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AF356 tailstrike in yyz

Old 25th Jan 2024, 11:43
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Two points,
1. Why are some of you mentioning the previous aircraft not clear of the runway the cause of this go around, when the pilot repeatedly told ATC it was due to a long landing. A long landing would be a piloting issue, an aircraft not clearing the runway would be a non pilot cause. So why would the pilot blame himself? Am I missing something?

2. In the military we did 10-15 Touch and go landings in a heavy 4 engine jet on every training mission. It was not a rushed maneuver and the procedure was methodical and paced. After touchdown, the flying pilot lowered the nosewheel to the ground, the Instructory pilot would set “takeoff flaps” and takeoff trim. Once flaps and trim were set, he would state, “Flaps and trim set, set takeoff thrust”. Once thrust was set, the IP would call “rotate”. This took at least 6-10 seconds. Not comfortable, as the end of the runway was rushing towards you. It was not a rushed maneuver and if a pilot was is familiar performing this, and rushed the procedure, it could lead to dire consequences .
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Old 25th Jan 2024, 13:10
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Originally Posted by Chiefttp
Two points,
1. Why are some of you mentioning the previous aircraft not clear of the runway the cause of this go around, when the pilot repeatedly told ATC it was due to a long landing. A long landing would be a piloting issue, an aircraft not clearing the runway would be a non pilot cause. So why would the pilot blame himself? Am I missing something?

2. In the military we did 10-15 Touch and go landings in a heavy 4 engine jet on every training mission. It was not a rushed maneuver and the procedure was methodical and paced. After touchdown, the flying pilot lowered the nosewheel to the ground, the Instructory pilot would set “takeoff flaps” and takeoff trim. Once flaps and trim were set, he would state, “Flaps and trim set, set takeoff thrust”. Once thrust was set, the IP would call “rotate”. This took at least 6-10 seconds. Not comfortable, as the end of the runway was rushing towards you. It was not a rushed maneuver and if a pilot was is familiar performing this, and rushed the procedure, it could lead to dire consequences .
In answer to your first question, that came about from a post (post #18) from one of the passengers on board who said that the PA announcement from the cockpit referred to the runway being occupied. Since that post it has become evident that was not the case. Cockpit probably thought it was the simplest excuse to give the pax.
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Old 25th Jan 2024, 15:00
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Originally Posted by Uplinker
So sometimes there might appear to be a counterintuitive response of the flight control surfaces to a side-stick input, but one that actually makes sense when ALL factors are taken into account. For example the underslung engines spooling up to TOGA will cause a strong pitch up, which Airbus FBW will compensate for with elevator movement, invisible to the pilots.
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I disagree. I looked up the 350 FCOM and it does very clearly state that the flare law is a direct law.
If you say that a flight control surface behavior makes sense "when taking all factors into account", okay, but that only applies if you consider that the law is not direct anymore.
Originally Posted by fdr

The DFCS-FBW system operates at its own proprietary sampling frequency, which is certainly faster than the DFDR/DFDAU/QAR sampling. Looking at any system with a time domain analysis where sampling rates are different, or where there is a sequential data sampling within the sentence can introduce artefacts.

For the case that you bring up of the A350 @ Heathrow, (see below) The apparent "anomaly" that shows up is able to be explained as a sampling artefact. The only random momentary FCS anomaly that I am aware of was Kev Sullivans wild ride near Learmonth AUS, in a Qantas A330. In that case, the FCC had a suspected bit corruption from a possible cosmic ray badness. That resulted in a wild ride. If there has been any other momentary anomaly, I am not aware of it offhand, but they probably have occurred.

In the view below, the second red, double ended arrow is the time range where the elevator has an artefact of its position. This is within the error margins of the timing for the ground contact, and has a potential that a very short order SSC pitch command was sent, and not sampled, but responded to in due course by the FCC and the elevator hydraulic actuator, leading to what looks like an anomaly.
I'm sorry to disagree again.
You're talking about sampling error, you also talk about a 0.25 or 0.5s sampling interval for pitch parameters.
Yes, but, if there is a two second discrepancy between two parameters that are supposed to be linked, it can't be attributed to sampling error.
Two seconds being much larger 0.5s.

