AF447

[surplus1]

Originally Posted by safetypee

surplus1, in recognizing your conjecture I believe that some of the emphasis and the correlations with previous accidents are misplaced; I covered some of these points in #3014.

Thank you for your reply. I have read and now re-read your posts # 2825 and #3014 carefully.

First let me state that I am not at all married to the idea that AF447 was in a flat spin. I am fairly convinced, due to the BEA report of compression evidence (if accurate) that it was not in a spiral or conventional spin. What I wrote was no more than one hypothesis among many.

I am also not married to the idea that this aircraft fell out of the sky due to some monstrous fly-by-wire system that failed totally causing the aircraft to plunge into the ocean.

Do I have a ‘favorite’ theory? As a matter of fact I do but as yet I have not stated it and do not plan to. You may be assured that it does not include any agendas re: Boeing v Airbus.

I do not actually discount anything, especially not the idea that the aircraft could have been fully stalled with its wings level. In fact I suspect that it was. Since I’m not aware of any stall that would stay that way for so many minutes - other than a stall in a flat spin - that is what made me postulate that possibility. Perhaps I failed to state my views with enough clarity.

But, I confess that I am not an engineer or an engineering test pilot. I’m just a retired line pilot with more than four decades in transport cockpits - enough to readily admit that I do not know everything, never have, never will, and do not have all the answers, or even a majority of them.

Over time I’ve learned a few things about stalls but unfortunately for me that does not include any knowledge of anything known aerodynamically as a “stable stall”. If you would be kind enough to tell me what that is, I would be grateful.

In those four decades of flying the line I’ve never heard of anything known aerodynamically as a “stable stall” to which you referred in all three of your posts. Of course that doesn’t mean that it does not exist, it could well be my ignorance. Again, would you please tell me what a “stable stall” is?

While I do not know factually, I am reasonably certain that the pilots of AF447 were not experimenting with stalls and did not intentionally stall their aircraft. Therefore, if the aircraft did stall on that night – something caused it.

I am extremely confused as to how you could define it by the term “stable” as well as by how that aircraft would be kept in a full stall, of both wings, wings level, over a descent of some 35,000 feet in a [near] vertical trajectory until impact. Please help me to understand what would keep one of the wings from gaining some lift and the other from rolling off during the descent. Something other than ALTN law, please.

If what you’re going to tell me is “ALTN law did it”, then tell me also why, when ALTN law makes the required control input to keep a wing from rolling off, would that not be likely to induce rotation of some type [what we call a flat spin]. What control surface(s) would you expect the computer to move in its effort to keep the wings level?

While some of the correlations with previous accidents, specifically those of the Russian aircraft both of which were TU-154 T-tail machines may be misplaced because they have different stall characteristics; the reason that I and others referenced them is because the stalls resulted from upsets due to turbulence. I think that’s relevant.

As far as I know, stalls are created only by alpha in excess of the critical angle for that airfoil. They can occur at any speed or in any attitude but always for the same reason – excessive AOA.

I’ve never flown the A330 but as far as I know it is equipped with conventional wing and empennage design and control surfaces typical of large transports. While computers normally send signals that actuate and move the control surfaces, FBW, they are not otherwise unconventional as far as I know.

To the best of my knowledge this aircraft is not inherently unstable. Therefore it would not be susceptible to ‘deep stall’ and would not require any unusual control inputs by its fly-by-wire control system. It is not an F-16. It’s just a state of the art transport with nothing unique other that its FBW features. If I am correct, it will behave in stalls just like any other large swept-wing aircraft certified in the transport category. It’s an airplane, not a starship.

Should a stall occur in a level flight attitude, I assume that the pilot will add power and attempt to lower the nose. If the pilot does not deliberately lower the nose, the application of power, due the slung position of the engines, will raise the nose even further, increasing AOA and deepening the stall. Any asymmetry at all in power application will cause a rolling moment. Application of up-aileron – either by a computer or the pilot – to stop the roll will likely induce and flatten the rotation. Since the rudder is limited by ALTN law lockout any effort to use it will be quite limited.

As I understand it, if ALTN law is removed and we go to Direct Law, the aircraft control is like any other. The only difference is that the controls are displaced as the result of an electronic signal to the actuators – rather than cables or rods porting the hydraulic fluids. If that is not correct, I’m more than willing to learn.

The one thing that I do not know is: How does the pilot force the aircraft to go from Alternate Law to Direct Law? Is that possible by a conscious act of the pilot or does the software have to make that decision for the pilot?

