A320 Dual input and side-stick priority
 

Hi everyone,

I'm new to this forum so I do apologise if I have posted this in the wrong section. I have tried searching for an answer but to no avail.

I'm currently doing my FYP at University which is looking into the Airbus flight control systems. This also means that I need to include information regarding the dual input and side-stick priority system. I have a few questions in which any response from anyone with experience on the Airbus flight deck will be massively appreciated!!!

1. I understand that the dual input system is in place, however how does it work exactly in regards to the warning messages on the glareshield/PFD?

2. How does the autopilot/priority button on the side-stick work (in terms of how long it needs to be pressed for)?

3. If anyone has ever encountered a dual input situation, is the system in place effective enough and clear to gain an immediate understanding of who has control?

I am currently doing some training/work experience at a base maintenance facility so I have studied the Airbus flight deck in person on an A330, however I have never seen the system in action. Any input would be greatly appreciated.

Thanks again!

If you go here(smartcockpit web site) A319 320 321 Flight Controls and select the Flight Controls choice, you should get a downloadable .pdf with descriptions of the flight controls. Please note the caveats. Starting about page/slide 58 the extended coverage of the priority choices begins. If that doesn't answer your questions, it may help you refine the questions you do have.

If you have already taken a look there, or have an up to date manual as reference, apologies.

Have you considered inquiring whether or not you could observe an A320 simulator session at a training facility through your university? Not sure how things like that work in the UK, but you may be able to get a look at how it works during a sim session if your university has contacts in the industry.
Best of luck to you.

In the event of a simultaneous input on both sidesticks (2° deflection off the neutral position in any direction) the two green SIDE STICK PRIORITY lights on the glareshield come on and "DUAL INPUT" voice message is activated.

A pilot can deactivate the other stick and take full control by pressing and keeping pressed his priority takeover push button.
For latching the priority condition, it is recommended to press the takeover push button for more than 40s.
This allows the pilot to release his takeover push button without losing priority.
However, a pilot can at any time reactivate a deactivated stick by momentarily pressing the takeover push button on either stick.
If both pilots press their takeover push buttons, the pilot that presses last gets priority.
Note: If an autopilot is engaged, any action on a takeover push button disengages it.

In a priority Situation:

- A red light comes on in from of the pilot whose stick is deactivated.
- A green light comes on in front of the pilot who has taken control, if the other stick is not in the neutral position (to indicate a potential and unwanted control demand).

In a pure dual input scenario both stick inputs will be algebraically added and the sum of both inputs will be used by the FBW system. Therefore they both are in command. That system is very unintuitive in my opinion but difficult to built otherwise without force feedback. A priority situation is pretty clear on the other hand, but takes some getting used to especially if coming from a conventional airplane.

Thanks for the replies! It makes much more sense now.

One more question though. If a Pilot continues to press the priority button, the other side-stick can not be reactivated and take control?

A330 flight controls - EFCS

Priority logic

• Normal operation : Captain and First Officer inputs are algebrically summed.

• Autopilot disconnect pushbutton is used at take-over button.

• Last pilot who depressed and holds take-over button has priority ; other pilot's inputs ignored.

• Priority annunciation : - in front of each pilot on glareshield - ECAM message - audio warning.

• Normal control restored when both buttons are released.

• Jammed sidestick : - priority automatically latched after 30 seconds - priority reset by depressing take-over button on previously jammed side stick.

Yes its all clear now :eek:


I'll take my Boeing thank you.

Most common case of 'Dual Input' scenarios, IMVHO, is during the training of new convertees to the airbus.

As the aircraft has no sensory feedback from either the side stick or the throttle quadrant the instructor in the LHS must 'gauge', 'judge' or 'anticipate' what the trainee is going to do, especially in the flare.

It is an odd system that provides additive input as one of the most common requirements for assisted input is in the landing flare when you need to be extra careful that you are not feeding potential recovery input into the stick for a badly judged flare at the same time as the trainee 'hoicks' in a handful of stick to arrest the ROD.

You get used to it pretty quickly though.

Another question (s) ... :O

I'm now slightly confused with pressing/holding the priority button haha.

So if a pilot presses the priority button, the autopilot is disconnected but BOTH side-sticks have control? (algebraically summed together)

If a pilot then presses on HOLDS the button, he/she has has full priority? If so, can the other side-stick take control whilst the opposing button is pressed?