Then, if you look closely on the curves, I count sixteen datapoints per each 2 seconds on the pitch SSC. As well as on the elevator deflection curve. Which means a 0.125s sampling interval. I don't have the A350 calibration files to decode the DFDR data, but I'm pretty sure that you will find 8 words per second for both these parameters.
The A350 being a modern airplane, there are many more parameters than on the older 320s and it's really not surprising to have such precision.

Basing all your reasoning on something, that, by chance, was not recorded, does not sound very serious. It can happen but it's very unlikely.
And even if what you said happened, that is, a jolt from the pilot flying at the tailstrike, it does not change the problem at all. It reduces a bit its seriousness, but the deed was already done at this time.
It does not explain why the elevators had been staying so much nose up even while the PF was, almost steadily, releasing his nose up input.

I don't know if you've flown airbuses, but I've never seen anyone making alternating inputs quicker than 4Hz...
Due to the internal damping of the sidestick, it would be very difficult to make significant oscillations at a significant rate. I'm pretty sure the 0.125s interval would capture any involuntary jolt, if it was significant, that is, if it was large enough to produce an elevator deflection visible on the curves. I'm completely sure if I try to pull a small jolt on the airbus sidestick, I can't make it quicker than 0.25s (even very small deflections like 10% of travel) and even less so quicker than 0.125.

If the report was as thorough as an accident report, it could have a paragraph somewhere stating that they did tests like so and concluded. I'm 99% sure it's impossible to make a jolt without being detected (shorter than 0.125s).

Lastly, I don't believe the artifact story. How come the sensor doesn't have any problem later on ? If there is a data point at some value, it means it was measured at this value. The shock did not impact directly the sensor nor any of its components, the acceleration alone was not very large (yes it's measured at the C of G for the one we have, but seeing the very steady increase in pitch, if we computed, we wouldn't find a very large acceleration at the tail, as computed by Nz + q'*length)

So, to sum up, I don't really buy your arguments, and even if they were to explain some things, they wouldn't explain the main problem, it being the 2s desynchronisation between elevators and SSC.
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Old 25th Jan 2024, 15:13
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Originally Posted by Uplinker

I suspect that what might have happened here is that PF was committed to landing - hence reverse was momentarily selected, but then PM saw that the exiting aircraft had stopped or slowed with its tail fin still obstructing; so ordered a baulked landing or took control.

We weren't there in the cockpit but it appears on the face of it that they took this landing much too close and should have gone around earlier.
This had nothing whatsoever to do with another aircraft on the runway. The crew was asked twice the reason for the GA and responded that it was a "long landing" issue.

I wonder what the discussion was in the cockpit during this event . . .
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Old 25th Jan 2024, 18:55
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I thought it was already fairly obvious that the 'blocked runway' was just the usual excuse rolled out for the PA to the pax. I've very rarely heard one of my colleagues admit that they screwed up the landing. However, I was once in the back as pax when the Captain admitted taxiing too far forward for the jetway to dock, so we'd have to get off via stairs at the back
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Old 25th Jan 2024, 20:02
  #66 (permalink)  
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Originally Posted by Uplinker
I haven't studied the data streams but I would just caution that the FBW has accelerometer and attitude feedbacks to inform the FBW how the aircraft is responding to inputs in terms of attitude.

So sometimes there might appear to be a counterintuitive response of the flight control surfaces to a side-stick input, but one that actually makes sense when ALL factors are taken into account. For example the underslung engines spooling up to TOGA will cause a strong pitch up, which Airbus FBW will compensate for with elevator movement, invisible to the pilots.

I suspect that what might have happened here is that PF was committed to landing - hence reverse was momentarily selected, but then PM saw that the exiting aircraft had stopped or slowed with its tail fin still obstructing; so ordered a baulked landing or took control.

