TAP008: Flight Control Laws

05 Dec 2016 21:00

Flight Control Laws

The flight control law is basically the relationship between the pilot's input on the side stick and the resulting aircraft or flight control surface response.
There are 3 flight control laws Normal Law, Alternate Law and Direct Law

 

As a general rule, normal law deals with single failures of a system and alternate law deals with double failures.

Within Normal Law we have three sub categories,

Ground Mode
Flight Mode
Flare Mode

Ground Mode was designed to make the aircraft behave more naturally when rotating at liftoff. The relationship between the side stick and the aircrafts response is much more like a conventional aircraft.
For pitch control - there is a direct relationship between Side stick deflection and elevator deflection. Once the aircraft reaches 75kts the maximum elevator deflection is reduced from 30 degrees to 20 degrees. If we haven't manually set a trim position using the trim wheel then the THS or trimmable horizontal stabiliser will automatically set to 0.
For lateral control - The side stick demands aileron and spoiler deflection as opposed to a roll rate but its not a direct relationship, the amount of deflection is dependant on the aircraft speed. As a extra bit of information for you, only spoilers 2 to 5 and the ailerons are used for roll. The rudders being a mechanical linkage aren't affected so you just have to remember that they become more sensitive the faster you go. There are no protections at all when in ground law.

Flight Mode
The aircraft will then start to blend smoothly from ground mode into flight law once the pitch attitude reaches 8 degrees. In roll this takes half a second and for pitch it takes five seconds.
There's a good graphic in the FCOM with this information on. Its in
Descriptions - Flight Controls - Flight control System - Normal Law - General

 

once the aircraft has been airborne for more than 5 seconds we are then in flight mode. This is obviously the one we are exposed to 99% of the time we are operating. As we mentioned a minute ago, normal law keeps us within the aircraft envelope and prevents us from doing manoeuvres that could potentially endanger the flight. It also gives the aircraft certain characteristics when manually flying.
In pitch the sidestick demands a load factor as opposed to an elevator deflection. So an input on the sidestick will give a pitch rate at low speed or a g-load at high speed. This is designed to give an aircraft response that the pilot would naturally expect. One of the first things you notice about the Airbus is the lack of trimming which is for me one of the best features. Therefore if there is no input on the stick the aircraft will maintain its flightpath even if the speed changes. In fact even if you change the thrust or the configuration, the aircraft will compensate for the pitching moments. This makes manual flying very easy and frees up lots of capacity.
With Roll, Again, unlike a conventional aircraft, lateral inputs on the side stick don't demand aileron deflection directly. They demand a roll rate and full side stick deflection will demand 15 degrees per second.
Just like with pitch, the aircraft will auto trim so the bank angle will be maintained when you let go of the stick up to 33 degrees, and will also automatically provide a pitch compensation and perform a coordinated turn using yaw. The maximum bank angle the aircraft will allow you to do is 67 degrees.
Beyond 33 degrees, the aircraft won't auto trim and if the side stick is then released it will return back to 33 degrees. In addition to this, above the 33 degrees, spiral stability is introduced and pitch compensation isn't available. The reason they've written this into the software is because there is no reason to fly at such high bank angles for a prolonged period.

Protections

Angle of Attack -
Autopilot out at Alpha prot, then from Alpha prot to alpha max side stick demands Alpha directly. Alpha floor trigger TOGA thrust and speed continues to decrease until we get back to Alpha max which the speed won't go below.

Load Factor -
+2.5G to -1G clean
+2G to 0G in any config other than clean

Pitch Attitude-
-15 degrees all configs
+30 degrees config 1, 2 and 3
+25 degrees config Full

High Speed Protection-
Autopilot out at VMO/MMO, master caution and overspeed ECAM at VMO/MMO +4 kts, then at VMO/MMO +6 kts, pitch trim is frozen, max bank angle is reduced and a nose up demand is triggered.

Bank Angle -
Max 67 degrees
Reduced to 45 degrees in Alpha protection and 40 degrees in high speed protection
Side stick pressure required to maintain bank angles greater than 33 degrees unless in high speed protection when its zero.

 

Alternate Law is generally for situations where there has been a double failure of a system which results in either lack of redundancy or integrity of the protections found in normal law. Auto pilot and auto thrust are still available.
You can get alternate law with protections and alternate law without protections.
With protections - has the following characteristics and protections,

Load Factor still has the same protections as normal law (+2.5G to -1G clean and +2 to 0G configured).

Pitch has no protections, the green equals symbols are replaced by amber ones.