If a pilot presses and HOLDS the button for more than 40 SECONDS, does this lock the other side-stick? If so, how long for and when does it return to normal mode?

Thanks!

... and will the system ever IGNORE both inputs in a certain situation?

Follow-up to BMATB...

If pilot A presses the button for 40s (isn't that a REALLY long time in an emergency btw?) and takes priority, can pilot B overrule that by pressing his/her button for 40s? (Assuming now that the B sidestick will go back to NORMAL mode after a while.)

And, reading Dave Clarke Fife, is there one autopilot disconnect button and one sidestick priority button (total sum of two buttons) or are those one and the same? I have looked at Airbus sidesticks but never really thought about this before.

No! When a pilot continues to press his priority button, the a/p is disconnected and his side stick and only his is in control. When no button is pushed but both sidesticks used at the same time, the inputs are summed together. In such case the pilots will gett a dual input warning.
Correct!
Yes, he can, by pushing his priority button again. It has been said before: Last pilot who depressed and holds take-over button has priority
Yes. The other sidestick is deactivated.
Indefintely, unless the priority button on this deactivated sidestick is pressed. Then it's active again.
No, dual sidestick faults are lethal
Why would you ever disconnect the other sidestick? When there's a sidestick fault (false internal contact with unwanted input or when a pilot drops dead on his sidestick. 40 seconds seems reasonable. It can be returned to "normal" mode by pressing the priority button on that sidestick. Something you wouldn't do if there was a fault, but might want to do if the fainted pilot is removed from the controls. ;-)

Here it is
 

http://i58.tinypic.com/v2uv5i.jpg
http://i60.tinypic.com/2u9kb9k.jpg

Thank you very much Mr Sabena.

All clear now, gracias!

I think I understand it a little better now and I know i've asked this question before but I never got a reasonable answer.


Apart from weight savings what are the advantages of non linked sidesticks with no feedback and non back driven autothrottles ?


Trying not to be so biased against Airbus here but I just don't see the point of this design.

There's no real advantage to "passive" sidestick systems except simplicity: less things to go wrong. And by now this design has been proven to be highly reliable and successful.

Yet despite its success, the A320 sidestick is literally 1980s technology. A bunch of springs and dampers -- it's ancient tech by today's standards. But there were no other options back then. Airbus had to fight hard to get digital FBW accepted by civil aviation authorities. They needed to demonstrate a fail-proof system. And a big reason Airbus pushed so hard for certification was because the chief proponent of the sidestick system was none other than the CEO's son, who had been a military pilot.

Only now in 2015 has technology advanced to the point that "active" sidestick systems with full force-feedback are becoming a commercial possibility.

Gulfstream will become the first civil aircraft manufacturer in history to use an active sidestick in the all-new G500 & G600, which will incorporate a new system designed by BAE.

It's just a matter of time until other commercial aircraft manufacturers will adopt this technology as the new standard.

Stilton, try asking your question the other way. What is the point of linked controls? In the event of incapacitation or a dispute over control they present a serious problem. There's no perfect answer of course, but the Airbus fbw approach is surely better than the straight trial of strength that would ensue in, say a 737?

I've been flying the A320-series for 10 years+ now. The current system does work, but I still believe it would be a major improvement if the PNF's side stick was somehow linked to and moving in the same way as the PF's one. Nobody can deny that would be a welcome feature. Obviously this should not necessarily be a mechanical linkage. An electronic feedback, combined with today's "dual input" warning and priority button would be optimal in my opinion. There's little doubt in my mind that that's what the future will bring. No problems in case of incapacitation in this way either.

(If you want to eliminate the risk of ONE rogue pilot wanting to do something wrong with the plane, I can't think of any other solution then introducing a 3 pilot cockpit each with their own sidestick or yoke where two inputs will have priority over the third pilot. But something tells me that's not going to happen. :rolleyes: )

The new Gulfstream and Bae sidestick system looks very promising to me!

This is the sole aim of my University project. Summed it up nicely!

That's exactly how the new Gulfstream active sidestick I mentioned above works!

The active sidesticks are not direct-linked mechanically, but rather uses a system of electronic servos via a computer to provide positioning and force-feedback. So an Airbus-style "priority button" can still be implemented if desired -- or alternatively a "split control" configuration can be applied, where in a contention situation each sidestick controls half of the control surfaces a la some Boeings.

