Airbus Fly-By-Wire Hard Limits

Features
#1

In all modern Airbii with the FBW system (A320 onwards aircraft), there are certain hard limits that the plane will not allow the pilot to exceed, even with continuous pitch inputs in the sidestick.
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Okay…so what does this mean?

Read on!

There are three primary flight control laws - Normal Law, Alternate Law and Direct Law. Alternate Law is further subdivided into Alternate Law 1 and Alternate Law 2. The degradation to one or the other of the Alternate Law options is dependent upon the type of failure. Each of the three laws has different sub modes inclusive of ground mode, flight mode and flare mode. Mechanical Back Up is designed to allow the pilots to maintain control of the aircraft while restoring flight control computers after a complete power interruption.
Normal Law
Flight control Normal Law provides three axis control, flight envelope protection and manoeuvre load alleviation. Normal Law operates in different modes depending on the stage of flight. These modes include:
Ground Mode
Flight Mode
Flare Mode

Ground mode
Ground mode is active whilst the aircraft is on the ground. The autotrim feature is turned off and there is a direct relationship between sidestick deflection and elevator response. The horizontal stabilizer is automatically set to 4° up but manual settings (e.g. for center of gravity) override this setting. Immediately after the wheels leave the ground, flight mode progressively takes over from ground mode. The reverse occurs after touch down during the landing phase.
Flight Mode
The flight mode of Normal Law provides five types of protection: pitch attitude, load factor limitations, high speed, high-AOA and bank angle. In addition, Low Speed Protection is available in certain phases of flight. Normal Law flight mode is operational from take-off and remains active until 100 feet above the ground during the landing phase. Failure of certain systems or multiple failures will result in degradation of Normal Law to Alternate Law (ALT 1 or ALT2).
Unlike conventional controls, in Normal Law flight mode the sidestick provides a load factor proportional to stick deflection which is independent of aircraft speed. When the sidestick is neutral in manual flight, the system will maintain a 1g load factor and the aircraft will remain in level flight with no requirement for the pilot to change the elevator trim, even during a speed or configuration change. For manual turns up to 33° bank, no sidestick back pressure is required as the system automatically trims the aircraft to maintain level flight. The system freezes the auto-trim when the angle of attack becomes excessive, the load factor exceeds 1.3g or when the bank angle exceeds 33°. If these situations occur as the result of a deliberate manoeuvre, the pilot must apply back pressure on the sidestick to maintain the selected attitude. In all cases, Load Factor Protection automatically limits the control inputs so that the aircraft remains within AOM “g” limitations and Pitch Attitude Protection limits the aircraft attitude to a maximum of 30° nose up or 15° nose down.
High Angle of Attack Protection, which protects against stalling and the effects of windshear has priority over all other protection functions. The protection engages when the angle of attack is between α-Prot and α-Max and limits the angle of attack commanded by the pilot’s sidestick to α-Max even with full sidestick deflection. If the autopilot is engaged, it is automatically disengaged with activation of High Angle of Attack Protection. α-Floor (automatic application of TOGA thrust) may be activated by the autothrust system if engagement parameters are met.
High Speed Protection will engage to automatically recover from high speed upset. There are two speed limitations for high altitude aircraft, VMO (Velocity Maximum Operational) and MMO (Mach Maximum Operational). The two speeds are the same at approximately 31,000 feet, below which overspeed is determined by VMO and above 31,000 feet by MMO. Activation of High Speed Protection results in reducing the positive spiral static stability of the aircraft from its normal 33° to 0° which means that if the pilot releases the sidestick, the aircraft will roll to a wings level attitude. It also reduces the sidestick nose down authority and applies a permanent nose up order to help reduce speed and recovery to normal flight. Activation of High Speed Protection results in automatic autopilot disengagement. Once the speed has decreased below VMO/MMO, Normal Law is restored and the autopilot can be re-engaged.
Bank Angle Protection limits the maximum bank angle of the aircraft. Within the normal flight envelope, if the sidestick is released when bank angle is above 33°, the bank angle is automatically reduced to 33°. With full sidestick deflection, the maximum achievable bank angle is 67°. If either Angle of Attack or High Speed Protection are active, full sidestick deflection will result in a maximum bank angle of 45°. With High Speed Protection active, release of the sidestick will cause the aircraft to return to a wings level (0° bank)attitude.
Low Energy Protection is also available while in Normal Law when the aircraft is between 100’ and 2000’ with flaps set at config 2 or greater. The low energy warning is computed by the PRIMs using parameters of configuration, airspeed deceleration rate and flight path angle. The aural warning “Speed Speed Speed” indicates to the pilot that aircraft energy has become too low and that power must be added to recover a positive flight path angle. α-Floor protection is available and will engage if pilot actions are inappropriate or insufficient.
Flare mode
This mode is automatically engaged when the radar altimeter indicates 100 feet above ground and provides for a direct sidestick to elevator relationship. At 50 feet the aircraft trims the nose slightly down requiring the pilot to progressively move the sidestick rearward emulating a conventional control input for landing.

