How does the control sidestick in Airbus aircraft work?

sidestick

What an easy intuitive and great way to fly that robust joystick is, called sidestick on the Airbus 320/330/340/350 and 380 models and submodels.

The best thing is that it’s not in the way to install a stowable table in front of the pilots, which makes eating and doing paperwork in flight all the easier. Hell, you can even have a flight attendant on your lap if you fancy a macho way of getting fired from your airline.

Yet there are a lot of misconceptions about this side control joystick, mainly by hardline Boeing fans who just don’t want to let go of that huge misfit between their legs.

The big control column of course is what were you thinking of?

control joystick

What a great way to fly this sidestick is. And it leaves room between the pilot’s legs for more important objects, like a table for example.  Note the red button which will be mentioned later.

The sidestick works from the pilot’s point of view just like a control joystick would work when you’re playing your favorite computer game:

You pull it and the nose of the airplane moves up – You push it forward and the nose drops – You turn it to the left and the airplane banks to the left – You turn it to the right.

However, what goes on behind the scenes

It is actually a bit more intriguing and it turns out that sidestick doesn’t really just do what pilots may ask of it.

Welcome to the world of fly-by-wire, where there are several independent and fancy computers that interpret what the pilot’s input is on this sidestick and deflect the control surfaces according to what they are best deflected to.

Contrary to conventional flying, the sidestick’s inputs don’t directly control a deflection of the control surfaces.
Also unlike with conventional steering the flight control laws of the Airbus use “g”, the acceleration vector of earth’s gravity, as a demand norm in pitch.

When a pilot lets the sidestick rest in neutral, the airplane will therefore assume he’s demanding 1 g from it. That can be level flight but it can also be a constant rate of climb or so.  [note 2]

In roll, the side control joystick asks a roll rate and in pitch, it asks a load factor (like 1.05 times g), rather than a direct reflection of the surfaces.

This is great for pilots who fly a smaller A318 one day and a bigger Airbus A330 the next: they feel the same, they handle the same and so you can be rated on both types at the same time – a big selling point for Airbus.

sidestick

The control joystick or the autopilot is the basis of the control chain and their signals are interpreted by some flight control computers before these computers send further orders to the control surfaces

These computers don’t let pilots bust their safety limits on the Airbus. In fact, the pilots are treated like kids: they can only go that far before mother Airbus has to stop them from harming themselves and others.

One of the things Airbus doesn’t let pilots do with the sidestick is going outside of the flight envelope

That’s a fancy way of saying that they won’t be able to fly too fast or too slow, a bank too much or pitch the airplane too much, or be too sporty and pull too many g loads.

In short: if the pilots don’t behave the airplane won’t blindly follow the stick inputs.

So if you hold the sidestick full-back and keep it there, the airplane will only let you pitch up so much before it knows you’re better suited for a desk job and starts ignoring your further demand to pitch up.

Similarly, when you yank a full deflection to the left out of the stick, the airplane will first make it hard for you to maintain that by exerting a feedback force in the stick once you pass 33 degrees.

And second, you won’t be able to exceed 67 degrees, even if your mother is waving at you at 68 degrees bank and you want to wave back.

These rules are abandoned when the airplane somehow ends up in a really unusual attitude, so you can still recover.

These rules work miracles when the pilots need to get away from the ground fast, in wind shear or in-ground proximity warnings, because they can just go full back stick without worrying about a stall.

Under the sidestick are a bunch of levers and artificial feel mechanisms that enable all this. The wires then continue to the flight computers from where other wires go to the hydraulic actuators of the flight controls

(Fly by WIRE, remember. Although you can’t fly without hydraulics). Note that this artificial feel is not feedback from the forces on the flight controls, as computer games may do.

Below the sidestick is a bunch of springs and rods and solenoids and dampers and screws and bearings, that make this magic possible.

sidestick

Now what happens when your fellow pilot spills his hot coffee over the control joystick

Short circuits the thing somehow or even makes the stick give weird and faulty inputs?
Or what if he’s not the sharpest tool in the shed and he’s flying in a way that’s really starting to work on your nerves?

Well, you can press your little red round takeover push button then. A sexy voice will come on saying “priority left/right” and there’s a red light that comes on in front of the pilot whose stick is deactivated so that he sees he’s out of the loop.

If the other stick is still giving inputs then there’s a green light coming on in front of him with his job title (Capt or F/O); this is the way to know when that spilled coffee is causing faulty inputs even though he’s pleading innocence and he isn’t touching the stick anymore.

If one pilot presses the red button for more than 40 seconds continuously, the other sidestick remains deactivated until the red button on it is pressed.

If both pilots are in a heated fight for control and both keep on pressing that button uninterruptedly then the captain in the left seat wins the fight and he’ll have control, otherwise it’s the last one who presses who gets control.

sidestick

Some flashy bulbs in the frontal view of the pilots are related to the side control joystick priority.

A red arrow indicates your side is out of the loop. A green light tells you that you’re giving inputs.

If both are on and you’re not touching your stick, it’s time to tell the other pilot he should press his button for 40 seconds because your stick is faulty.

What if nobody presses any priority button and both pilots just do their own thing with their own stick?

Then that sexy voice comes on again and says “dual input” and both the green lights with the job titles come on.  (No red arrows because nobody took priority).

What actually happens then?

It is that the inputs are algebraically added and sent to the flight computers, who will still limit the airplane to the normal maximum bank, pitch, etc.

So if one pilot goes full left and the other full right: nothing happens except the flashy bulbs and the sexy voice “dual input”.

If both pilots think the full left is the way to go and both go full stop to that side the airplane will still limit its bank to 67 degrees, but now with lights and a sexy voice.

The red button also works as an autopilot disconnect button.

Even though the red button is called by Airbus the “instinctive takeover pushbutton”, a comment from pilots is often that it’s not all that instinctive to press in a stressful situation that requires prompt action, like the other guy messing up the landing.

Since there’s no reflection on the other side of the cockpit you can’t feel what his inputs are. I’ll give Boeing guys the satisfaction to agree with them that this is the (only) criticism that has some ground.

1. Interesting fact that even most Airbus pilots don’t know: if you fly at cruise levels where g, the acceleration vector, has a smaller magnitude because you’re further away from earth’s center of gravity.

The airplane actually enters a very shallow climb with a neutral stick and manual flight to go after the 9.81 m/s^2.

2.  I completely forgot to mention that there’s no need to trim the airplane either.  While you fly the airplane is trimmed automatically for you.

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