Clandestino

Gentlemen, you are discussing the dynamics of aeroplane falling with low speed and extreme, never-before-achieved AoA for the type, using assumptions valid for normal, unstalled, flight. Your theories might be correct... by pure chance.

Why should they be prepared to fly the beast outside the envelope? Key issue is how the aeroplane got out of the envelope in the first place.

Maybe there are not test pilots but rather ordinary line jocks out there who would keep on flying reasonable pitch+power when faced with unreliable airspeed. Does anyone know any other case of loss of all IAS on A330?

Yup. If only we had some other case of A330 losing all airspeeds for comparison. If only...

mm43

As Gums has implied, the Vertical Stabilizer coupled with the Rudder is the only airfoil capable of behaving with any degree of reliability at such an extreme AoA when dropping like a stone with limited IAS.

So when outside the known envelope, some g-estimates are inevitable, and further study of the DFDR gives evidence that some of the roll component is being driven by the Yaw Damper, i.e. the YD is leading the roll by between 1 and 2 seconds.

Clandestino;

Yes, your post is noted, and the input of those with experience in military types may actually be of value in understanding the aerodynamics experienced by this unlucky A330.

HazelNuts39

Turbulence is not always random and perhaps its effects should not be dismissed so easily. The 'bump' around 02:10 lasted about 10 seconds, as shown below.

P.S. The derivation of the formula for vertical gust velocity can be read here .

philip2412

i`m no pilot,nor an engineer or pc expert.and english is no my native language as you will easy notice.
and excuse if i am too much OT.
there is sometong i`ve been wondering about from the first time i have read the cvr as everybody has done it a few times:

PNF: you`re going up !

PF: i`m going up ?

how comes that he did not expect the a/c to go up when pulling the stick backwards ?
better said,what was he intending when pulling back
he seemed surprised and not expecting to go up

answers ?

P.s. my only "experience" is, i`ve read about all NSTB and britisch acc.inv.reports from 1950 -2011

jcjeant

Hi,

There are several similar dialogues ... here is another just little time before the end CVR
2 h 14 min 05 4,024 The pitch attitude is 14°.

As I already pointed out several times .. these are very strange dialogues
And they only add lanes and deadlocks in the maze leading to the discovery of what happened to those pilots that night

DozyWannabe

I was referring to the theory being put forward by Machinbird that this *particular* A330 was poorly-rigged. I don't see it myself, but on the other hand, don't want to appear dismissive of possibilities just because I don't happen to agree with them**.

[** - Unless they're *really* stupid and/or offensive... ]

gums

Thanks for nice words, mm43.

Only reason I joined the discussion was my interest in any FBW system that had a crash or unusual flight condition.

Once the wreckage layout became available, my first impression was a fairly level impact at relatively slow speed. The Viper deep stall came instantly to mind. But I soon found out that the 'bus did not have the area of negative pitch moment coefficient that we had. Back to what happened. I then figured that the jet was very stable in yaw and roll at extreme AoA. So why hold the sucker there? Why not either let go or push forward a bit and see what happens? Never happened.

My point is that we folks here ( 'bird, Smilin', Retired and I) have seen the phenomena and are here to talk about it. And to contribute to an understanding of how many swept-wing jets behave at "abnormal" AoA, and some possible recovery techniques. Our second input has to do with recognition of the jet's actual flight condition.

The AF447 crew did not recognize their condition. Period. Whether they figured the system would prevent a stall or provide warnings (which it seemed to do), was prolly due to their training and experience at "normal" flight conditions.

I certainly do not advocate the same training that we had in our actual jet, but seems the carriers could get the crews up in a trainer such as NASA used for the Shuttle and let them see what it feels like ( jump on that comment Doze and Cland and...), and then what to do.

respectfully submitted

roulishollandais

@linktrained

Vous êtes un connaisseur !

Giraud did not officialy nor not officially teached low-g flying. But among his many many glaciers and mountain pilot-pupils, some copied his complex way of doing, without really understand wath he was doing. I only discovered that in 1999, and discussed with him about that : He looked around him if nobody could listen what he was saying, and told me about 3th "regime"...

Great great ART of Monsieur GIRAUD !

DozyWannabe

Jump on it? In an ideal world I'd be 100% behind it!

I think more realistically (on cost grounds), putting line crew back in a Cessna to revise the basics immediately after hiring, and thence one every couple of years would be a good start. The bean counters would probably have an aneurysm going even that far, and some experienced pilots may even balk at the idea, finding it demeaning.

However - the status quo at the moment is that even fresh recruits may not have actually practiced recovery from a real stall since they got their PPL - which can be months or years before they're put into the right-hand seat of an airliner, and then training becomes all about mastering the systems of these wonderfully complicated beasts (I'm not just talking about Airbus FBW types here), and practicing rote drills using those systems to avoid trouble.

