A320 Dual input and side-stick priority

[peekay4]

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.

[ShotOne]

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.

[Denti]

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

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?

[peekay4]

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.

[Denti]

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.

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.

[Goldenrivett]

Hi Denti,

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.

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.

[Denti]

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.

[speed freek]

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.

On an airbus that would only show the control input, but not the control surface deflection as those are not the same

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


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

[ShotOne]

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?

[FlyingStone]

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.

[Denti]

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.

[stilton]

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.

[friend007]

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

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...

[peekay4]

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).

[fizz57]

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

[Goldenrivett]

Hi fizz57,

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

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

[peekay4]

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

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.

[safelife]

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.

[FCeng84]

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.

[FCeng84]

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.