We weren't there in the cockpit but it appears on the face of it that they took this landing much too close and should have gone around earlier.
The pitch channel is in a direct law at this time. The output is proportional to the signal from the SSC at that point.
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Old 25th Jan 2024, 20:14
  #67 (permalink)  
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Originally Posted by CVividasku
I disagree. I looked up the 350 FCOM and it does very clearly state that the flare law is a direct law.
If you say that a flight control surface behavior makes sense "when taking all factors into account", okay, but that only applies if you consider that the law is not direct anymore.

I'm sorry to disagree again.
You're talking about sampling error, you also talk about a 0.25 or 0.5s sampling interval for pitch parameters.
Yes, but, if there is a two second discrepancy between two parameters that are supposed to be linked, it can't be attributed to sampling error.
Two seconds being much larger 0.5s.

Then, if you look closely on the curves, I count sixteen datapoints per each 2 seconds on the pitch SSC. As well as on the elevator deflection curve. Which means a 0.125s sampling interval. I don't have the A350 calibration files to decode the DFDR data, but I'm pretty sure that you will find 8 words per second for both these parameters.
The A350 being a modern airplane, there are many more parameters than on the older 320s and it's really not surprising to have such precision.

Basing all your reasoning on something, that, by chance, was not recorded, does not sound very serious. It can happen but it's very unlikely.
And even if what you said happened, that is, a jolt from the pilot flying at the tailstrike, it does not change the problem at all. It reduces a bit its seriousness, but the deed was already done at this time.
It does not explain why the elevators had been staying so much nose up even while the PF was, almost steadily, releasing his nose up input.

I don't know if you've flown airbuses, but I've never seen anyone making alternating inputs quicker than 4Hz...
Due to the internal damping of the sidestick, it would be very difficult to make significant oscillations at a significant rate. I'm pretty sure the 0.125s interval would capture any involuntary jolt, if it was significant, that is, if it was large enough to produce an elevator deflection visible on the curves. I'm completely sure if I try to pull a small jolt on the airbus sidestick, I can't make it quicker than 0.25s (even very small deflections like 10% of travel) and even less so quicker than 0.125.

If the report was as thorough as an accident report, it could have a paragraph somewhere stating that they did tests like so and concluded. I'm 99% sure it's impossible to make a jolt without being detected (shorter than 0.125s).

Lastly, I don't believe the artifact story. How come the sensor doesn't have any problem later on ? If there is a data point at some value, it means it was measured at this value. The shock did not impact directly the sensor nor any of its components, the acceleration alone was not very large (yes it's measured at the C of G for the one we have, but seeing the very steady increase in pitch, if we computed, we wouldn't find a very large acceleration at the tail, as computed by Nz + q'*length)

So, to sum up, I don't really buy your arguments, and even if they were to explain some things, they wouldn't explain the main problem, it being the 2s desynchronisation between elevators and SSC.
There is no 2s desynchronisation between elevators and SSC.
  • The SSC is not rate limited
  • The control surface is rate limited
The difference between control input and response always has an error unless the system is static.
The control surface actuator can be and is saturated in this condition, which is a normal system condition when a high amplitude input is made over a short time frame. The only time that doesn't occur is where the linkage between the two is rigid, as in aerobatic aircraft with push pull control rods with high rigidity, and zero pivot slack.
The FCC output in these cases is a proportional response, it is whatever the pilot is inputting by the SSC. When in normal law, the FCC will make control outputs as required to meet the demand, and those are determined by a programmed response table for the output, these tables are dependent on configuration.

The time delay from the saturation of the actuator is approximately 1.0s. For the ~36s immediately before the tail strike, the lag in the commanded position of the control and actual is minimal, and what is normally observed where there has not been a high amplitude, short time period input. Saturated control actuators are observed on various occasions, but are not the general manner by which the aircraft is flown, In this case, the controls have gone from near full nose up to reversed to full nose down in about 2s, and then in the next 2s have been reversed towards ~1/2 nose up again, then over the following ~2s to half nose down, before being nulled. With the gyrations of the high amplitude inputs which saturate the controls, position lag occurs. That is completely normal system function, the control inputs are less normal, they are unfortunately an over control of the required inputs by the pilot.

