direct relationship between side-stick input and control deflection). 30 degree/second roll rate.

    Y - Yay damping only (turn coordination is lost).

    P - A pitch up will occur at 320Kias (to prevent overspeed).

    A - Low speed stability is provided depending on malfunction.

    L - All previously mentioned load factor protection is intact.

    S - High speed stability is provided depending on malfunction.

    Note; Auto-trim is still avaiable.

    DIRECT LAW; Once again, will happen through either further failures, or upon lowering the gear when in alternate law.

    In direct law ALL flight control protection is lost. Auto-trim is lost (so you have to use the manual trim wheel). IT'S IMPORTANT TO NOTE THAT IN ESSENSE, THE AIRCRAFT IS NOW A CONVENTIONAL AIRCRAFT (kind of like a Boeing design without a trim switch). There's a direct relationship between side-stick input and aircraft response. YOU CAN NOW STALL THE AIRCRAFT THROUGH PILOT ACTION AND THE COMPUTERS WON'T (CAN'T) INTERVENE.

    MECHANICAL LAW; No flight control computers are available. The aircraft has to be flown using the rudder pedals, trim wheel, and differential throttles as the side-stick is inop. Hydraulics are required. It is highly unlikely that mechanical law would ever occur as it would take failure upon failure of systems to get to mechanical law. None-the-less, United trains it's pilots to be able to control and land the aircraft (in the simulator) using mechanical law. It's not easy, but it's possible.

    Lastly I'll address ABNORMAL ATTITUDE LAW. Abnormal Attitude Law takes effect in the unlikely event the aircraft exceeds certain flight envelope parameters.
    These are;
    -Greater than 125 degrees of bank.
    -Greater than 50 degrees nose up or 30 degrees nose down pitch.
    -Greater than +30 AoA or less than -10 AoA.
    -Greater than 440Kts/0.91 mach or less than 60 Kts/0.1 mach.

    The Active Law that occurs in an abnormal attitude is roll direct, pitch alternate (limited to 320Kts), and yaw alternate (without yaw damping). The intent is to positively recover the aircraft back to normal flight parameters. After the aircraft is recovered, the aircraft resorts to Pitch Alternate law, Roll Direct, and Yaw Alternate. As you can imagine, the crew will want to land the aircraft as soon as practicle after an abnormal attitude.

    I've only addressed the fundamentals of the Airbus control laws as implemented in their fly-by-wire aircraft. There are of course numerous other details that I haven't covered as it would take a flight manual and training course to properly explain everything. In addition, I've purposfully limited my discussion so as not to allow any more information than needed that might confuse the layman too much (I fear I've lost that battle already).
    As you can see, Airbus has designed an aircraft with many layers of protection not built into most other aircraft. Having flown the C-130, B727, and now A320 series, I can honestly say that I am both a fan and convert to the Airbus design philosphy. That of course doesn't mean I wouldn't fly other aircraft designs in the future (re. Boeing). However, based on my experience, the Airbus design philosophy is excellent and safe (just what you want in an Airliner).

    Thanks for bearing with me,
    Dave








  2. Default RE: Airbus Control Laws - LONG and technical

    Dave,

    THANK YOU! Excellent explanation.

  3. #3
    tailboom Guest

    Default Dave, have you tried Antti's new Dauphin.....

    yet? You'll feel right at home in that 'lil scutter...:-lol:-lol:-lol
    https://www.flightsim.com/images/noimage.png

  4. #4

    Default RE: Dave, have you tried Antti's new Dauphin.....

    Hey,
    Tried it, but couldn't get my throttle to work on it. Gave up out of frustration.
    Dave

  5. #5
    tailboom Guest

    Default What stick are you using..........?.....


  6. #6

    Default RE: What stick are you using..........?.....

    Thrustmaster Top Gun Afterburner II.


  7. #7
    tailboom Guest

    Default Do you still have the Dauphin on your HD....

    ?
    number_of_devices=1;
    cyclic_pitch=1,Y;
    cyclic_roll=1,X;
    yaw=1,rZ;
    collective=1,SLIDER0; <---try this line in your Dauphin.cfg
    force_trim_release=1,BUT1; file....
    beep_trim=1,POV0;
    toggle_landing_gear=1,BUT7;
    toggle_cycl_fd=1,BUT5;
    toggle_coll_fd=1,BUT3;
    toggle_coupler=1,BUT4;
    go_around=1,BUT2;
    brakes=1,BUT6;

    Did you disable your j'stick, then hit SHIFT+2 to bring up the pedestal?

  8. #8

    Default RE: Do you still have the Dauphin on your HD....

    Thanks,
    I'll give her another go.
    Dave

  9. #9
    Join Date
    Mar 2005
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    Hastings, MN, 55033
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    Default RE: Airbus Control Laws - LONG and technical

    I think the Airbus system is smart, and I also think the Boeing system is too...

    Thanks for explaining all that...:-)

    -E

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  10. #10

    Default RE: Airbus Control Laws - LONG and technical

    Nice explanation.
    Joe Apostolidis
    Supporter of FS, quality add-ons, and real aviation
    I Fly all planes eVerywhere, Very Fun Recreation

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