The cool thing is that the position and force feedback gradient of each sidestick can be altered in real time. E.g., they can be made more sensitive or smooth during certain phases of flight, or alternatively the feedback can be made stronger when approaching aerodynamic or safety limits. Commands from the auto-pilot also move the sidesticks so they can be observed immediately, reducing "surprises" in cases where the A/P suddenly disconnects.

With 18 years on fbw airbuses I can see the point of a feedback system, if I understand you right, of the other pilots control inputs. (What happens in a dual input?) But what's the point of the stick moving with the autopilot? That's arguably worse than pointless since the pilots should be monitoring what the aircraft's doing not stick position.

Having the sidestick move with the autopilot provides additional visual and tactile feedback to the pilots.

This does not detract from aircraft monitoring -- in fact autopilot backdrive enhances monitoring and situational awareness, by giving extra (and natural) cues to the pilots.

That is a very arguable point. It does give additional cues. But they might be misleading ones. Airbus philosophy rightly lays stress on monitoring the actual state/attitude of the AIRCRAFT, not the stick or switch position. Ditto for earlier comment re. Thrust lvrs.

On an airbus that would only show the control input, but not the control surface deflection as those are not the same. Mainly due to the flightpath stable architecture.

Reduce speed with without changing altitude, the feedback would show exactly nothing. Although the pitch attitude will change considerably. What use is the feedback in that situation?

Hi Denti, that's not quite correct.

With speed reduction, the autopilot will command pitch changes to maintain altitude. These pitch changes (up/down) will be seen on the active sidestick.

In fact, you will naturally expect to see these pitch changes, because if you were to fly the plane manually you will make those same pitch adjustments.

Over time, you start to develop an intuition of how active the sidestick should behave in different flight conditions when on autopilot.

E.g., in good weather straight-and-level flight, you would naturally expect the sidestick to only show continuous small movements corresponding to small autopilot corrections.

So if you happen to notice the autopilot commanding large and unexpected sidestick movements, then that might be an additional cue to you that there's something which might need your attention.

Today you might not have any indication of trouble until the autopilot gives up and disconnects.

This is not to suggest that you monitor the sidestick. However, as mentioned, having an active sidestick can provide additional visual and tactile feedback that's natural to the pilot -- almost at the intuition or sub-conscious level.

In a conventional aircraft that would be correct. In an Airbus it isn't. Remember, the airbus is flight path stable, not speed stable. It will maintain the flight path without pilot input within the normal envelope.
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The same thought experiment in manual flight, manual thrust. Pull back thrust and maintain the stick centered (just don't touch it). The aircraft will slow down and maintain (roughly) the same altitude with the same pitch change to do that.

The FBW system, not the autopilot, not the human pilot, will command the necessary control surface deflections. And if you have seen the "dance of the ailerons" after take off on an A380 for example you would know that any feedback of those control surface deflections would be very counter productive.

Hi Denti,
The feed back position of the master aileron (the internal aileron) is all that would be needed to be felt.
see: Page 12 of http://www.ukfsc.co.uk/files/Safety%...uly%202012.pdf

"A similar differential deflection was also implemented on the two rudders and was called “VDR” or “Valse Des Rudders” (rudders waltz), a typical Airbus “British – French” acronym, as rudder is not a French word!"

I'm sure the crews don't find the feed back position of the rudders through their pedals counter productive.

Flight control surface deflection however serves no purpose as pilots do not control the flight controls surfaces themselves, except in direct law. The pilot controls the flight path (g-load and roll rate demand) and the FBW does what it needs to follow those demands.

Control surface feedback would be counter productive therefore, except if the bus would be reconfigured to fly in direct law all the time, by which time it is absolutely useless to have the FBW in the first place.

Now, a feedback of the stick position of the other pilot would serve a purpose.

But isn't that what people are saying with backdriven controls?

If you treat the autopilot as a third pilot in the flight deck, then surely you'd like to see what it's commanding alongside what the instruments are saying. I agree, it's pointless having the FBW feedback through the sidestick.

Many conventional aircraft over the years have had low and high speed ailerons, with the change-over happening automatically and no indication in the flight deck. The control wheel still moves as normal though :ok:


And surely what you say is exactly what the set up is on the 777/787.

