Why do Airbus sidesticks add their inputs?

Question

There have been a couple of Airbus disasters where, in the confusion leading up to the crash, both pilots have attempted to control the aircraft with their sidesticks at the same time. In the absence of either pilot using the priority button, the effect of this is that both inputs are added, and the result clamped to the maximum single input.

On the face of it, this is a very curious design decision. Obvious alternatives include prioritising the left-seat stick (unless the priority button is used), or even averaging (instead of summing) the inputs.

Are the original reasons for this design decision documented? Is there an advantage to summing the inputs?

Answer 1

I believe that there is some logic in the design, actually, even if I don't really fancy it. If you think of the traditional yoke as a control element where a certain force (or moment, rather...) results in a certain change of flight path, then if both pilots apply the same force (rather than targeting the same deflection), the resulting aircraft reaction is more pronounced than if only one pilot applies that force, although probably not with linear relation. The sidestick controlled flight control system does the same.

What is missing much more, to my mind, is feedback to one pilot about what the other one is doing with the stick. I believe that having them interconnected or back driven would probably not have been such a bad idea.

Answer 2

Why do Airbus sidesticks add their inputs? I do not work for Airbus and am not privy to their design documents, but here is what I reckon based on the over 50 real-time simulation models of aircraft flying controls I've written and tuned:

• Summing the outputs makes most sense in avoiding discontinuities in position input, and provides the greatest flexibility if only one pilot at a time handles their side stick. A stick in centered position gives zero output. If a pilot transfers control to the other pilot, and no priority button is pressed, the flight control inputs from the two sticks have no discontinuity. Input from any pilot plus zero input from the other pilot works both ways, regardless of priority button status.

• Only one pilot at a time should operate their side stick. If the non-operating pilot does provide a stick input, summing the two outputs results in a change in attitude that alerts the PF of the action of the other pilot. Averaging the inputs would do this to a much lesser amount, or not at all. Giving priority to left stick may be counterproductive: what if the captain has a heart attack while the FO is flying?

As long as proper procedure is followed and only one pilot at a time handles a stick, summing the outputs works best. However the situation breaks down when proper procedure is not followed: in times of emergency and urgency, in the absence of clear heads. Two pilots each continue to provide very different stick outputs - which one should the aircraft use? How can it make a decision of which of the two inputs is the valid one, if the humans have not instructed it? There is no fool proof solution to this, so picking the best solution when procedure is followed (summing the outputs) makes sense and provides flexibility and continuity.

There have been a couple of Airbus disasters where, in the confusion leading up to the crash, both pilots have attempted to control the aircraft with their sidesticks at the same time.

Picture source

Consider the case of QZ8501. The captain is an ex military pilot, recognises the stall and provides proper recovery input to the stick. Had he instructed the FO to not touch the stick, he might have recovered successfully. But his ambiguous use of English (not his first language) caused the FO to follow perceived instructions and give a pull up input, of which the captain was unaware.

Summing the stick outputs did not save them, nor would have averaging the outputs. Transferring authority to the captain would have in this case, but now we would be waiting for Murphy's law: the captain panics while the FO gives proper inputs. I've heard anecdotal evidence of a case just like that, in gusty conditions at landing with an FO highly experienced in bad weather while the captain was from a warm and sunny country.

The basis of your question is a situation where proper procedure is not followed due to an emergency, and in this situation none of the options work very well. The only real solution here is to couple the sticks, and provide one output as a result of conflicting force inputs, like in physics. If there is no time for the rational brain to act, the only solution is to make use of the rapidly reacting sensory organs built into all humans.

Answer 3

This will be a theoretical answer, but unless the site is visited by someone from the Airbus control design team, that is probably the best we are going to get. Let me list for you the extremely desirable characteristics of the 'sum' approach.

• In the normal modes of operation, i.e. when one pilot is 'hands on' and the other 'hands off', the sum function means that the controls behave exactly like the pilots expect them to. I cannot emphasize how important this characteristic is. More importantly there isn't any switching' or 'special case' involved. More will be said on this absence of 'special case' later.
• It's suggested in the question that the inputs be 'averaged' instead of summed. However this means that a pilot attempting to fly the plane under normal circumstances gets only half the input he expects - full deflection of the stick gets only half deflection of the controls (because the other stick is centred). This is obviously completely unacceptable, and means that we would need either to always use the priority buttons, or to detect that the other stick is centred as a 'special case'. The reasons why this is a bad idea will be explained below.
• The 'special case' approach, where the stick behaves normally if the other stick is centred, but averages with the other stick if it is not, introduces some potentially catastrophic problems. If the 'centred' stick were to be moved very slightly - perhaps because the non-flying pilot accidentally brushed it with his sleeve, or there was a bump of turbulence - then the mode would switch. The response of the controls would suddenly change from the input requested by the flying pilot, to half that input (average of the flying pilot's input with the almost-centred other stick). A sudden large change of control position as a result of a tiny change like that is absolutely unacceptable, and under some circumstances might be life-threatening.
• The other advantage of the 'sum' approach is that any move of each stick always produces a similar response in the plane, no matter what the other pilot is doing. A certain deflection of the stick always produces approximately the same control response, whether or not the other pilot is also using the controls. This is extremely important. To have the controls respond differently depending on what the other pilot is doing has the potential to be extremely confusing.
• The 'averaging' function (proposed in the question) makes it impossible for one pilot to get full deflection of the controls when both sticks are active. Both pilots' maximum input are required (unless the 'priority' button is used - more later on why that is a bad idea). In the scenario where the non-flying pilot sees an emergency situation that immediately requires maximum control input, anything that prevents his ability to do that is life-threatening.
• The 'sum' function allows one pilot to make minor corrections to the other pilot's input. For example, if the First officer is flying, and is keeping the nose slightly too low, the captain can correct by making a small control input. As we saw above, if the 'averaging with mode switch' approach is taken, then the captain's input would result in a large response from the aircraft, not the small one he wants.
• It may be argued that alternative modes can work if the 'priority' switch is always used. However this requires that the button be pressed everyone time a pilot takes or gives control. Introducing an additional action - even one as simple as pushing a button - into an already well-established procedure is going to be dangerous. No other planes require you to push a button to gain control, and in a crisis it is likely that the additional action will be omitted. When a pilot-not-flying suddenly sees another plane on a collision course, or sees that the pilot flying has collapsed, their reaction will be to move the controls immediately - not to push a button and then manipulate the controls. On every other plane they have flown this action will have worked. The cost of that difference could be measured in lives lost.
• The idea that the left seat be favoured if the priority button is not used is also very problematic. Airliners are flown nearly 50% of the time from the right seat. The priority button would have to be used 50% of the time. The problems with the priority button would still be there, although admittedly half as often. The only certain way to make a control input in an emergency would still be to push the priority button (with the problems we listed above), unless you went through the additional step of thinking 'which seat am I in and do I have to push the button?", a step which would only slow response, with potentially catastrophic results.