Quote from CONF_iture:
"During high AoA demo where full back stick has to be maintained the FCS is constantly working at maintaining alpha max, therefore the FCS will not use a thrust increase or additional aport of energy to increase the speed but to increase the pitch and the FPA."

Okay, then what increases first - the FPA, or the pitch?
If the pitch, how can that be done without initially exceeding alpha-max?

Hard to tell ... who came first, the hen or the egg ?
The FCS mission is to maintain alpha max whatever the amount of energy brought to the equation. The goal is alpha max and punctual under or overshooting of the alpha max value is part of the process too, the time for the FCS to absorb a variation of energy.

That's only partially true - you're looking at it from the assumption that Alpha Max has already been achieved, in which case the system will maintain it as speed continues to wash off, though eventually if airspeed is not sufficient to fly, ground contact is a certainty (however as the Habsheim and Hudson incidents showed, the aircraft will "fly" right up until impact). This is why, as OG correctly pointed out, Alpha Floor is an integral part of the combined systems protections - and why disabling it effectively transfers the responsibility for maintaining airspeed and thrust solely to the flight crew.

The part you've now skipped over twice is how the EFCS works to *achieve* Alpha Max if it's not there already, and the rate at which it does so seems to be contingent on the mode (pitch normal vs. high AoA protection) and how the latter mode brings the phugoid damping aspect into the equation.

It seems to be generally accepted that the TOGA thrust was applied too late, as was the SS back-stick command. In the case of the latter, HN39 brings up the interesting fact that the back-stick command was not only late, but somewhat hesitant as well.

To me, OG's discovery regarding the phugoid damping system and HN39's careful parsing of the DFDR data regarding application of back-stick both provide good technical explanations of why acquisition of Alpha Max was slower than it might have been with more favourable conditions (as you pointed out, Bechet achieved it much more quickly and HN39 reckons that was because he was still in pitch normal at the time). [EDIT : It also raises the slightly ironic possibility that if AF296 had pulled the SS back to attain Alpha Max earlier in the sequence (before crossing the threshold), the more rapid loss of airspeed would have necessitated thrust application sooner and might have put them in a better position.]

Add to that the low-energy state of the aircraft in general, and I'd say we now have more technical detail, but that ultimately it comes back to the original conclusion - the aircraft was too low, too slow and corrective action was applied too late.

Yo!

Direct hit, Doze.

It all comes back to a poorly executed, poorly planned/practiced "display".

Sheesh.

Even 200 feet and adding power to maintain decent speed should have "satisfied" the crowd, most of whom did not understand what the 'bus was trying to demonstrate.

The guy screwed up. All the flight control laws discussion is "noise".

I would have been fired had I done something like that unless I demonstrated the manuever with HHQ review. The Birds and Angels do hundreds of practices, and all of their maneuvers are reviewd/approved by HHQ. Look at the Angels' loop with the gear down. Then see the inverted passes with a wingie tucked in real nice on the other plane.

The flight control laws on the 'bus seem very realistic for a heavy. The engine performance is about like what I saw back in 1971 when moving to a "fan" ve5rus a pure turbojet motor. Big deal. Spool up time is a bit longer, so you must anticipate, and not wait until you realize that your power setting is too low for what you are trying to do - go around, maintaining proper descent on a precision approach, etc. Sheesh.

Heh - while I must admit that I was going for a collegial tone rather than aggression, your support and sentiment are very welcome all the same.

To be honest, and I've said this before, I've gained so much more understanding of the systems design as it relates to flight mechanics from this thread that the discussion was definitely worth having anyway in my book.

And to play a bit of Devil's Advocate, while "screwed up" is a reasonable - if strongly-worded - conclusion to draw if you're looking purely at the conduct of the flight, if I'd been looking at it at the time I would have been deeply troubled by the apparent lack of oversight from - and shoddy preparation by - the airline. Obviously he was one of their top pilots on the A320 programme, and that implies a certain amount of trust and leeway - but he wasn't the only AF Captain in that position (indeed Capt. Bechet, who headed the investigation, was of equal seniority on the same programme). To me it just beggars belief that management didn't seem to require a peer review of the intended flight plan, because I'd be very surprised if such a review wouldn't have considered the level of risk unacceptable. At the very least I'd expect such a review to have triggered discussions with Airbus as to how it compared to their own practices.