This is fine to some extent, because there's no doubt that automation and modern systems have by and large improved safety. What bothers me is that because basic flying skills are rarely revised once an ATPL is granted, the piloting "instinct" seems to suffer. In the case of AF447, at no point did the crew do the basic calculation:

I would hope that most pilots with an understanding of the principles of flight should be able to perform that calculation, but the truth is that these skills need to be exercised fairly regularly to stay current in the ol' grey matter - and the longer you do a job without needing that information the more likely the knowledge will disappear further into the recesses of long-term memory if not forgotten altogether.

Take my job as a software engineer - we have these wonderful tools and languages these days that abstract away all the nitty-gritty of understanding how these machines work underneath, and except for very specialised parts of the industry, the basics of understanding low-level code and the principles of a von Neumann machine are never used, because all the business is interested in is the speed of delivery and quality of the final product. The tools turn out final code that's *almost* as good as properly hand-optimised assembler, and because machines are so fast these days "almost as good" is usually more than enough. But on the rare occasion that it isn't, you can find yourself galloping up Diarrhoea Drive without a saddle as you try to remember all that stuff about memory pointers and opcode cycles. This has only happened to me once, but from then on once or twice a year I fire up a C64 or Amiga emulator and make myself do some rudimentary assembler just so it's relatively fresh in my head - and the first time I did I was genuinely aghast at how much I'd forgotten.

All this, and at no time was I in danger of injuring anything more than my pride. Airline pilots are risking considerably more than that, even if it's unlikely they'll draw the short straw.

HazelNuts39

When the PF made that remark his PFD was on ADR3 and had no airspeed display. Shortly after that the angle of attack 2 was temporarily valid at 41°, and the stall warning was triggered. That means that ADR2 speed was temporarily greater than 60 kts. His PFD on ADR3 indicated temporarily also above 60 kts. The actual speed was then about 140 kts CAS.

Owain Glyndwr

MM43,

Sorry, I have to disagree with you on fin and rudder remaining effective at high AoA. In fact the increased sweep coming from the geometric sweep plus the AoA means that the fin and rudder lift curve slopes fall sharply as AoA get large - see Fig9 on the figure. OK, this is obviously a Boeing design, but fifty plus years in aerodynamics tells me that the A330 isn't going to be a mile different. So although the yaw damper was working the rudder pretty hard, I doubt it was really driving anything since at 40 deg AoA it was down to about 1/3 power.

In addition, when a swept wing aircraft gets to that sort of AoA the lateral behaviour changes from a classic dutch roll to something more like a simple roll oscillation. This isn't sudden, and (of course) the A330 at 30~40 deg AoA lies in the transition zone. But a feature of this change is that the relative phasing of yaw rate and roll rate alters, so it is impossible to say that the yaw damper is driving roll - it is more likely to be lagging it considerably.

Clandestino;

You are right to express caution on using classical unstalled equations to explain AF447 behaviour when stalled, but there are other theories that may throw some light on it - but treat with care! - and they will only give an indication of what might be happening.

Amongst other things there is a little matter of asymmetric shedding of vortices off the forward fuselage at high AoA (see picture), and this can be time variant, so it is a brave man who says he can explain what is happening (or one with a lot more data and computing power than I possess)

DozyWannabe

I think what mm43 was saying was that rudder is *more* effective than aileron in the stall regime, which is undoubtedly true. Obviously at extreme AoA this effectiveness will decrease - possibly rapidly.

That graphic you've posted (thanks by the way - very illuminating) shows that rudder effectiveness on that design approaches 0 at around 60deg AoA. AF447 maxed out in the 40s. Fig. 9 suggests that rudder remains significantly effective at that point.

Linktrained

Roulishollandais #228

Hi,

Thank you for your praise - I wish that it was deserved. At Montaigne Noire this technique was taught and practiced without MY understanding, I think - but that was 62 years ago. I was to make my longest duration flight without FOUR engines whilst there. (I had seen the eight engined 100 Passenger "Trans Atlantic" Bristol Brabazon at Farnborough in 1949. Things have changed !)

bubbers44

Hopefully no pilot will get into a situation like this again. Losing airspeed at FL350 should not put you into a 14 degree nose up attitude because the overspeed was sounding putting you into a full fatal stall. All airliners have UAS charts to set power and attitude to deal with that. I am sure AF did too. I am sure they do now.

bubbers44

To clarify, no airliner at FL350 can go over 2 degrees nose up at cruise power without stalling. What they did was way out of the envelope. We need to hire pilots that can fly and not just button pushers that can't handle a simple autopilot disconnect.

gums

@Bubbers

C'mon, two friggin' degrees nose up or you fall outta the sky? BEAM ME UP!

