Accidents and Close Calls Discussion on accidents, close calls, and other unplanned aviation events, so we can learn from them, and be better pilots ourselves.

AF356 tailstrike in yyz

Old 23rd Jan 2024, 20:23
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Originally Posted by tdracer
Does Airbus actually say "Never"?
Boeing has a similar statement, but it's somewhat weasel worded - IIRC it's along the line of "Go around after selecting reverse is not recommended" - it doesn't say "never".
If you touchdown, select reverse, then discover another aircraft/vehicle blocking the runway, you have pretty limited options...

Some moons ago a TAP Bus at Copehangen did a Go Around and all the circuit pattern with nr1 full shells opened in Reverse config...successfully

https://samchui.com/2022/04/21/serio...ing-go-around/

https://avherald.com/h?article=4f73f634

Airbus then stated as conclusion:

The SOP for landing also states that as soon as the flight crew selects reverse thrust, they must perform a full-stop landing. This is also highlighted for a go-around near the ground in the FCTM, which states, “the PF must not initiate a go-around after the selection of the thrust reversers.” Adherence to this SOP will avoid any repeat of the event described in this article.



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Old 23rd Jan 2024, 21:25
  #42 (permalink)  
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Originally Posted by CVividasku
I explained everything but you don't believe me.
They're telling in on the radio that the cause was a long landing. They touched down slightly after the 600m mark. Some airlines won't allow pilots to touch "in the TDZ" which can sometimes extend for more than one kilometer.

Why they pulled so hard is probably explained here :
AF356 tailstrike in yyz

Although the AAIB report doesn't explicitly identify the elevator delay and PIO, as a contributing factor, they are clearly visible on the curves. If we get curves for this incident, I expect to see a similar thing. I don't think they've updated the flight control laws between the two incidents.
?

You have the video, you know which airport it is, there is the Demi god, google. The video show fairly well when they touched down, and when they added thrust, and when they rotated, when they hit their tail and when they finally stopped de-dagging the butt of the bus.

"The radio"... ATC? I've added some points of interest to a screen capture of the location. They were well within the normal TDZ, but that is their choice, up until they have selected reverse thrust at least. There is no magic about doing a GA, the aircraft follows the SSC input intros case, AS IT DID AT LHR, and neither of those events has indication of a PIO, or APIO or anything similar, BEFORE the tail strike. Have another look at the SSC trace for the LHR event that you have linked to, the tail strike occurs before the pilot reverses the SSC pitch command, and that is a high amplitude input as a consequence of the attitude achieving ANU sufficient to achieve the tail strike. During these events, by memory and assumption of Airbus architecture, the pitch is in a direct law, the pilot is not commanding a g load, he/she/it is commanding a deflection of the elevator.

The thrust line is providing a considerable ANU moment, and what is interesting is the sequence of the pilot input; When increasing the pitch up moment from the engines, adding full aft SSC at the same time is going to always result in a wild ride until the gain of the input and the direction of the input is resolved by the pitch moment attained by the engines. For a normal takeoff, the thrust moment is nominally stable (nominal, as the ATR iis commanding an EPR or RPM, which is gross thrust, but net thrust reduces as TAS increases, (the -mdot.V0 component)). In the GA case pulling back while thrust is increasing will hive a total ANU pitch rate that is undesired. stabilising the engine at GA and then putting in the SSC command would avoid the excessive pitch rate that occurs, and there is nothing novel in that, it is what we have done for 75 years with underwing pylon mounted engines.

Having an excessive resultant pitch rate is not a "PIO", it is the wrong applied control deflection given the total moments that are being generated. LHR was not caused by a PIO, nor is YYZ.

There is no time criticality in a GA from the TDZ unless it is related to an obstruction such as could have been the case in HND 34R or LAX 24L. In a GA for a long landing, you have far more runway available than a takeoff case from the same runway, I would think that getting the thrust applied first while limiting ANU pitch up inputs is rational to the point that is what we used to teach; hold the attitude if before TD, and when stable, pitch to a normal TO attitude. No magic read, just not adding limit control inputs needlessly.





G-XWBC [AAIB ] [1]

The co-pilot initiated the go-around, selected Take Off Go-Around (TOGA) on the thrust levers and applied a pitch up demand on his control column, briefly reaching full aft control movement. Engine response from idle to go-around thrust takes several seconds and with the low energy state the aircraft briefly touched down. As it did so the pitch attitude was increasing in response to the co-pilot’s control inputs and reached a maximum of 15° nose up.

