Is Dual Input feature necessary during T/O & LDG?

[vk757320]

Originally Posted by Uplinker

I have always wondered about this. Why do Airbus side-sticks arithmetically add if they are never supposed to be operated together ? I think the intention is so that a TRE can grab the situation and pick up a wing in the event of a bad gust near the ground, or a missed flare, or a bounce, that a trainee does not react appropriately to, to avert a prang. If they also press the red button, fine, but even if they don't, they can still save the situation. I suspect that the "Dual Input" warning was provided for the trainee to be aware that their instructor was correcting on the non linked side-sticks.

Some years ago I was taxiing out at JFK and my Captain suddenly slammed on the brakes because I had slightly strayed off the centreline in a turn. The whole 230 tonne aircraft dipped and shuddered and lurched to a stop which was far too dramatic. If he had simply used his tiller to steer back onto the line and said something appropriate, it would have been far smoother and less alarming for all concerned, including the passengers.

.....Indeed as I myself did to another Captain when he went wrong - I just steered him back to the centreline with my tiller and asked if he was OK. No dramatic emergency stop required, just a gentle tweak.

Obviously, you absolutely do NOT want both pilots applying simultaneous inputs to their side-sticks routinely during normal flying or taxiing, but the facility is surely there for very occasional emergency corrections.

Originally Posted by sonicbum

The only reason the inputs are algebraically added is because there is no better solution; it is the one that makes more sense from a design point of view.

The “Dual input” is a big no-no. It’s a warning rather than a caution like: “you are not supposed to be doing this”.

In your example of a bad gust near the ground you push the TakeOver PB and you correct. the sentence “I have control” is simultaneous to pressing the red button.

Trainees of any experience level have to be taught by the instructor from Day 1 that on Airbus FBW there is no room for “a little help in flare”. So if needed the instructor/cpt/FO (incapacitation) will takeover and no hard feelings. It’s absolutely normal.

Originally Posted by BUSSPilot

The Airbus recommended techniques for taking over during take off and landing is by pressing the take over push button and keeping it pressed - irrespective of how many tenth’s of a second away we are from the main wheels touching the runway. From my personal experience I have found that it has worked well enough. I suppose it’s prudent to ensure you are not too late to exercise the use of it. Airbus golden rule - Take action if things do not go as expected. I am sure using the Prompt-Assist-Takeover model or its equivalent at your airline should help mitigate these issues.

As far as discussing the disadvantages of your proposed “software update” - why should it be deactivated for a training flight and not regular line flights? Are these tail strikes happening only during training flights with first officers on the CM2 seat?

It should be applicable for all flights. And the software update obviously isn't for CM1. The priority can be assigned in advance. Tailstrikes are more likely after a bounce followed by a landing or go around when both pilots give control inputs. Also, the Capt can correct for only what he sees at that moment and not cater for what his/her FO is about to do. FO still has the TO pb as an option to cancel the priority logic.

Question is, what is the disadvantage of this logic? In what situation, can it go wrong?

[AerocatS2A]

Question is, what is the disadvantage of this logic? In what situation, can it go wrong?

The other pilot unintentionally applies an input to the side stick. No “DUAL INPUT” warning means neither pilot knows it is happening.

[Speed_Trim_Fail]

Originally Posted by AerocatS2A

The other pilot unintentionally applies an input to the side stick. No “DUAL INPUT” warning means neither pilot knows it is happening.

Bumpy approach, skipper has hand covering their sidestick, for radio or SOP, knocks the sidestick, control is transferred to the LHS…. FO goes to flare and there is literally no response. 50ft from the ground is not a place for any control ambiguity or the possibility for the PF’s sidestick to be silently locked out…

This is a solution looking for a problem - exactly the same logic is applied during an autoland, if you don’t like it press the big red button? If you are holding the sidestick in the right way then it is exactly what it is named, instinctive.

[Uplinker]

Originally Posted by sonicbum

The only reason the inputs are algebraically added is because there is no better solution; it is the one that makes more sense from a design point of view.......

Well, actually PM's side-stick could be designed to be ignored by the FBW unless and UNTIL the take-over button was pressed. On pressing the take-over, PM's 'stick would become active and PF's; inactive. That would exactly prevent dual inputs happening, but still allow PM to take control.