A nice, slow pull without violating the control law AoA limits or gee limits can be done, then a gentle nose down should not be a big deal. The situation you describe reminds me of a U-2.

If the 'bus or any other airliner design requires that much attention and skill, then we're outta luck as SLF.

Sorry to be harsh, but I think the 'bus design and other jets have a lot more tolerance in attitude and such.

bubbers44

Not 2 degrees but 14 degrees nose up at FL350 doesn't work at FL 350. Anything much over 2 degrees will catch up with you rapidly and put you in a stall at that altitude. These two managed to do it in less than a minute with their maneuver. I think they really screwed up. Competent pilots don't do that.

mm43

Thanks for the graphics (think HN39 posted a link to them some time back) and your explanation.

Not being an aerodynamics expert by any stretch of the imagination, I had concluded that the lateral/transverse stability of the aircraft at such a high AoA and low IAS was balanced on a knife edge, and even with the reduced efficiency of the rudder it would still be possible to induce a roll moment. You doubt that that was the case, but as I stated in my earlier post, the DFDR data is pointing toward it doing just that.

To emphasize what I am getting at, I have blown up part of the Roll trace and and superimposed the Rudder trace on top of it. In addition, an expanded (replotted) 30 second section is shown where (if the traces have been synchronized by the BEA) the rudder is constantly leading the roll and not the other way.

I'm still puzzling over the constant yaw to the right and its cause. Were the ailerons responsible, asymmetric drag, or a side-slip component? Though the more I look at it, I have come to realize that the PF had one hell of a fight on his hands just keeping the wings level, though through mostly of his own doing.

Owain Glyndwr

Dozy
Since the aileron effectiveness is just about zero in fully stalled conditions this has got to be true.
But I didn't mean to say that the rudder was completely useless, just that it was operating at about 30% effectiveness relative to normal conditions.
Incidentally, I forgot to mention that the effectiveness is further reduced by the fin being swept by the low energy wing wake at some AoAs

MM43
Yes, I see where you are coming from and understand how you might have come to that conclusion; but I am coming, quite literally, from the opposite angle.

When you put an aircraft up at those sort of AoAs funny things start to happen to the lateral behaviour, of which perhaps the most important is that you develop a direct kinematic interchange between roll and sideslip - bottom left in the graphic. When this really gets going the dutch roll disappears and is replaced by an oscillation which depends on roll-sideslip-rolling moment due to sideslip- roll - sideslip. This oscillation really doesn't involve any significant yaw motion. The top right picture in the graphic is the nearest I can find to an A330 situation - the relevant CL would be in the range 1.2~1.4.

I don't think that AF447 got fully into this state, but if you look closely at the estimated sideslip trace then when it is not saturated the peaks in sideslip coincide timewise with the peaks in bank angle, so there is an element of this phenomenon present I think.

Again, if you look at the rudder traces you see that in the timeframe you cite there was no rudder pedal movement worth a damn and that all the rudder movement was commanded by the yaw damper. Philosophically, the yaw damper is a follower and shouldn't be driving the motion unless the system designers really fouled up and produced an unstable system (which they did not).

More practically, the yaw damper is just what it is called - it applies rudder proportional to rate of yaw, so in this case if you are thinking about the relationship between rudder angle and bank angle you really need to consider the relationship between bank and rate of yaw.

In a conventional dutch roll the rate of roll lags the rate of yaw by about 160deg, so that bank angle will lag rate of yaw (or yaw damper input) by 250 deg. When you get into the roll oscillation mode this may increase a little.

I don't want to sound dogmatic about this because we really don't know where the A330 might sit in the spectrum of possibilities, but it seems to me that the observed relationship between rudder angle and roll could be a big lag rather than a small lead (which would anyway imply some sort of system malfunction).

So in the time interval you quote I see an aircraft with saturated, but ineffective, roll control operating without any effective pilot contributions so that the response is something like the 'natural' one, which in this case is a very lightly damped oscillation more or less confined to roll/sideslip variations. Experience with other aircraft suggests that although the roll oscillation is lightly damped, it won't go unstable.

So far as the drift to the right is concerned I share your puzzlement. It looks like some sort of out of trim coupled with slightly negative spiral stability and if you digitise the estimated sideslip trace ignoring the obvious saturation errors you see an intriguing drift from zero to about -1 deg (left slip) over about 3 minutes. At the same time the rudder trim trace shows a constant 0.5 deg right turn deflection which is at least consistent.

So it is possible that there was a standing out of trim which was being held by the A/P, but whether the numbers stack up I just wouldn't know.

HazelNuts39

It didn't. It went to 60 - 70 but became NCD above 40.