t54

Mr Optimistic
Hardly possible? (speaking entirely as a theoretician only).

You want to point the nose into the wind, more or less so the wing can start working "proper" again. If the wind isn't coming from below it soon will be, assuming you are flying the right way up.

Nose down a bit should give you more horizontal velocity too, Going faster horizontally should send the AoA in the right direction. These guys needed a horizontal velocity of over 1000mph if they were going to get out of the stall by "outrunning" it, so to speak, As it was, they had the engines on idle, I believe.

t54

auraflyer
How true. Methinks there should have been a warning that the system had gIven up and the last time it felt confident enough to speak (only a second before!) they were in or approaching a stall.
The guys made a tentative nose down to get out of a stall, got blasted by a warning, and thereafter tried to get out of a dive,

Jazz Hands

Sounds like you didn't read the articles properly. Or the article which contained Air France's blatant catalogue of technical problems, without any mention of the crew's actions.

Perhaps you're trying to inject your own "spin" with your accusation, for which you appear to provide no evidence.

Mr Optimistic

If the stall warning was perceived as real, you would expect the nose down input to be sustained but with some puzzlement (to be solved later) as to what delayed the warning. If the warning was considered spurious then there is still no reason to pull back on receipt of the warning because it is, erm, spurious. I can't imagine a train of thought which would cause a pull back on receipt of the warning, purely because of the warning.

On the subject of the intial climb, really can't progress without the CVR can we ? If the tape in the early part of the incident happens to record something along the lines of ' I am going to try and get out of this icing' or ' jeez, have you seen the height, how did that happen' the mystery of what was intended evaporates. Mind you, that sort of clarity may also be the reason the CVR hasn't been reported in full at this stage.

gums

Thanks for the link Franz, it should help more here to consider stall recovery and recognition and corrective actions.

The first graphic in the file shows the curve for a typical straight wing, large camber wing. Swept wings generally have a lower slope and an extended curve at the top of the curve. So as Franz, 'bird, Smilin' Ed and I have seen, you can fly around up at the top of the curve when you need to, but you have to be very careful!!! Besides, the plane is usually buffeting/shaking like crazy - it's telling you it doesn't like it there, duuuh?????

As PJ has noted, the mach buffet and overspeed have a different "feel" than a no-kidding aerodynamic stall. Personal opinion, but I suspect the 'bus is very tolerant of mach overspeed until maybe 0.9M or so. So get the nose down and worry about mach later.

Observation: The displays I have seen for the 'bus show a small AoA indication on the left scale. Is that correct? My preference would be a larger range and not worry about the "low" end, but the "high" end. Comments?


Edited: the main Pprune forum has this excellent link concerning stalls. I wonder if the AF447 story prompts this... Hmmmm.

http://www.ukfsc.co.uk/files/Safety Briefings _ Presentations/Airbus Safety First Mag -January 2011.pdf

t54

Mr Optimistic
You are quite right, of course.
In the info released so far there is no mention of what the PF did after that stall warning caused by his nose down inputs. It's just surmise that he stopped nose down inputs at that point.

The released statement is obviously a highly selected set of the whole data. I would imagine selected in order to prepare us for a conclusion, which those in the know have already come to. After all, with the voice recorder available, and precise timings of inputs, it may be obvious that the resumed stall warning had a direct reponse from the PF - or not as the case may be. I guess with all the data you could come to a conclusion in the time it takes to listen the the voice recorder.

Mimpe

Jazz Hands - apart from all the other problems, you can be assured that spatial disorientation was a major part of this accident. The initial pitch up would have been misconstrued by a stressed and distracted pilot handflying in IMC/Storm as both acceleration and pitch down ( somatogravic via decelleration), with the persistence of pitch up commands the likley result in the absence a contribution from instrument scan.

Not knowing which raw data to trust, of which the attitude indicator and the altidude indicator were I presume the most trustworthy, and which stall warning to trust, must have been horrifying..

A best guess approach on known cruise setting perhaps with a very gentle descent..

antpp

Isn't it rather a DOWNWARD wind that would explain pitch = aoa at horizontal flight they had at FL380? Should be about 10 kts for 185 kts airspeed and 3 deg chord angle. Could the down draft also explain the compensating NU sidestick pilot's input, which in turn sent them (without alpha protections and reliable airspeed info) to FL380 with loss of airspeed and finally to a stall?

t54

antpp
My error. Thanks for that.

DJ77

HN39,
That’s right. I figured the stall warning could also activate in normal law. Same reason probably for not having low speed longitudinal static stability in ALT law. Or am I missing something?

While there may be a logical answer I am still wondering about why activation of the stall alarm does not simultaneously prevent further auto-trim induced nose up THS move as does AoA protection activation in normal law?

Mr Optimistic

Gums, what does Issue 5 have to say on unreliable airspeed ?

OK465

HN 39:

I may not see exactly what you’re getting at. If I look at the triangle I think you’re referring to, i.e. aircraft longitudinal axis and FPA and I close it vertically, I get a near equilateral triangle, not a right triangle, so the cosine function of 61 degrees is N/A? If you close it either of two ways to make a right triangle, I’m not sure what that other side represents?