Roll is now a direct stick-to-surface relationship. To help reduce the roll rate, in alternate and direct law only ailerons and spoilers 4 & 5 are available. As a note, if spoiler 4 has failed number three will replace it and if the ailerons have failed, all roll spoilers (2 to 5) become available. There are no bank angle protections and the green equals signs are replaced by amber crosses.
Yaw control, as its a mechanical linkage isn't changed although only yaw damping is available.

Angle of Attack protection is no longer available and is now replaced by low speed stability. It's available for all configurations and is active from 5 to 10 knots above the stall speed. Somewhere in this range (as it depends on weight and config) a gentle nose down signal is introduced but this can be overridden. The speed scale now shows VLS followed by a black and red barber pole below V Stall Warning. At V Stall Warning, you get....... the stall warning! which is the words STALL STALL STALL repeated until the speed is back above V stall warning. With this you will also get the associated master warning. The warning can't be cancelled by pressing the master warning button on the glareshield Its important to remember that the aircraft can be stalled.

High Speed Protection is replaced by high speed stability instead. The speed tape looks the same and has the same warnings and sounds as normal law. The only difference is that there's no protection to stop the overspeed. Instead we have a nose up demand from the aircraft but this can be overridden. It's worth noting that VMO is reduced from 350 to 320 knots. I was once asked by a trainer what speed would I select for an emergency descent. There's no right answer here but he said he always selects 320kts because if then for some reason you go into alternate law, you won't have an overspeed to deal with on top of everything else which I thought was a good little tip.

Alternate law with protections lost is the same as alternate law but you don't get the high speed and low speed stability. So basically you only have the load factor limitation which I'll say again as repetition is the key to remembering things, +2.5 to -1G clean and +2 to 0G in any other config.

Alternate law then automatically downgrades to Direct Law when the landing gear is selected down.
Once in Direct Law, all protections and stabilities are lost. We are now essentially flying a conventional aircraft. Pitch now joins Roll and Yaw in having a direct stick-to-surface relationship. Overspeed and stall warnings are still exactly the same as Alternate Law.

The most noticeable difference when going into direct law is the lack of autotrim. 'USE MAN PITCH TRIM' is displayed in amber on the PFD. This is why many of the procedures advise taking flap 3 before gear down if flap 3 is the landing config because then the autopilot will get the aircraft correctly trimmed before it has to be done manually. Any trim adjustments will have to be made using the trim wheels either side of the thrust levers.

 

Remembering how to draw the speed tapes in each configuration

  • Firstly, the high speed barbers pole is the same in all cases, so just remember its black and red - easy
  • At the slow end of the speed scale, every case has VLS, its always there, whatever - again, easy
  • Normal Law has alpha prot (the tiger tail) and then you can't go slower than the bottom of that so its a solid red bar (think of as signifying stopping)
  • Alternate and direct law look exactly the same as each other, they both just go from VLS to a barbers pole just like the high speed one.
  • All the equals signs are only green in Normal law, they are amber in alternate and direct.

 

And that's it. If you can remember those simple rules, next time you're asked to draw the speed scale you will be ahead of most of your colleagues.

 

Mechanical Backup. Although its mentioned in the same section in FCOM and the flight crew training manual, its not actually a programmed control law. It is used to manage a temporary total loss of electrics, a loss of all 5 fly-by-wire computers, a loss of both elevators or a total loss of both ailerons and spoilers. It's worth mentioning here that this is extremely unlikely and that even in emergency electrical config or a double engine failure, alternate law is still available. This is designed to be a temporary situation just until the affected systems can be restored. Pitch is controlled by the trim wheel and lateral control is done using the rudder pedals, both of which have mechanical linkages (hence the name!). When using the rudder like this there is a significant delay in getting  roll. You will also have to anticipate rolling out as this will be delayed too. You're not going to be able to fly this accurately but its just to keep you safe and stabilised. Unlike Direct law which says USE MAN PITCH TRIM in amber, the PFD will display MAN PITCH TRIM ONLY in red.

 

 

 

 

 

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Duration: 24:39


2 Comments

Raju
16 Nov 2017 03:15 PM
Hi. Thanks for that fantastic podcast. I have a question, which is not very well discribed in the books and also is not clear to me after listening to this podcast. In alternate law, when Vmo is reduced to 320 Kts does the speedtape actually change to show the barber pole at 320 KTS? Thank you for your kind help and I am looking foward to listen to your podcasts. Kind regards Raju
azry
27 May 2018 01:43 AM
hi guys, i agree with Raju, it is a fantastic podcast..I suggest a downloadable version of this..thanks guys.cheers!

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