This bit is not a PIO, it is a poor technique to apply which arises from either lack of understanding or knowledge of the dynamics of the aircraft. Just because the aircraft has smart systems don't mean that it is smart at all times. Having a big pull on the prong and adding thrust moments thereafter is going to be sporty, when the driver is controlling the elevator deflection proportionally to the SSC, as this and all Bus' do on/near ground. At the time of the tail strike, the driver d'busses has grabbed a handful of back stick, and plonked the thrust levers (switches? toggles? variable resistors) to the loud setting, and the blenders take some time to crank up, and when they do they give an additive ANU pitch moment, and stuff happens. Holding an attitude, and applying thrust, getting thrust set and then pitching thereafter by input to achieve some semblance of a rational target works pretty well, but is regretfully far less exciting. We do not teach GA in the flare, or touchdown sufficiently, these drivers are not the problem, the problem is the assumption that without proficiency in this fairly simple control requirement, when it occurs, the crew react and potentially overreact in a state of anxiety.

From a great bit of reading [2]:

4 . 8 . 3 Pilot-Induced Oscillations
The pilot-induced oscillation (PIO) can be defined as sustained oscillations or instabilities resulting from the pilot being in the control loop. These oscillations would not occur if the pilot had not closed the loop, since with few exceptions the airplane alone is dynamically stable. It follows that control system dynamics as well as airframe and pilot dynamics enter into this phenomenon. In other words, it is the total system that must be considered when evaluating PIO.


[1] AAIB 27939
[2] USNTPS FTM103 FIXED WING STABILITY AND CONTROL
[3] Flight Investigation of Longitudinal Short Period Frequency Requirements and PIO Tendencies, by Dante A. Difranco, Cornell Aeronautical Laboratory, Inc., Buffalo, New York, AFFDL-TR-66-163, June 1967.
[4] Frequency Response Method of Determining Aircraft Longitudinal Short- Period Stability and Control System Characteristics in Flight, by Henry A. Klung, Jr., Captain, USAF, Aerospace Engineer, AFFTC-TR-66-24, August 1966.

Last edited by fdr; 24th Jan 2024 at 00:36.
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Old 23rd Jan 2024, 22:03
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For those interested

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Old 23rd Jan 2024, 23:21
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Originally Posted by India Four Two
I thought it became "never" after the Cranbrook 737 crash, where the crew tried to go around, after selecting reverse thrust, when they discovered a snowplow on the runway. The FO was a friend of mine.

https://en.wikipedia.org/wiki/Pacifi...nes_Flight_314
Even after Cranbrook, I don't believe it stated "Never". I don't recall the exact wording (and no longer have access) but I reviewed a lot of AFM revisions and I'm reasonably sure that it makes it clear they don't want you to initiate a go-around after T/R deployment, it does't specially say you can't.
I know that the Boeing T/R design does account for the possibility that a go-around may be initiated after T/R deployment and the design now insures that the T/R will complete its stow cycle even if the aircraft lifts off without it stowed (which is what doomed the 737 at Cranbrook - one T/R hadn't completed the stow cycle before the air/ground transitioned to 'air' - and that removed hydraulic pressure from the T/R. The aero forces on the clamshell -combined with the loss of hydraulic pressure - allowed the clamshells to be pushed back to full deploy and the pilots couldn't handle the combination of yaw, loss of thrust, and loss of lift due to the deployed reverser.
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Old 24th Jan 2024, 08:59
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From our B737FCTM. Copy and pasted.

WARNING:After​ reverse thrust is initiated, a full​ stop landing must be
made. If an engine stays in reverse, safe flight is not possible
Doesn’t say ‘never’. But pretty strongly worded.
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Old 24th Jan 2024, 09:13
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Originally Posted by tdracer
...the design now insures that the T/R will complete its stow cycle even if the aircraft lifts off without it stowed ...
Thanks for the explanation, tdr.
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Old 24th Jan 2024, 09:40
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Originally Posted by Fursty Ferret
You can get a land-after clearance in the UK. ...
I haven't had a "land after clearance" in the UK for decades. And that was most likely in the GA world.
Originally Posted by Seat4A
Oh dear! The Canadians are 'as bad' as CDG, clearing an aeroplane to land before the one ahead has yet landed. (I do some part-time instructing at a 'busy' UK airport and I always emphasise to students to taxi clear of the runway and past the holding point markers expeditiously as ATC will not, and does not, clear the next aeroplane to land until you are fully clear of the runway.) But... it appears that this is not an issue with this incident.