Good practice, no question. But that dreadful B-52 accident demonstrated that even military practice can lapse if the person involved has a track record of making things difficult.

Anyways, nowt more to add - cheers again!

Quote from gums:
The guy screwed up...

As a general observation, I think even the A320's strongest critics in this discussion have never disagreed that no flypast should be conducted with passengers, that 100 ft is too low at any speed, and that this one was ill-conceived, badly planned, and poorly executed - in a number of ways. The Karsenty video proves that beyond reasonable doubt each time we replay it.

...All the flight control laws discussion is "noise".

Given the general consensus that has been evident since the first couple of pages of this thread, some of us have taken the opportiunity to try and improve our understanding of flight at high AoAs in general, and the characteristics of the Airbus alpha-prot mode in particular - including aspects of its certification. Not only was this a unique accident in public transport, but all the gathered evidence provides unique opportunities for study.

Quote from Dozy Wannabe:
"... ultimately it comes back to the original conclusion - the aircraft was too low, too slow and corrective action was applied too late."

Assuming you are referring to the execution in relation to the plan, you are right on your first and last "conclusions", but wrong again on the second. The reason the crew decided to keep the thrust at idle - and then left it there until too late - was that the a/c arrived over the display area too fast - not too slow. :ugh:

Important observation. Agreed.

Of course - perhaps I should have qualified the statement as "*ended up* too slow". I've certainly never argued that the aircraft wasn't fast and high on the approach initially!

Agree with Chris and Doze.

He was too fast, and had to keep power low to finally reach max AoA or whatever some want to call it. Then was slow getting the power back in.

As with others here, I have learned a lot about the "laws" and details about the nuances of the "laws". IMHO, they ain't so straight forward as some might think, huh?

Well, from a technical standpoint they have to be complex by their very nature - they were designed to be as transparent and easily-learned as possible for the end-users (i.e. pilots) though. Of course, there's only so much simplification one can do!

That said, I think it's fair to state that at or below 100ft RA at an unfamiliar airfield is definitely not a suitable time to start probing around the limits of the systems, one's experience with those systems or both - regardless of whether or not you've got pax behind you!

Quote from Dozy Wannabe:
" - perhaps I should have qualified the statement as "*ended up* too slow". I've certainly never argued that the aircraft wasn't fast and high on the approach initially!"

Even that statement would require to be qualified with something like "for the current power setting". During 35 pages of reading and contributing enthusiastically, you have evidently not fully grasped the fundamentals of the basic scenario, nor those of the developing argument.

The game plan was to display the a/c in stabilised flight at an AoA of alpha-max, meaning that - using a fairly high thrust-setting - the airspeed would have remained roughly constant at Valpha-max in level flight and during a spectacular go-around.

The essence of the accusation repeatedly made by CONF_iture is that the a/c failed to achieve alpha-max after the PF called for it. That means that its IAS remained ABOVE Valpha-max until the treetops slowed it down. He argues, understandably, that the slight, residual surplus of speed above Valpha-max could have been traded more immediately for height.

Contributors who are expert in aerodynamic performance have explained that the refusal of the EFCS to respond immediately to the stick-command ** was due to the lack of thrust, and the decaying IAS. The latter may have been exacerbated by a slight loss of headwind component at about t -2: that crucial moment when the N1s were still low, although spooling-up.

Therefore, the unqualified statement "ended up too slow", is simplistic, to say the least. Strictly speaking, it is incorrect and misleading. I'm sure that's not what you intended?


** (that is, increase the AoA as soon as physically possible to the commanded value of alpha-max)

A fine and for me definitive response Chris Scott, thank you.