But Airbus did not engineer it that way - why ? Nor did they provide a take-over button for the tillers, and the "dual input" warning does not activate when dual tiller inputs are made. Why is that ?

.....The “Dual input” is a big no-no. It’s a warning rather than a caution like: “you are not supposed to be doing this”.

In your example of a bad gust near the ground you push the TakeOver PB and you correct. the sentence “I have control” is simultaneous to pressing the red button.

Yes but remember in your very initial PPL training, when the instructor occasionally applied pressure or even firm corrections to their yoke when you were learning and feeling how to flare for the very first times - without the "I have control" full dramatic take-over, just gentle guidance and finesse.

Completely taking over is not gently correcting, and the cadet is left feeling that they screwed-up big time rather than simply being guided.

I was often invited to "follow me through on the yoke" when learning to land my first commercial types, and deal with crosswinds etc. You cannot do this with un-linked controls, and I think Airbus engineered the arithmetical addition and keeping both 'sticks active to allow this sort of assistance, and the ability to quickly "nudge it" to avoid a prang.

Cadets are now going from a flight school Diamond 42 straight onto an A320 large jet, rather than gradually building up skills and experience via small turbo-props, larger turbo-props, small jets and then large jets. So the first flares and landings on the A320 family are going to be a huge step from what they are used to. The first few might therefore be a bit sketchy, and I suspect that Airbus designers allowed both 'sticks to be always active to allow a TRE to prevent it going bad, or gently nudge it, but without all the drama and stress of "I have control", and taking over, which would destroy confidence.

.......Trainees of any experience level have to be taught by the instructor from Day 1 that on Airbus FBW there is no room for “a little help in flare”........

Why not though ? You have that facility on every other aircraft type, why should it be denied on Airbus FBW ? The Airbus side-sticks are not mechanically linked of course, so this arithmetic addition is probably the closest way of simulating it.
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[sonicbum]

Originally Posted by Uplinker

Well, actually PM's side-stick could be designed to be ignored by the FBW unless and UNTIL the take-over button was pressed. On pressing the take-over, PM's 'stick would become active and PF's; inactive. That would exactly prevent dual inputs happening, but still allow PM to take control.

But Airbus did not engineer it that way - why ? Nor did they provide a take-over button for the tillers, and the "dual input" warning does not activate when dual tiller inputs are made. Why is that ?

Think about risk assessment. If You want to exclude a primary flight control You must willfully do it, this applies to all the airplanes where there is a possibility to exclude the other Crew Members primary flight controls. Simply acting on a flight control cannot exclude the other.

Originally Posted by Uplinker

Yes but remember in your very initial PPL training, when the instructor occasionally applied pressure or even firm corrections to their yoke when you were learning and feeling how to flare for the very first times - without the "I have control" full dramatic take-over, just gentle guidance and finesse.

Completely taking over is not gently correcting, and the cadet is left feeling that they screwed-up big time rather than simply being guided.

I was often invited to "follow me through on the yoke" when learning to land my first commercial types, and deal with crosswinds etc. You cannot do this with un-linked controls, and I think Airbus engineered the arithmetical addition and keeping both 'sticks active to allow this sort of assistance, and the ability to quickly "nudge it" to avoid a prang.

Cadets are now going from a flight school Diamond 42 straight onto an A320 large jet, rather than gradually building up skills and experience via small turbo-props, larger turbo-props, small jets and then large jets. So the first flares and landings on the A320 family are going to be a huge step from what they are used to. The first few might therefore be a bit sketchy, and I suspect that Airbus designers allowed both 'sticks to be always active to allow a TRE to prevent it going bad, or gently nudge it, but without all the drama and stress of "I have control", and taking over, which would destroy confidence.

Why not though ? You have that facility on every other aircraft type, why should it be denied on Airbus FBW ? The Airbus side-sticks are not mechanically linked of course, so this arithmetic addition is probably the closest way of simulating it.
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It is all tied to a different design philosophy, whereas we can achieve the same results but through a different process.

Your inputs also serve as an opening for a very important aspect: it is of utmost importance that instructors properly teach the landing technique to the trainees at SIM stage -which is often not the case- and then the issues are transferred to the line. We have, more or less, all been there.