Alternate Law
There are three basic reconfiguration modes for the Airbus fly-by-wire aircraft, Alternate Law, Direct Law and Mechanical Back Up. Alternate Law is subdivided into two somewhat different configurations dependent upon the specific failure(s). The ground mode and flare modes for Alternate Law are identical to those modes for Normal Law.
Alternate Law 1 (ALT1) combines Normal Law lateral mode with Alternate Law pitch modes. Low Energy Protection is replaced by Low Speed Stability meaning that the aircraft no longer has automatic stall protection. At low speed, a nose down demand is introduced based on IAS (instead of AOA) and Alternate Law changes to Direct Law. In addition, an audio “STALL” warning is introduced. α-Floor protection is not available so conventional pilot stall recovery action is required.
Load Factor and Bank Angle Protections are retained. High Speed and High Angle of Attack Protections enter Alternate Law mode. Pitch Attitude Protection is lost.
ALT1 control law degradation will result from some faults in the horizontal stabilizer, a single elevator fault, loss of a yaw-damper actuator, loss of slat or flap position sensors or a single air data reference fault. Dependent upon the failure, autopilot may not be available.
In Alternate Law 2 (ALT2), Normal Law lateral mode is lost and is replaced by roll Direct Law and yaw Alternate Law. Pitch mode is in Alternate Law. Load factor protection is retained. In addition to those protections lost in ALT1 (Pitch Attitude and Low Energy Protection), Bank Angle Protection is also lost. In some failure cases, High Angle of Attack and High Speed Protections will also be lost.
As is the case with ALT1, some failure cases that result in ALT2 will also cause the autopilot to disconnecnt. ALT2 is entered when both engines flame out, with faults in two inertial or two air-data reference units, with faults to all spoilers, certain aileron faults or with a pedal transducers fault.

Direct Law
In Direct Law (DIR), lateral modes are the same as ALT2; that is roll Direct Law and yaw Alternate Law. Pitch control degrades to Direct Law and automatic trim is inoperative requiring stab trim to be adjusted manually by the pilot. Control surface motion is directly related to the sidestick motion. ALL protections are lost.
In Direct Law, autopilot function is always lost. DIR is entered if there is failure of all three inertial reference units or all three primary flight computers, faults in both elevators or flame out of both engines concurrent with loss of PRIM 1.

Mechanical Back Up
In the Mechanical Back Up mode, pitch is controlled by the mechanical horizontal stab trim system and lateral direction is controlled by the rudder pedals operating the rudder mechanically. This mode is intended to allow the pilots to maintain level flight while resetting flight control computers after a temporary total loss of power.
(credit to http://www.skybrary.aero/index.php/Flight_Control_Laws)

This would be a great addition to a simulator that also prides itself on realism, as this is a huge defining feature of modern ABs.

#2

Interesting request, I feel we’ve been missing this. Boeing aircraft are very different to Airbus especially because of this safety feature.