The sampling rates are required to be no less than 0.25s for both of these chanels, the rate that is recorded in the DFDR is 0.125s.

I don't know if you've flown airbuses, but I've never seen anyone making alternating inputs quicker than 4Hz...
Yes, I have, A320, 330, 340, and all of it as flight test. Of more relevance is I have conducted incident review on A320, A330 and various other Airbus products, as well as B737, B747, B777 types.


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Last edited by fdr; 25th Jan 2024 at 23:35.
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Old 25th Jan 2024, 21:39
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We don't seem to reach a common ground on this matter.
There cannot be a (significant) 0.125s artifact on an airbus sidestick, with the huge force required to move it and the damping that is has to come back to its position.
If there was really an artifact in the data, it could easily be shown when re-computing the flight mechanics equations.
One artifact doesn't change the whole picture.

The elevator has a maximum deflection rate, yes, this is very clearly visible on the triangles that happen after the tailstrike.
However, nothing was preventing them from following the released order of the PF, nothing was making them mandatorily be 2 seconds late.

The flight control law is supposed to be direct, and there is no direct relationship between input and output. It is not acceptable, as is. We are lacking an explanation.
I am this close to sending an email to the AAIB asking about this matter.
The least they could have done is ask airbus to compute and plot the "elevator command" parameter. And continue to analyse as to why it didn't release in a timely manner.
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Old 25th Jan 2024, 23:41
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Originally Posted by CVividasku
We don't seem to reach a common ground on this matter.
There cannot be a (significant) 0.125s artifact on an airbus sidestick, with the huge force required to move it and the damping that is has to come back to its position.
If there was really an artifact in the data, it could easily be shown when re-computing the flight mechanics equations.
One artifact doesn't change the whole picture.

The elevator has a maximum deflection rate, yes, this is very clearly visible on the triangles that happen after the tailstrike.
However, nothing was preventing them from following the released order of the PF, nothing was making them mandatorily be 2 seconds late.

The flight control law is supposed to be direct, and there is no direct relationship between input and output. It is not acceptable, as is. We are lacking an explanation.
I am this close to sending an email to the AAIB asking about this matter.
The least they could have done is ask airbus to compute and plot the "elevator command" parameter. And continue to analyse as to why it didn't release in a timely manner.
There cannot be a (significant) 0.125s artifact on an airbus sidestick, with the huge force required to move it and the damping that is has to come back to its position
there is no "huge force" required to move the SSC, it has a mild breakout force and very little else, with an increasing resistance to large deflections, but the loads to move the SSC are minor.

The control surface loads alter as a result of aerodynamic loading, but the rate is also constrained by the flow restrictions of the hydraulic systems, they can only move at a certain rate.

​​​​​​​If there was really an artifact in the data, it could easily be shown when re-computing the flight mechanics equations.
Quite so, and there is nothing in the behaviour of the aircraft at LHR that suggests that the aircraft was not responding correctly to the input by the pilot.

​​​​​​​However, nothing was preventing them from following the released order of the PF, nothing was making them mandatorily be 2 seconds late.
What do you mean by released order of the PF? The FDR gives the sampled data, it does not give an exact representation of short timebase changes. The input by the pilot is continuous, the response of the controls are as well, within the sampling rate of the FCC, and the command signal rate to the actuator.


The controls move at the rate that they can be actuated. If you move your controller from full up to full down in 1s, that does not mean the control follows in the same time. They cannot. They have a saturated rate of actuation, and that is a design reality on every jet transport that has hydraulic actuated controls. This is observed on Airbus and Boeings. For good reasons, Apart from the fact that it is a physical constraint on the actuator response in various load conditions, it avoids generating excessive loads on the structure that the control is related to. AA587 rates unfortunately should have been lower to protect the structure from the bending-torsion-bending cycle that was achieved. That did not work out well as the design by the OEM was not rate limiting, it only limited the throw angle, and that paradoxically increases the sensitivity of the system when least desired. Boeing acts by reducing the actuator pressure, and that results in lower response rates, which also reduces the throw of the control as aerodynamic loads will match the actuator force at lower deflections.