As discussed before, the trainee must be aware that during LIFUS the takeover is absolutely normal, especially at early stage. What becomes important afterwards, during the debriefing, is that the instructor identifies the root cause of the takeover, otherwise the one who fails at the job is the instructor, not the trainee. Within the next few landings the root cause will be tackled and the problem solved. Providing the trainee with "a little help" during flare is a short-term fix (again a no-no on Airbus FBW) that does not address the root cause.
During instructor training, the instructor trainers have to highlight the most common "mistakes" made by trainees of different level of experience -cadet, experienced changing type, etc..- and raise awareness to find solutions. We are looking at permanent fixes as quickly as possibile, that is the idea.

[Uplinker]

Simply acting on a flight control cannot exclude the other.

OK, but you also say that an Airbus TRE must only correct a situation by pressing the override button - which does exclude the opposite side-stick - and "I have control".

Yes, of course training must be clear and understood, but completely taking control from someone (instead of gently assisting), is a big deal and puts the cadet on the back foot, thinking they have done something bad. That Captain PM at JFK who dramatically emergency stopped rather than simply gently correcting my taxiing caused me a lot of concern and alarm and put me right off at a time when we both needed to be focussed and very aware of ATC and everything going on at that airport before we took off.

You might be right, but when someone has the wrong picture of the flare in their mind, a gentle correction there-and-then by a TRE might sort it, rather than discussing it in the classroom afterwards, when the picture is not there in front of you.

[Vessbot]

Sorry but I'll be blunt, any suggestion to violate the "stick makes the airplane move" principle is absurd. That is the most basic bedrock, catch-all foundation of what flight control inputs are supposed to do, that any situation can immediately filter down into.

Sometimes, especially in a split second stressful situation with bigger fish to fry, people forget to press a button. Aviation history is full of crashes where people forgot to fly the airplane because they were tumbling down a logic tree of automation modes. One more obstacle in this path is not what the world needs. Quite the opposite.

[sonicbum]

Originally Posted by Uplinker

OK, but you also say that an Airbus TRE must only correct a situation by pressing the override button - which does exclude the opposite side-stick - and "I have control".

Yes, of course training must be clear and understood, but completely taking control from someone (instead of gently assisting), is a big deal and puts the cadet on the back foot, thinking they have done something bad. That Captain PM at JFK who dramatically emergency stopped rather than simply gently correcting my taxiing caused me a lot of concern and alarm and put me right off at a time when we both needed to be focussed and very aware of ATC and everything going on at that airport before we took off.

You might be right, but when someone has the wrong picture of the flare in their mind, a gentle correction there-and-then by a TRE might sort it, rather than discussing it in the classroom afterwards, when the picture is not there in front of you.

Let’s consider the fact that taking over is obviously the last resort in the chain of corrective actions that are put in place between the pilots. If we consider a training flight, there are stages where a takeover is normal, as we can’t decide the weather like in the Sim, and we can’t have the freshly Base-Trained or ZeroFlightTimed trainee wait for Variable 2kt, CAVOK for the first landing as PF. So at early stages, despite the thorough briefings that can be put in place by the instructor, the skill based competencies are still “work-in-progress” and there will be room for intervention. The latter will be mainly verbal coaching but could result in a takeover too. No big deal.
As the trainee progresses, ending up in a takeover at an advanced stage of the line training means quite often that some instructors have not done their job properly (we are talking, of course, about non-challenging airports and WX) . Sometimes there are personal issues of course, but that’s a different scenario.

In Your JFK episode it looks like your colleague did not communicate effectively with you to give you advice before ending up slamming the brakes and taking control. If a dual side stick input is a big no-no, a dual NWS input is the mother of all no-no.

Of course after a takeover the debriefing has to be extremely gentle and effective. There is nothing to worry about. If a flight goes well for 2 hours or so, where there is not much to correct, then we will correct those final 2/3 minutes for sure. We are 95% done. If you pass this message across, the all thing takes a different atmosphere.

Just to give You an idea: in my outfit FOs are allowed to exceed their X-Wind limitations when flying with a trainer. Luckily our training culture is such that each and every single FO will take advantage of it -should the case arise and if conductions are not too dramatic- with no fear of being considered “not good enough” if the Capt. has to takeover. That makes a big difference.

