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Airbus Sidestick Command

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Airbus Sidestick Command

Old 22nd Apr 2010, 08:26
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Airbus Sidestick Command

Hey Everyone!

Before im referred to the search function, Ive already done my research but I have a follow up question below:

Call me a bit sad, but I was pondering this last night in bed . In regards to the Airbus Family sidesticks, I understand the logic as the following (correct where necessary):

Lets first consider 2 seperate logics, Logic 1, where we do not consider the Takeover buttons ("TOB", excuse my improvised acronym) and Logic 2, where the TOB has been used.

Logic 1
  • PIC is assigned to whichever pilot inputs a command from their sidestick first.
    • i.e. P1 moves his stick, he is PIC while he makes a commanded input, he reurns his stick to the centre position, to which P2 then provides an input and subsequently becomes PIC and so on....
  • If there is an input from both sticks, (who inputs first is PIC) the command from each are algebraically summed, but the total command is limited to the maximum command of 1 stick.
    • i.e. P1 moved 75% displacement port (currently PIC), P2 then moves 60% displacement Port, System only registers 25% displacement of the 60% from P2?
    • P1 moves 25% displacement port (currently PIC), P2 then moves 25% starboard, total command = 0% displacement?
    • Is the command from the sticks an attitude or a rate??
Logic 2
  • If P1 is PIC, and P2 wants some action, he hits the TOB and becomes PIC while the TOB is in detent.
  • If the P2 TOB is held for 30-40s, the logic becomes latched with P2 as PIC until either takeover buttons are pressed again.
    • if given the above, P1 then presses the TOB for 2s, for that duration P1 becomes PIC, but after the 2s detent it reverts the system back to original Logic 1??
    • if given the above, P2 (PIC) presses the TOB for 2s, for that 2s P2 still remains as PIC for the 2s, but after it reverts the system back to Logic 1??
PLEASE correct any of the above...my main question is below:

If P1 is PIC (under Logic 1), he has a heart attack/gets shot and slumps on his stick...consequently a 100% displacement left is input to the system....P2 thinks "oh crap" grabs his stick, moves 100% right displacement, then hits the TOB, there is an instantaneous input from extreme left to extreme right. How does the system cope with this instant "jump" as im pretty sure its not acceptable to the system. Is the command rate limited? if so, how? is the mm displacement rate limited to the FCC or is the FCC output rate limited?

Please dont debate the probability/relevance/ins & outs of the above scenario, im more interested in what the system would do!! Sorry if I have over-thought such a small detail, BUT im an Engineer by trade its in my nature! :P

Thankyou for reading the above. I would be really grateful to relevant contributions! Thanks for your expertise
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Old 22nd Apr 2010, 08:52
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This just further confirms my deep founded respect for any Airbus driver (absolutely no punt intended). As someone that never flies one, I too am profoundly interested to know if there's a proper explanation to your queries.
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Old 22nd Apr 2010, 09:01
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Simple, really.

In normal law, pitch command is blend of rate+G, roll command is roll rate. It doesn't feel awkward, I have quickly adapted to it and equally quickly forgotten it, once I was back on classic controls.

Simultaneous inputs form both sides are actively discouraged during training. If they are made, both inputs are simply summed and result passed to flight control computers, limiting value being 100% displacement. E.G. both full left sidesticks give maximum left roll rate command that is achieved with one sidestick in neutral and other fully deflected. If sidesticks are displaced at the opposite ends, sum iz zero and no command is given to FCCs.

When practicing incap in the sim, procedure was for the pilot simulating incapacitated pilot to push stick fully outboard and forward. Pilot taking over would have to press priority button and correct the attitude,. if he forgot the appropriate sequence and used dual input to stop the excursion, nothing much would have happened when the priority finally got pressed. Both controls and aeroplane have a lot of inertia and G-protection would prevent excessive load anyway.

IIRC, if one stick was latched out by holding the other takeover button, it could be restored by pressing its button, not any.

Oh, and PIC = pilot in command, independent of who flies. The gal/guy that handles the aeroplane's controls is PF(lying) or H(andling)P.
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Old 22nd Apr 2010, 09:13
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Thanks guys!

Thanks for your replies so far, especially clandestino for your experiences from training. I should appologise for my PIC definition getting mixed with the PF definition!!

Quote
"if he forgot the appropriate sequence and used dual input to stop the excursion, nothing much would have happened when the priority finally got pressed."

Nothing much would have happened?? Can anyone expand on this?? I understand nothing much would have happened BEFORE the TOB was pressed, but afterwards i would expect the new PF command to take priority and the aircraft to take corrective action according to the new command??

I understand the Airbus protects itself from excessive loads, however, my question is what happens with the instantanous change in command from extreme left/forward to extreme right/backward from a control law point of view?

Can anyone shed any light on this??
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Old 22nd Apr 2010, 09:24
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I understand nothing much would have happened BEFORE the TOB was pressed, but afterwards i would expect the new PF command to take priority and the aircraft to take corrective action according to the new command??
Correct. That's exactly what would have happened and it's nothing much. Aeroplane takes corrective action but it's very remote from the snap roll. It takes a while for controls to move to their commanded position, aeroplane inertia dampens response even more, roll rate is limited to 15/sec ang G to +2.5/-1 gear up, +2/0 gear down.
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Old 22nd Apr 2010, 10:26
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Call me a bit sad, but I was pondering this last night in bed
Er.....you actualy pondered ALL THIS last night in bed?

You BADLY need a girlfriend mate!

PS: Im not an Airboos PC fanatic but I think your confusing PIC with PF. If my
P2 pushed the red button and said he was now PIC Id give the little bugger a
firm backhander!
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Old 22nd Apr 2010, 10:37
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Er.....you actualy pondered ALL THIS last night in bed?

You BADLY need a girlfriend mate!

PS: Im not an Airboos PC fanatic but I think your confusing PIC with PF.If my P2 pushed the red button and said he was now PIC Id give the little bugger a
firm backhander!
ahaaa!! LOL...thanks for the advice slasher!! Like i said, call me sad! That said, I do indeed have a gf, but unfortunately im working out of the UK for the forseeable future!! Hopefully she is visiting this weekend if her flight is ok!!

I also already appologised for mixing PIC with PF!! I made the confusion becaus for me as an Engineer im not interested in who is Captain (PIC) or first officer...i just see 2 seats with commands originating from both!! hence why for me, PIC was whoever is making the command from the sidesticks!! Appologies for my ignorance!

It takes a while for controls to move to their commanded position,
Thanks again Clandestino!! So we can assume a rate limitation is infact present for the instantaneous jump from extreme left/fwd to extreme right/back (disregarding the aircraft dynamics)?? The next question would be is this done on the Control Law (CLAW) output or CLAW input? I would suspect the latter??

Might be worth moving this to the Engineers/Technicians Forum?? Might be some FBW Design Engineers in there?!
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Old 22nd Apr 2010, 10:59
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I was actually referring to powered flight controls lagging behind their inputs, which is not just Airbus specific but yes, roll rate limitation is always active in the normal law. Flight controls always give their best to satisfy roll/G demand but the actual way they do it is way beyond me, humble former A320 RHS pilot.
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Old 22nd Apr 2010, 13:57
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ah ok! Thanks for your help Clandestino

Now...I wonder where Airbus Flight Control System/FBW Engineers hang out....
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