Originally Posted by fdr
The touchdown was not long ... something spooked them into tossing away the landing ...
Air France do have a history of running off the end of a runway in YYZ. Maybe that is what 'spooked' them??
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Old 24th Jan 2024, 10:22
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Originally Posted by fdr
?
"The radio"... ATC? I've added some points of interest to a screen capture of the location. They were well within the normal TDZ, but that is their choice, up until they have selected reverse thrust at least. There is no magic about doing a GA, the aircraft follows the SSC input intros case, AS IT DID AT LHR, and neither of those events has indication of a PIO, or APIO or anything similar, BEFORE the tail strike. Have another look at the SSC trace for the LHR event that you have linked to, the tail strike occurs before the pilot reverses the SSC pitch command, and that is a high amplitude input as a consequence of the attitude achieving ANU sufficient to achieve the tail strike. During these events, by memory and assumption of Airbus architecture, the pitch is in a direct law, the pilot is not commanding a g load, he/she/it is commanding a deflection of the elevator.
No indeed. There is no PIO before the tail strike. But the triangle shape in the elevator deflections is a very clear indication of PIO.
The PIO appeared because the aircraft was slow to react, which prompted the pilot flying to maintain too big of an order, leading the aircraft to overreact, and himself to overcorrect. This puts too much demand in elevator deflection speed, which is the slope of the curve. When the curve is a steadily descending and ascending line, in a triangle shape, it means you reached the elevator maximum deflection speed, which is a very clear indication of PIO. It didn't last for long as only a few triangles are visible. But it's a very clear sign. You won't find anything similar on the curves for a normal go around.

Reaching maximum elevator speed means the elevators are chasing a target that they can't reach. It means that the aircraft cannot respond timely to the pilot's orders.

As for the long landing, you can find the recordings on the youtube video just below your post... I'm not going to write the exact same thing that I already wrote. Just wait for the final report if you don't believe me. Or look for another similar incident report if you're impatient.
Originally Posted by fdr
The thrust line is providing a considerable ANU moment, and what is interesting is the sequence of the pilot input; When increasing the pitch up moment from the engines, adding full aft SSC at the same time is going to always result in a wild ride until the gain of the input and the direction of the input is resolved by the pitch moment attained by the engines. For a normal takeoff, the thrust moment is nominally stable (nominal, as the ATR iis commanding an EPR or RPM, which is gross thrust, but net thrust reduces as TAS increases, (the -mdot.V0 component)). In the GA case pulling back while thrust is increasing will hive a total ANU pitch rate that is undesired. stabilising the engine at GA and then putting in the SSC command would avoid the excessive pitch rate that occurs, and there is nothing novel in that, it is what we have done for 75 years with underwing pylon mounted engines.
If you look at the curves, it's not what happens. The engines are still almost at idle when the tail strikes the runway.
So the elevators are responsible for the pitch up. If you use an image processing software to put the two curves, pilot input and elevator response, in relation to each other, you will see that there is a delayed response from the elevators, of almost one second. Then, while the elevators are not following the order they're given, the pilot waits for them while maintaining his input... If the elevators had responded, surely the pilot, who is not incompetent, would have given a smaller overall input. and he might not have stricken the tail.
During these events, by memory and assumption of Airbus architecture, the pitch is in a direct law, the pilot is not commanding a g load, he/she/it is commanding a deflection of the elevator.
Yes. It should be. But if you look at the curves, again by superposing both, you will see that if most of the time the elevators follow closely the input, in what is believably a direct law, they do not follow properly between 2 and 4 seconds before the tailstrike.
Having an excessive resultant pitch rate is not a "PIO", it is the wrong applied control deflection given the total moments that are being generated. LHR was not caused by a PIO, nor is YYZ.
LHR is not caused by PIO. (in all I'm writing I'm not talking about YYZ because I don't have any curves obviously)
However, the fact that PIO occurred shows that the aircraft wasn't responding as expected by the pilot. And if you look closely in this 2-4 seconds before event timeframe, you will see that the elevators indeed responded with a delay.
the crew react and potentially overreact in a state of anxiety.
Before saying things like this, you should at least go in a simulator, perform a go around. Then, when the curve analysis will have shown that you successfully performed a go around in spite of a one second delay in elevator response, you will be in a better position to criticise... If the elevator delay does not occur, you've proven nothing, and you're left with looking for why the elevators responded nicely in one case and late in another case.
​​​​​​​The pilot-induced oscillation (PIO) can be defined as sustained oscillations or instabilities resulting from the pilot being in the control loop. These oscillations would not occur if the pilot had not closed the loop, since with few exceptions the airplane alone is dynamically stable. It follows that control system dynamics as well as airframe and pilot dynamics enter into this phenomenon. In other words, it is the total system that must be considered when evaluating PIO.
PIO occurs easily when the aircraft response is approximately one second. Because it is also the human response time.
Have you ever flown a delta wing microlight ? Like this one :
https://images.virginexperiencedays....ompress,format
Especially in roll, these things are a bit difficult to control, at least at first. Inducing a roll requires a large force in your arms, so it takes time to move it. Then it's not ailerons that you're moving but the entire fuselage under the wing. So it's even longer to get a roll angle and a turn. Since there is a delay, it's easy especially if you over correct to get into PIO.
Indeed, the aircraft will not enter oscillations by itself, because it's dynamically stable. Dynamically unstable aircraft would be very difficult to fly and not certified.