A large amount of nose down control input, regardless of who/what applies it, plus the additional power will break the stall but it will also produce excessive high speed very quickly, especially at high altitude. To stop that will take a lot of nose up control – quite likely to produce another high speed stall. While in normal circumstance this should not be completely beyond recovery, with high loss of altitude – does that remain unaffected if the stall was caused by extreme turbulence? If you believe that to be so I can only conclude that you have been fortunate enough to never to have encountered severe turbulence – or for some other reason you consider it to be insignificant. [The differentiation between severe and extreme is intentional on my part.]

It is true that we cannot prove that the aircraft ever entered any type of spin. Neither can we prove that it entered any type of stall. We can’t even prove that it “upset” at any time. We cannot prove that it entered a Cb or two and if it did, we do not know what level of turbulence may have been encountered or its duration. That is all speculation at this point - so take it as such.

We also cannot prove that it impacted the surface intact and right side up. Yes, there is ‘compression’ evidence in some recovered parts – and the BEA theorizes as much. That proves that those parts were compressed – but absolutely nothing else. We also cannot prove that the aircraft descended in a near vertical trajectory – regardless of whether it was right side up, wrong side down, tail down or nose down, or rotating.

Perhaps most important – at this point in time – we cannot prove how or when the VS separated from the aircraft. We only know that it did. The rest is theoretical.

We may all speculate and conjecture ‘till the cows come home but none of us has enough evidence to make a definitive conclusion, and that includes the BEA [unless they are withholding evidence for some reason.]

Originally Posted by ”safetypee”

Aircraft are built and certificated to withstand inadvertent Cb encounters; they do not automatically stall, nor is a spin an automatic consequence of a stall. Large aircraft tend to depart controlled flight relatively gracefully in comparison small aircraft or high performance fighters. The engines do not flame out in Cbs without other contributions, and all aircraft, with their natural stability can transit the relatively short duration of these events in a reasonable stable manner when the controls are used to maintain attitude.

Candidly, I find that statement extraordinary. I do not subscribe to the idea that airliners are “built to withstand inadvertent Cb encounters”. I sincerely hope you do not plan to penetrate Cbs based on that premise. Airliners are built and certificated to withstand very specific positive and negative “g” forces. That is all. There design ‘strength’ is tested by computer modeling and on the ground. They are not intentionally flown into Cbs during flight testing. They all have specific certified ‘limits’ applicable to civil transports.

It is true that airliners do not automatically stall; it is also true that a spin is not the automatic consequence of a stall.

Whether or not transports depart controlled flight relatively gracefully or not is open to depate. What is not open to depate is that large transports do stall; large transports do depart controlled flight; large transports can exceed their certification limits. A substantial number of the pilots present at the time of departure from controlled flight are no longer available to give us their opinions.

Many transports have be lost due to stalls from which they did not recover. Many others have been lost due to encounters with Cbs. Many have exceeded their certification limits in turbulence and dives resulting from upsets; recoverd FDRs have confirmed the ‘g’ levels at which they experienced structural failure. Engines have been known to flame out as a result of turbulence – they have also been torn from their mounts in the process. Many transport airframes have been torn apart by extreme turbulence encountered in Cbs and also in clear air.

Jet engines have experienced compressor stalls and flameouts as a result of heavy turbulence. That is a matter of record. I don’t see how you could imply that it is impossible.

With all due respect, to believe that this “can’t happen” is not conducive to old age. Your postulation is a dangerous one, sir.

I readily admit that we do not know if AF447 penetrated a Cb or if it experienced severe or extreme turbulence. All that we know is that potentially severe weather existed in the area it was crossing on the night of the accident.

I offer this third party statement for your consideration:
"Current state-of-the-art in aircraft modeling cannot accurately predict aerodynamic and/or flight dynamic characteristics under departed and loss-of-control conditions." – Source: NASA, Aeronautics Research, Aviation Safety Program, IRAC (Integrated Resilient Aircraft Control, 2007)

IMHO your analytical logic fails with the assumption that the pitot systems would see different air masses on each side of the aircraft. From experience of tests and often due to the architecture of the pitot systems (cross balance pipes) this occurrence is most unlikely.

Sorry, we disagree. First of all there are no “cross balance pipes” between pitot systems. They are independent of each other. Cross balance is between static ports, not pitot tubes. Secondly, severe turbulence can disrupt airflow to pitot probes – both in updrafts and down drafts. Whether or not that will produce erroneous airspeeds and warnings will depend on the duration and intensity of the up/down drafts and the time delays built into the warning systems. I do not know what compensations may be built into the A330 systems for this purpose, if any.

Indeed there are many IF’s. Some of us are determined to associate the ACARS messages with the cause of the accident. Others believe there may be different reasons that triggered those messages after the upset was fete acompli (sp).

None of us will know what happened until the FDR and CVR have been recovered and analyzed. If in the absence of the recorders the Board still chooses to make a probable cause determination, for many of us, including me, the actual cause will remain unknown.