For a constant given inertial FPA (in this case -45), with vertical speed along the vertical axis & groundspeed along the horizontal axis, as you increase any postulated headwind component, for a given constant pitch attitude (in this case +16), it has the effect of lowering the existing aero AOA (higher KTAS & consequently higher KCAS for the conditions) while the inertial AOA (in this case 61 degrees, 16+45) remains the same. I think…

Lonewolf_50

MM43, in re final data:

My browser doesn't always display graphics. Now that the pix display, it is quite clear.

Thanks to both you and tailspin.

OK: roger.

gums

Sorry, Opto, but I can't access the directory for past issues. I had to save the file and then upload it to my personal domain so's all here could read, learn, think, theorize, et al

Maybe a 'bus driver here can help, as the magazine has a wealth of data from real folks and not the press or BEA.

syseng68k

It seems to me that the confusion (here anyway) about the various control
laws could be made a lot easier to dispell if there was a chart showing
the rules and input conditions under which each are transitioned
to and from. A nice big A3 sheet perhaps, as a picture is always worth
more than dry text.

To put it another way, to transition from one law to another depends on
a variety of input data, logical, boolean and variable. A boolean
variable might be something like the ap or ath dropping out (ie: on or
off), while a variable might be airspeed, aoa or similar. A logical
function being and / or / sum of 2 or more inputs to generate an output.
Apologies if this seems a bit abstract, but the transition between the
various laws does seem to be key to understanding what happened to
af447.

From what's been discussed here, it seems to me that the control law
logic is not smart enough by half. As someone else said, it looks rule
based, when what's really needed is a more fuzzy, or semi ai approach
that takes into account history and trends, as well as all the
variables to arrive at a conclusion. It then presents this to the pilot
as a best effort scenario of where it's going before handing control back.
More of a big picture view, rather than the fine granularity of individual
system failure alarms. Even with partial system failure, there should
still be enough data and history to get an idea of where it's all going.

Would guess that there is some of this already, but it does seem to give up
just at the point where it most needs to be providing a bit more intelligent
reasoning. Before anyone gets excited about the idea of yet more automation,
such a system would provide only information, not direct control of the
aircraft...

Graybeard

Sounds like a good project for you, SYSENG.

I for one can't understand why there are more than two Laws: Normal and Direct.

With more than two Laws available, I also don't understand why the roll automation and protections were given up when the bad data was in the pitch channel.

Machinbird

Mr. Optimistic
I saw my wingman push nose down stick (relative to the aircraft) at very low speed at the top of a loop maneuver (he was chasing the "bad guy" who was trying real hard to get away).

Result was a very stable inverted spin and a not so beautiful ejection.
He had been taught to forcibly "unload" the wing in training.

For airline flying, the lesson is that you need to be carefull about nose down inputs as well.

At very low speed (like AF447), if you generated a high nose down pitch rate, you could go right on through flying angles of attack to inverted stall angles of attack.

Suppose they had run the trim full down on AF447 and generated a strong pitch rate nose down. The pitch rate momentum of the aircraft could have easily taken them on through into an inverted stall. By the time you are feeling yourself get light in the seat, it can be too late to stop the pitch rate.

That is why jets need AOA indicators. So they know where they are in their AOA envelope.

AVLNative

I am a mere SLF but this helps me through all of this:

Airbus Flight Control Laws

Tailspin Turtle

There appears to be some differences as to the definition of airspeed. As far as I know, it's the speed in the direction that the airplane is going because that's what determines lift. By that definition, it's the diagonal of the ground speed and rate of descent vector triangle that I posted before, which is 152 knots. Note that the actual angle of attack and airspeed is dependent on the surface wind speed and direction relative to the airplane's heading. Also, the aircraft airspeed measurement system will not be able to accurately convey this speed to the pilots due to the very high angle of attack involved. Not that it matters, post stall.

Airspeed is not the the relative speed along the horizontal unless the airplane is in level flight (that's ground speed after correction for the wind aloft) or the relative speed along the axis of the body (I don't know what would be called but the hard-core aero guys probably have a phrase for it) unless the airplane angle of attack is zero or thereabouts, depending on the angle of attack reference versus the wing incidence on the fuselage. In other words, if the airplane was in level flight with a 16-degree nose-up pitch and there was no wind aloft, the air speed would be equal to ground speed, not reduced by the body angle relative to the horizon.

OK465

HN 39:

I should have used the term near "isosceles" rather than near "equilateral" earlier. (45, 61, 74)

TST: AIR speed parameters (KTAS, KCAS) are sensed by the aircraft onboard systems. You are correct that the aircraft has a trig calculated speed of 151 knots. Your making the assumption that all 151 knots of actual speed would be sensed by the on board systems at a 61 degree AOA. Maybe you're right, I don't know, but the other possibility is that at that AOA (61 degrees) there is some disturbed airflow across the pitot. The pitots are roughly longitudinal. Most "normal" large aircraft flight conditions are flown with the pitots quite a bit less than 60 degrees from the relative airflow.