What I'm saying since the beginning is contained here :
It follows that control system dynamics as well as airframe and pilot dynamics enter into this phenomenon
​​​​​​​Elevator maximum speed is part of control system dynamics

If you'd like to go into more details you can send me a PM
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Old 24th Jan 2024, 11:12
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Land after

Originally Posted by NoelEvans
I haven't had a "land after clearance" in the UK for decades. And that was most likely in the GA world.
Had one at Manchester on 05R last year - the FO had never heard of it and din't know what to do!

MATS Part 1:

19.4 When aircraft are using the same runway, a landing aircraft may be permitted to
touch down before a preceding landing aircraft which has landed is clear of the
runway provided that:
(1) the runway is long enough to allow safe separation between the two aircraft
and there is no evidence to indicate that braking may be adversely affected;
(2) it is during daylight hours;
(3) the preceding landing aircraft is not required to backtrack in order to vacate
the runway;
(4) the controller is satisfied that the landing aircraft will be able to see the
preceding aircraft which has landed, clearly and continuously, until it has
vacated the runway; and
(5) the pilot of the following aircraft is warned. Responsibility for ensuring
adequate separation rests with the pilot of the following aircraft.
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Old 24th Jan 2024, 11:17
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Originally Posted by meleagertoo
Ah! In YYZ was it! Now some know but probably 95% of the readership haven't a clue. And why should they?
How about using real names instead of esoteric codes to show how clever (not) and 'insider' you are.

Has it not occurred that people who don't operate in that area will have no idea where that is, and that many here are not pilots or exposed in any way to IATA codes either?
Not all of us have a global list of IATA codes in our heads, and suggestions to use google are just smug smartarsery.
This is (or was) a professional pilot's forum, so three letter codes of major airports would be known or easily looked up by professional pilots. Haven't we all got an old copy of the Aerad books, with all the 3 and 4 letter airport codes in the back, or is that just me.

Originally Posted by JW411
Which reminds me of a Canadian pilot we once hired:
"How the hell do you get Luton out of EGGW?".......!"
(engages nerd mode.....) E is north-western Europe, G is Great Britain, G is an area of Great Britain, and W is the airport. W might refer to Wigmore valley which is an area next to the airport in Luton, (although the airport is not in the valley). Sometimes the appropriate letter has already been assigned elsewhere, so a substitute letter has to be used.

Originally Posted by Intrance
I'd hope the decision to go around due to an occupied runway would come a bit before touchdown on a seemingly severe CAVOK day with great visibility to see this other traffic occupying the runway... No need to put it down first and then second guess if the guy in front has vacated yet or not........
My guess is that the AF was cleared to land as the previous flight was exiting, so the AF pilots continued, projecting the progress of the previous aircraft to be clear by the time the AF touched down. But maybe for some reason the exiting aircraft then either stopped or slowed right down, so the AF had to go around. Nobody wants to go around, especially after a long transatlantic flight, but they obviously got this a bit too tight !

And I was always taught that if reversers and/or brakes had been applied, then you were committed to land and stop.

What is concerning here is the excessive pitch-up though. Pilots are fully aware of the max pitch-up value they can apply during rotation.
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Old 24th Jan 2024, 12:23
  #51 (permalink)  
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Originally Posted by CVividasku
No indeed. There is no PIO before the tail strike. But the triangle shape in the elevator deflections is a very clear indication of PIO.
The PIO appeared because the aircraft was slow to react, which prompted the pilot flying to maintain too big of an order, leading the aircraft to overreact, and himself to overcorrect. This puts too much demand in elevator deflection speed, which is the slope of the curve. When the curve is a steadily descending and ascending line, in a triangle shape, it means you reached the elevator maximum deflection speed, which is a very clear indication of PIO. It didn't last for long as only a few triangles are visible. But it's a very clear sign. You won't find anything similar on the curves for a normal go around.

Reaching maximum elevator speed means the elevators are chasing a target that they can't reach. It means that the aircraft cannot respond timely to the pilot's orders.

As for the long landing, you can find the recordings on the youtube video just below your post... I'm not going to write the exact same thing that I already wrote. Just wait for the final report if you don't believe me. Or look for another similar incident report if you're impatient.

If you look at the curves, it's not what happens. The engines are still almost at idle when the tail strikes the runway.
So the elevators are responsible for the pitch up. If you use an image processing software to put the two curves, pilot input and elevator response, in relation to each other, you will see that there is a delayed response from the elevators, of almost one second. Then, while the elevators are not following the order they're given, the pilot waits for them while maintaining his input... If the elevators had responded, surely the pilot, who is not incompetent, would have given a smaller overall input. and he might not have stricken the tail.