What happens in a contention situation to this new side stick?


An earlier poster (peekay?) wrote on some boeings "each sIdestick controls half the control surfaces". So if one pilot commanded left and the other right, it would command up aileron/spoiler on both wings(!!)? Please someone tell me that's nonsense?

I know nothing of Boeing FBW, but...

1. Boeing does not have sidesticks.

2. Control columns on all Boeings are mechanically inter-connected, so even on FBW aircraft they are most likely to give the same commands to FBW system from either left or right control culmn.

There is usually a breakout mechanism, with enough force you can disconnect the columns and then you have indeed the case that different control surfaces get their commands from their column, or better said each side controls only a portion of them.

On Airbii of course you have either arithmetic summation of both control inputs or you can disconnect the other stick with the priority button.

Why on earth would you want 'arithmetic summation' of both sidesticks ?


The Airbus control philosophy seems counterintuitive, byzantine and needlessly complex with no consideration to human factors or simply
'whats it doing ?'


And for all its vaunted precision I read many accounts of its lag and the difficulties this causes in gusty crosswinds.


I don't believe it was outside the state of the art for the sidesticks to be linked and backdriven when the A320 was introduced and I know it wasn't an issue for the autothrottles to have had the same arrangement.

Because both sidesticks are not connected and this makes it intuitive for the other pilot to make/add corrections. Eg trainee does not pull enough in the flare. LTC's sidestick is neutral. He just wants to add a bit of nose up so he just pulls his stick a bit and his input is added to that of the trainee.

If you want to take control regardless of what the other guy is doing, just push the disconnect button to override him...

The need for summation is simply a legacy of sidestick technology that was available 30 years ago. With non-linked passive sidesticks, summation was (and is) the only logical arrangement.

Denti: you may want to check out how the autopilot and auto-throttle back-drives are integrated into the F-35 FBW. (And soon on the G500 & G600).

Don't the coupled yokes on a conventional system provide "arithmetic summation" of the force applied by each pilot?

Hi fizz57,
Correct. But the advantage of having displacement feed back is that you will have a much better idea of how much input was required.

Active Sidesticks: A New Way to Fly

Not exactly the same.

Scenario: FO applies 10 lbs back pressure causing the plane to start pitching up. At 3 degrees nose up, the Captain applies same magnitude forward pressure.

Airbus. Sum == 0, no commanded change, so the nose stops pitching up at 3 degrees ANU.

Conventional. Equal forces, so the yoke stops moving rear-of-center. Plane continues to pitch up past 3 degrees nose up.

Also note:

Airbus. FO's sidestick is now rear-of-center while the Captain's sidestick is front-of-center. But the plane continues "straight" -- contrary to both pilots' natural expectation! See AF447 to see why this is bad.

Conventional. Both pilots' yokes are at the same rear-of-center position, and the plane behaves exactly as both would expect.

Active sidesticks will basically restore what's already natural to pilots / human beings.

peekay4, that is exactly the reason why it is taught on the airbus not to make use of dual input, but rather take over control (via tha button) and gain exclusive control.

Boeing Column Input Clarification
 

As stated above, the Boeing columns are mechanically linked through a breakout that allows relative motion if the force applied to the link exceeds the breakout level.

Separate sensors measure the positions of the left and right columns. For the non-FBW models and the FBW models operating in Direct Mode, left column position commands the left elevator and right column position commands the right elevator. On the non-FBW models the command path from column to elevator actuator is via mechanical cables (plus the manual reversion path on the 737). On FBW models this command path is via electrical signaling.

For Boeing FBW models operating in either Normal or Secondary mode the flight control computer receives data from both the left and right column position sensors. The average of the left and right column positions is used to drive the longitudinal control laws. In Normal and Secondary modes both elevator surfaces are commanded to the same position.

Boeing FBW Controller Backdrive Objective
 

The objective behind pilot controller backdrive on the Boeing airplanes is to move the controllers as they would need to be moved by the pilot to achieve the same response as is being delivered by the autopilot were the autopilot disconnected and the pilot flying manually. On non-FBW models this indicates the position of the associated surfaces. That is not the case on FBW models where surface position is defined by maneuver demand, stability augmentation control laws.

One outcome of this approach is that only minimal airplane transient will result if at any time the crew grabs the controllers and disconnects the autopilot while maintaining controller position.