I am this close to sending an email to the AAIB asking about this matter.
It's a free world. There is nothing in the report there that is of great note, but you are free to act as you see fit. You will likely get a "we thank you for your input" or, "thank you for your kind advice" polite response. If you feel strongly that Airbus flight controls have a defect that you have uncovered, then go right ahead. I'm limited in my experience as the former lead of Operations, FDR and CVR groups for an accident investigation team, a team member of more accident and incident investigations than I care to recall, so defer to your system knowledge, having not flown the A350, I have only done flight tests on A320/330/340, MD11, B737, and B777 for the purposes of investigation, and R&D and certification along with military types.

Note that the data that is presented in the graph is the end result of a number of conversions. The flight controls are on a MIL-STD-1553B bus, the data recorded on the FDR is ARINC 717, and in between the data may be sampled after being converted from 1553 to ARINC 427, and then to 717. Each conversion takes a finite time to complete.

UK AAIB (state of incident, state of registration)

[email protected]Maladministration Complaints
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For the OEM, you appear to have a bee in your bonnet related to safety of design, which is a compliance with CS-LARGE AIRCRAFT (FAR PART 25) as far as you consider that the LHR A350 did something that was not normal response. That is a compliance matter then for the OEM:

​​​​​​​I would like to report a concern regarding ethics and compliance

Airbus takes concerns of ethics and compliance issues seriously. To that end, Airbus maintains a dedicated system to receive alerts called the Airbus OpenLine. If you would like to report a concern, please visit www.airbusopenline.com and click "Make a Report”. The Airbus Openline is available to employees and third parties (including but not limited to contractors, subcontractors, suppliers, and local communities around our sites and our suppliers' sites). Reports may be submitted anonymously.



Last edited by fdr; 26th Jan 2024 at 00:59.
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Old 26th Jan 2024, 11:28
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Originally Posted by tdracer
Sorry, nerd mode coming out...
The JT9Ds did not have an electrical actuator in the pylon, it was a mechanical block. The mechanical block was connected to the reversers by feedback cables - it prevented 'advancing' the thrust levers to 'higher' reverse thrust until the reverser was ~85% deployed, then prevented moving the thrust levers into the forward quadrant until the reverser was ~85% stowed (e.g. 15% deployed) (being a mechanical device, there was some variability in the exact T/R positions). The same mechanical block would move the thrust lever to idle if the reverser moved out of its commanded position for some reason. There was a similar system on all pre-FADEC Boeing installations.
With the advent of FADEC, that mechanical system would rather obviously no longer work, so an electrical actuator or solenoid was in the flight deck thrust lever quadrant that served the same function of preventing high reverse thrust until the reverser was deployed, and forward thrust selection until the reverser was stowed - feedback was from electrical sensors on the T/R actuators - which also allowed the FADEC to limit thrust if the T/R was not in the commanded position.
The aircraft was a SAS wet leased B747 classic of a good age, and now operating in hot to hot climates the problem arose: on subsequent start up and taxi the throttle could not be advanced as the actuator had failed in the T/R deployed mode. As Line Manager I instructed the crew: after a flight shut down, to move the throttles forward to check that the actuator was not failed, rather than later on next flight with a full pax load, finding out sooner allowed fixing during quite a long turnaround by the flying spanner crew.
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Old 26th Jan 2024, 12:21
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Originally Posted by fdr
The pitch channel is in a direct law at this time. The output is proportional to the signal from the SSC at that point.
Originally Posted by CVividasku
..........The flight control law is supposed to be direct, and there is no direct relationship between input and output. It is not acceptable, as is. We are lacking an explanation.........
Direct law for the pilot side-stick elevator inputs, yes, but there are also - unseen by the pilots - attitude and rate feedbacks to the FBW; could these modify the elevator position during that phase ? And I can't remember if the FBW will still compensate in pitch for engine thrust changes during a baulked landing ?