Yes. It should be. But if you look at the curves, again by superposing both, you will see that if most of the time the elevators follow closely the input, in what is believably a direct law, they do not follow properly between 2 and 4 seconds before the tailstrike.

LHR is not caused by PIO. (in all I'm writing I'm not talking about YYZ because I don't have any curves obviously)
However, the fact that PIO occurred shows that the aircraft wasn't responding as expected by the pilot. And if you look closely in this 2-4 seconds before event timeframe, you will see that the elevators indeed responded with a delay.

Before saying things like this, you should at least go in a simulator, perform a go around. Then, when the curve analysis will have shown that you successfully performed a go around in spite of a one second delay in elevator response, you will be in a better position to criticise... If the elevator delay does not occur, you've proven nothing, and you're left with looking for why the elevators responded nicely in one case and late in another case.

PIO occurs easily when the aircraft response is approximately one second. Because it is also the human response time.
Have you ever flown a delta wing microlight ? Like this one :
https://images.virginexperiencedays....ompress,format
Especially in roll, these things are a bit difficult to control, at least at first. Inducing a roll requires a large force in your arms, so it takes time to move it. Then it's not ailerons that you're moving but the entire fuselage under the wing. So it's even longer to get a roll angle and a turn. Since there is a delay, it's easy especially if you over correct to get into PIO.
Indeed, the aircraft will not enter oscillations by itself, because it's dynamically stable. Dynamically unstable aircraft would be very difficult to fly and not certified.

What I'm saying since the beginning is contained here :
It follows that control system dynamics as well as airframe and pilot dynamics enter into this phenomenon
Elevator maximum speed is part of control system dynamics

If you'd like to go into more details you can send me a PM
You called this a PIO and gave the G- rego tailscrape at LHR as an exemplar of the same. Neither of them are caused by a PIO/APIO. Now, if you provide evidence that Gz and pitch rate gets out of sync with the SSC and elevator deflection then it might be of interest. As some background, I provided expert witness testimony on PIO on an A320, so it is a subject that is of some interest. In that case there was PIO, however there was also limit cycle divergence in an attitude axis with the autopilot controlling the aircraft. In that case, the regulator, manufacturer and the accident investigation board did not look great in the courtroom. If this involved a PIO, I would be particularly interested in that. Within my investigations of the usual Monday morning funnies, PIOs were indeed noted in some events, for the electrons they invariably involved the PF not using the arm rest properly, and encountering a sharp Gz change at the pilot station, which can be from a vertical acceleration, or by static stability doing its thang, and rotating the attitude. The bus is pretty darn good however, when the schedule control deflects are correct for the aerodynamic state. In the flare, and on the ground, as well as after takeoff, the SSC gives a simple proportional deflection for the elevator deflection, (Boring added a pitch protection that reduced the elevator deflection where high pitch rates and high pitch attitudes happen on the ground (? + t buffer?), for the B773ER). As much fun as it is to evaluate control derivatives and the stability of the aircraft, looking at the longitudinal differential equations, transfer functions and Laplace transformations, they are redundant when the prong is pulled back to the stops, the plane is on the ground and the aircraft gets a cut-n-polish of its nether regions.

The following though is kind of nice as it avoids the pesky thrust couple which gets to be added the mix.

Don't get me wrong, having full backstick and then having a rapid longitudinal acceleration as the blenders come on song, increasing elevator effectiveness as a function of V0^2, and getting the additional whammy of the pitch couple from said thrust, even as a Net outcome, FN=((P19.V19)+(P8.V8))-(P2.V0) and that thus having a vertical offset from the CG of the bus, it adds a complexity to the control needed by the person holding the prong. As complex as that becomes if the seat occupier has gone to a full backtick, it does not constitute a PIO, it is just a really poor piloting practice. The guys that do well in such a case are often helicopter pilots, as they are trained in dealing with weird changes in control authority, stability and the whole inertia-aerodynamics deal that Locke number speaks to. Even then, helicopters are easier to manage when the variables are reduced to human manageable levels.

Wild ride PIOs are impressive, google the first inadvertent flight of the YF-16, and the Saab JAS-39 Gripen. Minor PIO is often observed in first sessions of sim on the bus,
and were a feature of the F-16 first up, which softened with the introduction of some flex to the side stick. We see a form of PIO with the ATR-72 landings sometimes, that introduces additional forces into the body response, enough to be interesting. The solution in most go these cases is to remove the P from the PIO, just momentarily is enough normally. The gain and lag in the control loop are the other variables to be played with to stop getting ugiies.