Because that's part of the alpha max demo and that's where our discussion with OG initially started.

Obviously you still didn't get what alpha max is ...

1. you simply cannot "wash off speed" and "maintain alpha max" at the same time
2. Airspeed is always sufficient to fly at alpha max

As a start, neither Habsheim or Hudson were at alpha max.

Alpha floor is an auto thrust function only and has nothing to do with maintaining speed for a flight crew.

I have not skipped anything, but the BEA obviously has.
The US and Spanish teams have underlined the lack of pitch authority given to the pilots - They made some research and proposed possible explanations.
The BEA did ... nothing : "normal functioning of the aircraft"

That part is a ALL chapter on its own as Asseline pretends TOGA was applied 4 seconds earlier than the BEA has concluded ... But we can keep that chapter for later as we have already enough to discuss on alpha max for now.

Actually it is more specific : the elevator movements show that the FCS had no intention to deliver anything closer than 2.5 deg short of alpha max.
And the BEA has not provided any reason for such restriction.
But I mainly agree with the way you write the conclusion :
"That means that its IAS remained ABOVE Valpha-max until the treetops slowed it down. He argues, understandably, that the slight, residual surplus of speed above Valpha-max could have been traded more immediately for height."

Then our "experts in aerodynamic performance" are in total contradiction with the BEA who conducted the simulated flight at the hands of Bechet.

We need something more substantial than an hypothetical 2 kt wind shear to justify a drastic difference in the FCS response between the simulated flight and the Habsheim one.

Confiture,

In my post #605 I noted the difference between Bechet's simulation and the accident flight. Bechet went to full side stick before entering alpha-prot, and Asseline after, 3 seconds later.

The difference between the two flight control modes is decribed in the Final Report, 1.16.1.2 Lois de pilotage de l'Airbus A 320:

If you see a contradiction with the BEA report please explain where you see it.

EDIT:: Phugoid damping is a feature of the alpha-protection mode, because an airplane is prone to develop phugoid motions when constrained to a commanded angle of attack. Flying an airplane to a commanded pitch attitude does not result in phugoid motion, and therefore pitch-command mode does not require phugoid damping.

HN39,
I do salute your effort to provide what the BEA and Airbus should have dealt with ... but as you quote part of the Report, absolutely nothing in 1.16.1.2 details or even mentions why the FCS response in its capacity to command alpha max should be different whether full back stick is applied before or after Alpha Prot is reached.
The paragraph in question had no intention to deal with such aspect but is purely an attempt to justify why what they call the de rotation law or the ground effect simulation law could not have negatively interfered during the event.

Confiture,

There is no discussion of alpha-protection phugoid damping in the BEA report, it doesn't go into that level of detail. Many more details have been discussed in later reports of BEA and other investigators.

I'm well aware of your views regarding the BEA, but where is the "total contradiction"?

Then why do you quote at length a paragraph that says nothing on the topic if you want to use it to justify your hypothesis ... ?

Before, at, and after thrust levers are advanced to TOGA position, both Bechet and Asseline suffer from the same "lack of thrust and decaying IAS"

• Our "experts in aerodynamic performance" explain that's the reason why the FCS did not comply
• The BEA demonstrates how the FCS fully complies

It is what I call total contradiction ...

The Bilbao and Hudson investigations were of a very different scope though. Bilbao highlighted an unforeseen 'gotcha' (for want of a better phrase) in the EFCS design when subjected to a tail gust - Airbus themselves admitted as much - and therefore a degree of research would have to focus on what precisely caused this unwanted behaviour. The Hudson investigation would have been interested in focusing on every detail of what was an exemplary bit of piloting, and therefore would want to go over the technical aspects with a fine-toothed comb. It's also worth pointing out that the behaviour of the phugoid-damping aspect of HAP mode was known in the industry by that time as a result of Bilbao, and would have been something they'd want to test anyway.