[vk757320]

Originally Posted by AerocatS2A

The other pilot unintentionally applies an input to the side stick. No “DUAL INPUT” warning means neither pilot knows it is happening.

Originally Posted by Speed_Trim_Fail

Bumpy approach, skipper has hand covering their sidestick, for radio or SOP, knocks the sidestick, control is transferred to the LHS…. FO goes to flare and there is literally no response. 50ft from the ground is not a place for any control ambiguity or the possibility for the PF’s sidestick to be silently locked out…

This is a solution looking for a problem - exactly the same logic is applied during an autoland, if you don’t like it press the big red button? If you are holding the sidestick in the right way then it is exactly what it is named, instinctive.

I am sorry to not be specific here. Obviously, the "Priority Left" would trigger to let both pilots know that the Capt has given an input. So, long story short, there is no dual input, just one input, Capts intervention disconnects the other, and priority left ⬅️ triggers. The FO still has the option to press the TO pb, just to takeover or deactivate the system.
​​​​​Also, in the debrief, atleast there is clear distinction between who did what.
Inadvertent inputs below 50ft makes no sense. But to protect that a 2deg threshold similar for dual input can be set.

[FullWings]

Originally Posted by Uplinker

Yes, of course training must be clear and understood, but completely taking control from someone (instead of gently assisting), is a big deal...

I have completely taken control at a fairly late stage on the line just once in 10,000+ hrs in the LHS and it taught me what might happen, which was flying solo for several minutes afterwards. If it’s a training sector you’re likely more mentally prepared for this kind of thing (as is the trainee) but with experienced pilots who are doing everything right and are attacked by the once-in-a-decade gust at the end of a long day it can come as a bit of a surprise.

[Uplinker]

I am still feeling confused as to why only Airbus FBW pilots are forbidden from making dual inputs. It is of course undesirable to do so if it can be avoided, but I feel sure that all of us at some point in our training have felt the instructor's pressure on the yoke or the pedals.

To be clear, dual inputs should only ever be to arrest a sudden attitude change or the wrong attitude, where not to intervene would result in a significant deviation or a very uncomfortable arrival, and when there isn't time to correct the situation by talking.

In such a situation, quickly moving the other side-stick to arrest or change the attitude might be more important than pressing the override and saying "I have control". Hence both 'sticks remain active even without pressing the override.
I thought the override was more for the incapacitation case when the other pilot has collapsed while still pushing the 'stick away from neutral.

My minor taxiing error was made unnecessarily dramatic, as well as literally disturbing and alarming the passengers. A gentle contra-input, combined with a "No, that line there....OK now? Good, All yours", would not have scrambled my brain at a very demanding and difficult airport at night just before take-off.

Airbus controls are averaged, (I think), so if one stick or tiller is fully left and the other is fully right, the result will be zero demand. And that way you can either cancel an incorrect demand or apply an absent demand.
There is no warning given for dual tiller inputs, so Airbus obviously do not consider that to be a problem.
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[Vessbot]

Originally Posted by Uplinker;11759325.

Airbus controls are averaged, (I think)

They are summed. For full left+full right, it leads to the same answer as averaging. But let's consider half left+half left. Summing gives full left, while averaging gives half left. Or, let's say one is half left and the other is centered (actual normal flight). Think about what averaging would do here. Every single input ever, would be cut in half.

[Speed_Trim_Fail]

The latest safety first seems rather apt given this thread.

https://safetyfirst.airbus.com

Only one flight crew flies at a time

Case study 2 shows that the PM may intend to take control in such dynamic situations. As per the FCTM chapter about the use of sidestick: only one flight crew flies at a time. If the PM intends to apply inputs using the sidestick, they must do the following actions:

• Clearly announce “I have control"
• Press and maintain the sidestick priority pushbutton in order to get full control of the Fly-By-Wire system.

The flight crew should keep in mind that sidestick inputs are algebraically added and the “DUAL INPUT” alert triggers if the priority pushbutton is not pressed and maintained.

In case study 2, the dual input of the PM on the Captain’s sidestick did not prevent the tailstrike. In other reported cases, the dual input from the PM in the same direction as the PF increased the inputs up to an equivalent of full nose-up, and the resulting increased rate of rotation contributed to the tailstrike.

The latter part seems to be particularly relevant, and explains Airbus’s logic on this matter.

[Uplinker]

OK then.