Tin Hat time: One event that I investigated which was pretty funny was attempted to be blamed as a PIO due to the OEMs control system. The driver hit the ground hard in his trusty 2 holer, second hard landing in 2 sectors. He/she remembered the no fault position of the aircraft operator where a GA was conducted, and so decided to GA, yelling out, "GoAround"... and pulled back on the prong. Unfortunately, he had already selected reverse, and tapping (pulling in this case) the TOGA paddles didn't increase the noise, both engines were obediently in reverse. The speed was still high enough for the boys room to be elevated, and the tail fortunately stopped the plane from tipping backward too far. After some time, a long time, with the plain still refusing to get airborne, the occupants of the flight deck realised the plane was decelerating, and realised they were still in reverse. Luckily, they got both engines to cancel reverse, and then bent the thrust levers forward mandraulicly. The attitude remained nice and stable, more or les, only increasing slightly as more and more bits o' plain got wored offn'' it. Eventually, the plain rejected planet earth and got airborne, however, it did perform a little better as the OEW was reduced by the tail cone ,APU, paint and sundry items that traveled the length of the runway behind our intepid aviators. The bolder plane got cleared for a circuit, much to the surprise of all concerned in the investigation, and as a sister ship had observed the debris being dropped on the runway and had advised tower, the lads landed on the parallel without further incident. On approaching the gate, the FO found he couldn't get the APU to start, so on chocks external power was applied. Plane signed off as APU INOP. By the time the crew got ready to depart the pointy end, there was quite a congregation of engineers who had seen a lot of stuff still falling off the back of the plane at its leisure. There have been many impressive tailstrikes, but this one was in a league of its own, bad enough that the aircraft never returned to service. In the investigation, there was a political line towards blaming the OEM for the "PIO" that had occurred, and not only did I object to that, so did the test pilots from the OEM. Remedial training was given and our recommendations on GA were taken up.

LONGITUDINAL CONTROL stuff
Spoiler
 

While the above equations give a nice way of describing and modelling the control laws, and are the right ones to be considering in this non PIO case, the problem arises when the pilots inputs get out of sync with the control system or aircraft response. That is the differential of the inputs and the responses. Humans approximate a PID controller if they are well trained, otherwise they are barely proportional, they don't integrate the response without training, and they don't do the differential bit without experience in the response of that system. Airbus's own control system (C*, which is C*= n + A*q) was in effect a PI controller, and there was some consideration on their enhancement to PID which may well have occurred by now, (I haven't messed with an A320/330 or 340 for a few years).

PID stuff
Spoiler
 


[Engine acceleration is non linear, it is.... a third order function with two asymptotes, one at each end. If the controller is hydromechanical. then it tends to look like a 5th order function with little wiggly bits at the max thrust limit. Thrust increase is itself a 2nd order function, it follows a rather common curve].


Last edited by fdr; 24th Jan 2024 at 12:34.
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Old 24th Jan 2024, 13:05
  #52 (permalink)  
 
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Originally Posted by Uplinker
This is (or was) a professional pilot's forum, so three letter codes of major airports would be known or easily looked up by professional pilots. Haven't we all got an old copy of the Aerad books, with all the 3 and 4 letter airport codes in the back, or is that just me.
Or, if all else fails, ask the guys who assign them:

IATA: Airline and Airport Code Search
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Old 24th Jan 2024, 13:35
  #53 (permalink)  
 
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Finger trouble?

What would happen if one of the crew simply accidentally pressed/bumped the GA button? A bit of a startle effect could result leading to some
mishandling.
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Old 24th Jan 2024, 13:50
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No GA button on Airbus aircraft.
enzino is offline  
Old 24th Jan 2024, 14:33
  #55 (permalink)  
 
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Well there is, but on Airbus FBW the TOGA buttons are inside the thrust lever quadrant - not external on the thrust levers themselves - so they can only be activated by clicking the thrust levers fully forwards.

So in an Airbus FBW you have to deliberately select TOGA with the thrust levers; you cannot bump the switches by accident.
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Old 24th Jan 2024, 15:14
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I was trying to work out the reverser stow time and acceleration of a wide cord high by-pass engine to TOGA ( I know a pure jet from just above idle to nearly T/O power is 4.5 seconds) and I come to a figure which would be about 0.35 miles of distance covered by an aircraft at 120knts.
That is a preponderance of a decision for a PIC in such circumstances; perhaps some knowledgeable comments/ corrections?

Out of interest, even the very classic B747 with JT9's had an interlock, by way of an electrical actuator in the pylon area, which prevented forward throttle movement whilst the reversers were not stowed.
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Old 24th Jan 2024, 15:48
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Originally Posted by fdr
You called this a PIO and gave the G- rego tailscrape at LHR as an exemplar of the same. Neither of them are caused by a PIO/APIO. Now, if you provide evidence that Gz and pitch rate gets out of sync with the SSC and elevator deflection then it might be of interest.
I'm not saying the tailscrape at LHR was caused by PIO.
I'm saying that there is something that caused the PIO.
Something that should not have happened, that caused the PIO.
And the same thing that caused the PIO, also caused the tailstrike.