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#3

To a certain extent, the 777 and the 787 have limits as they also are fly-by-wire, however in keeping with Boeing’s philosophy of the pilot always being in control, these are merely soft limits which can be overridden with more control input into the yoke. I couldn’t think of a way to simulate this so I can only suggest the AB style of hard limit.

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#4

This is why i love flying boeing and when i fly for the airlines in the next couple of years i will fly nothing but boeing, because the plane listens to every input i say, the pilot controls the plane not the plane controls the pilot.
I know it is implemented for safety, but there has been some serious fatal catastrophes for example when a plane goes into a dive and the pilot wants to get out of it but the plane’s computer FBW system thinks it is a dangerous menuver, i know the 787 and the 777 have FWB System as well, but soon every plane will sadly 😫

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#5

I’m a Boeing person myself, although I have to say there are merits of both systems. The Airbus system makes it such that as long as the plane is in Normal Law, it is nearly impossible without a huge oversight to crash by virtue of a stall. However this can cause problems with pilots being over reliant on the system, such as in AF447 where they tried to pitch up and add power, not realising that this would only work in Normal Law with all the instruments working (the airspeed pitot had frozen over if I remember correctly) thus the plane went into a full stall and crashed.

Regarding the dangerous manoeuvre, the system would recognise if the limits have been exceeded somehow and immediately go into Alternate Law where the bank and pitch angles are no longer limited by the computers.

FBW in essence isn’t such a bad thing, it saves on weight as it only has wires to a computer and saves on mechanical linkages to the control surfaces directly. I’ll be posting a topic about this soon, keep a look out for that!

1 Like
#6

I love this idea. Although I’m not sure how easy it will be to code this, it will be brilliant to see airbus aircraft flying or resembling (however small) how they fly in real life.

#7

Fly-By Wire is used by every new aircrafts today (Mostly known to Airbus and Newest Boeings) for safety purpose to keep the pilots doing the right thing. I would love to see some hard limits of the system that can be implemented to this to make it more realistic

FBW in other words means more COM control. Even though sometimes it would be safer to be controlled manually by pilots. But the computerization is going to change our lives, so we cant stop the growth of FBW system

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#8

This is quite a dead topic so I thought I’d give it some love.
This feature is definitely needed. At the minute, you can fly an A320 on the hus view, and then fly a 737 also on the hud view, and you would never know the difference. This is a must to make different aircraft… well, just that, different aircraft.

#9

Thanks for the bump. Haven’t been here in eons so was nice to see the IFC pop up in my email. Been switching between FlightGear and the A320-family planes with the 777 (Project Seattle) and IF’s 350/777 implementation as a comparison recently, and there’s no comparison to be made here between the two on the flight envelope protections implemented. In other news, I’ve gone to the dark side and embraced the 'bus driver’s philosophy! Once you learn how the Airbus FBW works, and actually go out and fly them for a bit (in sims, still can’t speak of the actual plane), its extremely intuitive in how it works. You really don’t notice that there’s any difference vs. the 737 and other more “manual” birds, since you shouldn’t be reaching the limiting cases anyways. Where you do notice it is in cases like with the autotrim and the roll not affecting pitch (rather roll rate), both features being extremely convenient and working well in the background. Plus it’s fun to lose an engine, come back round for a landing and see “USE MAN PITCH TRIM” on the PFD when you drop the gear as a challenge :)

The way that every Airbus from the 320 forwards flies very differently from many Boeings is really what makes an Airbus an Airbus. I do understand that if you wanted to put this in, it would likely require a lot of work, but right now the Airbuses in the game are missing their most distinctive feature in how they fly, and this would go a long way to furthering this simulator as a serious competitor to even some desktop sims. Would be nice to get some acknowledgement of this as a possible future feature, or at least that it was considered for addition but decided to be too troublesome/not worth the effort/unknowns about how to handle the degradation of laws with failures as some closure to this topic.

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