But either way; current line pilots should know all this, and be in current practice, so as not to over-pitch. What happened here ?
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Old 26th Jan 2024, 18:08
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Originally Posted by fdr
It's a free world. There is nothing in the report there that is of great note, but you are free to act as you see fit. You will likely get a "we thank you for your input" or, "thank you for your kind advice" polite response. If you feel strongly that Airbus flight controls have a defect that you have uncovered, then go right ahead. I'm limited in my experience as the former lead of Operations, FDR and CVR groups for an accident investigation team, a team member of more accident and incident investigations than I care to recall, so defer to your system knowledge, having not flown the A350, I have only done flight tests on A320/330/340, MD11, B737, and B777 for the purposes of investigation, and R&D and certification along with military types.
You use a huge argument of authority but then say things like

Originally Posted by fdr
there is no "huge force" required to move the SSC, it has a mild breakout force and very little else, with an increasing resistance to large deflections, but the loads to move the SSC are minor.
The force required to move the sidestick full up can be up to 20 kg (kgf of course), depending on how you grab it.
I call that a huge force. 20 kg is not "minor". Then, there is a damping. And I do confirm, I even tried this morning on a real airbus sidestick, I even filmed it. I would need a computer to properly analyse the video and see how long was the shortest significant input I could make. Maybe I will post here just to prove this point.
Originally Posted by fdr
The control surface loads alter as a result of aerodynamic loading, but the rate is also constrained by the flow restrictions of the hydraulic systems, they can only move at a certain rate.
There is no such thing at the time period we're talking about. The average position of the elevators remain almost full up, so there is no problem with elevator speed.
Just take the blue triangle, its linearly ascending part, move it to the left, you will see without doubt that the elevator remains far below where it could be. There is no problem with elevator speed (as there is later, after the tailstrike)
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Originally Posted by fdr
Quite so, and there is nothing in the behaviour of the aircraft at LHR that suggests that the aircraft was not responding correctly to the input by the pilot.

What do you mean by released order of the PF? The FDR gives the sampled data, it does not give an exact representation of short timebase changes. The input by the pilot is continuous, the response of the controls are as well, within the sampling rate of the FCC, and the command signal rate to the actuator.
Again, if you look at the curves, you will clearly see. It looks like we're not looking at the same curves.
https://i.gyazo.com/7805953bd1bc0dfc...89e5c86214.png
Just before and during the red event, it is clearly visible (sorry to say?) that the order of the PF is going from almost full nose up, to almost full nose down, in around three seconds.

What you're saying about the FDR sampling data is worrying. If all the analysis that we can make about the data, must be thrown out the window, just because there are "sampling problems", then why are we using FDR's in the first place ?

And, for the nth time, even if there are sampling data, they cannot explain a 2 second discrepancy, when the sampling is much quicker !
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Originally Posted by fdr
The controls move at the rate that they can be actuated. If you move your controller from full up to full down in 1s, that does not mean the control follows in the same time. They cannot. They have a saturated rate of actuation, and that is a design reality on every jet transport that has hydraulic actuated controls. (...)
Yes, and this rate of saturation was absolutely not reached before the red event. The controls remained at almost full up for around two seconds after the pilot started to release his input.
And there is nothing that can explain that, neither the sampling rate, nor the actuation rate.

Also, as an investigator or former investigator, maybe you can't tell everything you know.
Did the A350 pitch control chain never pose any problem whatsoever... ? I have some knowledge that it did

Lastly, I have no such ego as to think I discovered a problem before airbus. Airbus has the flight data of many incidents, including this BA one, long before the report is published. They have several teams analysing that kind of events, at the same time. They have a dedicated safety team (maybe even several..). So in the time that went between the incident and when I found it, it's almost certain they would have had the time to completely analyse, and even re-work on the FBW laws, and upload them in a new flight control computer software standard... It happens, last time I checked, the related FCC was at it's 8th standard.
Having 8 standards also means that they discovered problems or "underoptimizations" worth reworking the laws at least 7 times... (maybe more if they grouped the updates)
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Old 26th Jan 2024, 18:39
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Air France and "long landing" on 24L is not a good combination (Google AF358 August 2005) so I guess they preferred to be safe and GA.