First, do you agree that the triangles on the elevator deflection are clear signs of PIO ?

However, there is not a PIO problem, prior to the tail strike. There is a "flight control dynamics problem"
I zoomed in on the curves available here.
https://assets.publishing.service.go...XWBC_09-22.pdf
If you put the two curves together, you get this picture :
https://i.gyazo.com/7805953bd1bc0dfc...89e5c86214.png
You can see that the elevator follow closely the sidestick input, up until a point...
There is one nose down input, approximately 5 seconds (two and a half squares) before the red event (tail strike). Third black vertical line before the red event.
After that, there is one pitch up order. It is followed by the elevators, but late. And not to the extent that you would expect.
Then, the stick is released a bit. There is still some delay in that.
The stick is pulled again. This time there is a half second delay at the beginning of the pulling.
Then in the middle of the nose-up elevator movement, the blue line slows down. The delay is now one second between the elevators and sidestick pitch order.

As soon as the elevators reach full aft, the pilot releases his input up until the tailstrike. While the pilot releases his input, the elevators remain generally almost at full aft. You can even see, at the event, the pilot order is towards nose down. Has been in a nose down direction for 2 seconds. And the current position of the pilot order is neutral.
However, the elevators are near full aft, and are going full aft. Doing exactly the opposite of what the pilot had been ordering for two seconds.
In the flare, and on the ground, as well as after takeoff, the SSC gives a simple proportional deflection for the elevator deflection
Hence, this is clearly not the case.
Maybe in theory, not in practise.

The deed is done. The PIO occurs just after this moment, but it's already too late.

The triangles were a good indication that something weird was happening before them. And looking into it, it is the case.

There is a lot more to say about this. Notably, what would have happened if the elevators had followed the nose down tendency in the two seconds before tailstrike ? Since it's a very short duration, it's likely the pilot input would have been very similar.


It is possible to plot pitch, pitch rate, pitch rate rate (which is proportional to pitching moment), but I'm fairly sure that everything will be very consistent. It is aerodynamics, except if one aileron was broken in half, for sure everything is going to be consistent.
The discrepancy is between sidestick order and elevator response. The discrepancy during a PIO itself is due to elevator maximum speed being reached. The discrepancy before that is less obvious.

In any case, I think it wiser to pull up seriously when you hear the thrust reaching maximum. Not before that...


As for the rest, your post was really interesting. Some general comments :
I really like the APU missing incident.
We do not have the same notations so it's sometimes not easy to follow. What would V19 be, for example ?
You talking about the C* law shows in-depth knowledge of airbus FBW. It's fun to see how reality is different than the simple stuff told in the FCOM. It would be interesting to teach this in engineering school. Do you mean there is a PI filter on top of this equation, or that just having this equation plugged into the system is enough to make it work PI-like ?
For normal cases, going into all the matrix thingys and even the non linear effects is not really necessary. The simple stuff, pitch rate rate times pitch inertia equals pitch moment, without taking into account the other terms, is more than enough. In this case there is no "cross axis effect", all the angles are small..

Last edited by CVividasku; 24th Jan 2024 at 16:26.
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Old 24th Jan 2024, 19:01
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Originally Posted by aeromech3
I
Out of interest, even the very classic B747 with JT9's had an interlock, by way of an electrical actuator in the pylon area, which prevented forward throttle movement whilst the reversers were not stowed.
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.
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Old 25th Jan 2024, 00:07
  #59 (permalink)  
fdr
 
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Originally Posted by CVividasku
I'm not saying the tailscrape at LHR was caused by PIO.
I'm saying that there is something that caused the PIO.
Something that should not have happened, that caused the PIO.
And the same thing that caused the PIO, also caused the tailstrike.

First, do you agree that the triangles on the elevator deflection are clear signs of PIO ?

However, there is not a PIO problem, prior to the tail strike. There is a "flight control dynamics problem"
I zoomed in on the curves available here.
https://assets.publishing.service.go...XWBC_09-22.pdf
If you put the two curves together, you get this picture :
https://i.gyazo.com/7805953bd1bc0dfc...89e5c86214.png
You can see that the elevator follow closely the sidestick input, up until a point...
There is one nose down input, approximately 5 seconds (two and a half squares) before the red event (tail strike). Third black vertical line before the red event.
After that, there is one pitch up order. It is followed by the elevators, but late. And not to the extent that you would expect.
Then, the stick is released a bit. There is still some delay in that.
The stick is pulled again. This time there is a half second delay at the beginning of the pulling.
Then in the middle of the nose-up elevator movement, the blue line slows down. The delay is now one second between the elevators and sidestick pitch order.