Overall, what distinguishes those incidents from Habsheim is that they occurred on what were routine ops, and in both cases the airmanship from the crews couldn't be seriously faulted.

With Habsheim, what the BEA were faced with was a system-wide failure. Even setting the airmanship question aside, there should have been red flags raised within the airline and by the crew well before the aircraft ever left the ground.

Ultimately, the main aim of the investigation was to ensure that something like that never happened again. I'm happy to concede that an argument could be made that the BEA missed an opportunity to look further into the phugoid-damping aspect in the course of that investigation, however at the same time when looking at the circumstances of the accident holistically, that would have been a small, tangential issue at most when compared to the obvious shortcomings of AF regarding the handling of the operation and the conduct of the flight itself.

I would also be inclined to argue that Capt. Asseline himself bears some responsibility for the way that part was covered - after all it was he who made the decision to disengage from the investigation as soon as the scope moved in a direction he didn't like. I think it's a distinct possibility that if he'd been able to swallow his pride and continue to work with Bechet and the other investigators, then more technical detail may have been uncovered. But by retreating behind his lawyers, openly briefing the press against the investigation with claims of a cover-up and an increasingly bizarre veil of excuses, he left the investigators with no option but to try and work things out on their own.

You're parsing things again. The BEA said that the EFCS behaviour was *normal*, it did not (as far as I know) say anything about complying with the demand.

Unfortunately the report in that sentence uses the term "Alpha Prot" (which as you have stated is a value rather than a mode) to refer to High AoA Protection mode (to give the mode its correct nomenclature in English Airbus documentation), which can cause confusion. If you'll excuse Google Translate again:

If you combine this with what HN39 was saying:

It can therefore be deduced that - based on the assertion that it was the phugoid-damping feature which slowed acquisition of 17.5deg AoA - the difference between pulling the SS back in HAP mode versus pitch normal is that there is no phugoid-damping in pitch normal, thus less potential impediment to the rate the system can acquire 17.5deg AoA.

PS - @Chris Scott - many thanks for the clarification.

Quote from Dozy Wannabe:
"[...] the difference between pulling the SS back in HAP mode versus pitch normal is that there is no phugoid-damping in pitch normal, thus less potential impediment to the rate the system can acquire 17.5deg AoA."

No. The EFCS would never permit the AoA of alpha-prot (in this case 14.5 deg) to be exceeded in Normal Law without introducing High-AoA Protection mode. Therefore, to imply that alpha-max might be achieved in a shorter time from Normal Law than from an AoA in High-AoA Protection mode is a non-sequitur.

Perhaps I should elaborate. While pulling the SS all the way back in Pitch Rate Normal will, as you say, engage HAP mode upon reaching 14.5 degrees, the rate at which pitch-up is commanded will - if I have this right* - be somewhat more rapid than if the aircraft has been put into HAP mode with partial back-stick for some time before full back-stick is commanded. Based on discussions offline, I believe the phugoid-damping aspect of HAP mode relies in part on airspeed deltas recorded at certain time frames prior to that point. Because, as HN39 pointed out:

It is therefore reasonable to infer that the EFCS only uses these deltas (and possibly only stores them) after HAP mode is triggered.

One possibility is because the change in pitch is more rapid from Pitch Rate Normal, there is less time for these deltas to be interpreted as phugoid motion by the EFCS (and because pitch changes due to phugoid motion tend to be more gradual). Certainly in the sim, full back-stick from straight and level seemed to induce a very rapid pitch change.

Another (which could complement the first) is that the decaying airspeed along with the more minor pitch change due to the hesitant back-pressure prior to 3 seconds before impact could also be interpreted as phugoid motion. The damping would then progressively introduce pitch-down in order to counteract this, increasing the time required for the full back-stick command to deliver Alpha Max.

In any case, the damping feature requires a period of time to come into effect, and the faster the aircraft is rotated, the less time that feature has to make any immediate changes to the elevator command.

[* - As stick deflection in Pitch Rate Normal commands rate of pitch change rather than AoA or load factor]