Last edited by WingSlinger; 26th Jan 2024 at 18:58.
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Old 21st Apr 2024, 14:48
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F-HTYH still parked ?

It is 3 month since the tailstrike . Airfleets website shows F-HTYH as parked . How long time does it usually take to buff out scratches on the tail ?
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Old 10th Jun 2024, 20:04
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F-HTYH finally left YYZ on 10th June as AFR373V and flew south over the UK between 1900 and 2000Z at FL410 en route to CDG

Noticeable patch up at the back, observed through the bins. Bit more than buffing up I think...

https://avherald.com/h?article=513fc722


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Old 10th Jun 2024, 20:53
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Hmmm... "...tous les mains, tirez, tirez...!!"

Last edited by BugBear; 10th Jun 2024 at 21:03.
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Old 11th Jun 2024, 16:03
  #77 (permalink)  
 
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Originally Posted by physicus
flyrealtraffic.com not showing any ground targets that might have been in the way. ADS-B ground coverage at YYZ is gapless, so if whatever was on the runway had an ADS-B transponder it would show up here.

Perhaps a goose walking across.

For most planes, nothing shows below 100 kts on the ground. We've seen a few incidents where controller missed an slow speed RTO and cleared a landing aircraft to land because on the screen the runway was clear.
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Old 14th Jun 2024, 16:03
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Originally Posted by Uplinker
Direct law for the pilot side-stick elevator inputs, yes, but there are also - unseen by the pilots - attitude and rate feedbacks to the FBW; could these modify the elevator position during that phase ?
Not in direct law - that's what direct law means: the only thing that moves the elevators is the stick, same as a Cessna 172.
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Old 15th Jun 2024, 06:41
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Originally Posted by Vessbot
Not in direct law - that's what direct law means: the only thing that moves the elevators is the stick, same as a Cessna 172.
Sort of - there are still pitch rate modifiers to the elevator demand.

Tail strikes seem particularly common on Airbus types and I donít understand why there isnít a hard limit on pitch attitude close to the ground. You could still have a direct stick to elevator relationship with a pitch limit.
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Old 15th Jun 2024, 13:34
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Many of the responses are overly complicating this. The first thing to understand is the Airbus fly by wire system is a very basic FBW with a lot of limitations. Solutions to the limitations involve many modes and sub modes some of which are not even taught to pilots.

The landing phase below 100 feet has a lot going on in an Airbus. The key from a pilot standpoint is make decisions early. Not once was I taught or have I practised a go around after touchdown. Simulator training was either a go around or rejected landing. A rejected landing was always trained at 50 feet. You might or might not touch down depending on weight etc.. from 50 feet. You however had the advantage of spooled engines at 50 feet. You lose that advantage once below about 30 feet in two steps. At the retard point the engines go to flight idle. At weight on wheels the engines go to ground idle. Spool up is slow from ground idle.

If you touchdown in the bus you are in a direct law sub mode. Again it’s a bit more than straight direct law but from a pilot standpoint that’s what it is. You are at now at idle or worse transitioning to reverse thrust. As thrust is transitioned to go around power there is a significant amount of time passing and runway moving behind you. Pilots will often start rotating early in this situation. As you're rotating the engines are finally spooling. The last 50% of power comes on very rapidly causing a pitch up moment that in most Airbus FBW modes is compensated for automatically. It’s not compensated for in direct law. You initiate the rotation which will often be slow so you might command almost full back stick just as you also get a significant nose pitch up from the engines.

I have never trained for this nor have most pilots. That is why the bus has tail strikes.
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