As soon as the elevators reach full aft, the pilot releases his input up until the tailstrike. While the pilot releases his input, the elevators remain generally almost at full aft. You can even see, at the event, the pilot order is towards nose down. Has been in a nose down direction for 2 seconds. And the current position of the pilot order is neutral.
However, the elevators are near full aft, and are going full aft. Doing exactly the opposite of what the pilot had been ordering for two seconds.

Hence, this is clearly not the case.
Maybe in theory, not in practise.

The deed is done. The PIO occurs just after this moment, but it's already too late.

The triangles were a good indication that something weird was happening before them. And looking into it, it is the case.

There is a lot more to say about this. Notably, what would have happened if the elevators had followed the nose down tendency in the two seconds before tailstrike ? Since it's a very short duration, it's likely the pilot input would have been very similar.

It is possible to plot pitch, pitch rate, pitch rate rate (which is proportional to pitching moment), but I'm fairly sure that everything will be very consistent. It is aerodynamics, except if one aileron was broken in half, for sure everything is going to be consistent.
The discrepancy is between sidestick order and elevator response. The discrepancy during a PIO itself is due to elevator maximum speed being reached. The discrepancy before that is less obvious.

In any case, I think it wiser to pull up seriously when you hear the thrust reaching maximum. Not before that...
Spoiler
 
K. I note your issue on the elevator position vs the SSC. Some background. The data in the report of the LHR case is based on the DFDR data, and that has various recording rates for various data channels. The recording in accordance with ARINC 717, however that data is converted from the backbone of the aircraft that was originally ARINC 429 for Airbus, (the standard was upgraded to ARINC 629 in 1995 but not applied to all OEMS. A far faster and greater bandwidth protocol is proposed as the ARINC 664 (AFDX protocol)). The QAR will take data from the DFDAU for easy access. The DFDR takes the output 717 sentence. (Normally, data systems generally are designed to annoy everyone involved, particularly the habit of OEMs and operators not bothering to pass on the Frames definition for the system that is installed, despite this normally being an obligation on the sale of an aircraft fitted with a FDR).

Where there is an option on the time period, it is dependent on the date of certification of the aircraft, Aug 19, 2002 being the determinative date. [1]
  • Ground Air sensing; required at 1 sec resolution, or 0.25s for designs with TC issue post 19 AUG 2002.
  • Radio Altimeter: 1s
  • Control input, FBW system: 0.5, or 0.25s
  • Pitch control surface: 0.5 or 0.25s
  • Pitch attitude: 1.0 or 0.25s
  • Normal acceleration, (vertical, Gz): 0.125s (freqHz)
etc.

What arises is that when doing an analysis based on the data, what looks like a smooth line, may not be at all, it is possible to introduce artefacts. The timing of the contact is open to timing error depending on what the investigator used as his metric. Did they use ground air sensing? if so, there is up to 0.25s timing error in that value, from the granularity of the data. If using the vertical G, then it is even more problematic, as the question arises as to what component of the fuselage bending and shock transmissions impact that value. For the SSC, and elevator, there absolutely can be differences between the SSC command and the elevator position.... the elevators in this case of the direct law are commanded electronically to alter a position by a hydraulic system. The electronic process lag time is not significant, but does exist, and normally would be nulled by the recording system for post process analysis. The hydraulic system is very dependent on airloads, and rates in order to follow the command of the pilot. The hydraulic actuator has a finite time to respond to a change in the valve porting of the hydraulic controls in response to the electrical actuation of the control valve "bobbin". Thereafter, with a high order command applied, the elevators take a finite time to respond, it is a hydromechanics system with all of the delays that involves. There is additionally a high likelihood that a control input that is being sampled for the DFDR at a modest rate, will not reflect the human input fully, and a sudden change in the control input that is then nulled out or reversed will not necessarily show the control input, or the response of the control surface correctly. In this case, the actuator response is able to be determined, and it shows the elevators are not instantaneous in changing their position, nor would they be expected to be.

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. The human body has its own resonant frequency for various parts, and in response to a load from the impact of the tail on the ground, a force will be transmitted up what is a flexible beam, the fuselage, to the pilot seat, and gives an acceleration to the protoplasm that sits proudly in the seat, holding the SSC, with/without the aid of the arm rest. momentary involuntary inputs can occur. These may be unwanted, but the aircraft response is not itself a PIO, nor is the pilots, it is an unwanted short period input, and that is highly speculative, the acceleration at the flight deck from the tail contact is not determined, and is way outside of the scope of this conversation (the CFRP barrel of the airframe of the A350 is pretty rigid, but it still has flexure under loads, just like planet earth does).





[1] Appendix M to Part 121, Airplane Flight Recorder Systems. (EASA EU OPS 1 is similar standard)
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Old 25th Jan 2024, 08:37
  #60 (permalink)  
 
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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.
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