Entropy and ice thawing: Misconceptions.
 

The idea that water ice does not change phase ie thaw until +4C is simply wrong.

This is basic Second Law of Thermodynamics stuff.

Water has an equal probability of solid and liquid phases at 0 degrees C under standard (we can argue) conditions of T&P or, more accurately, the Entropy Change ("Delta S Total") is zero at 0 degrees C under standard conditions of T&P say 101.3 kPa et al.

Yes yes that means the Gibbs Free Energy Change Delta G is zero too.

Shifting the parameters of T & P will simply shift the position of the equilibrium.

At any temperature > 0 degrees C, including +4C, the probability of liquid phase water existing represents a positive entropy change over that of solid phase water. ie Liquid is more probable.

In simple language ice melts at > 0C and it's crucial to grasp that there is no single "switch temperature", its a continuous shift in equilibrium position and probability that's going on.

Do the sums, try different values.

Delta G = Delta H minus [T(in Kelvin) x Delta S of the particles]

Or if you prefer

Delta S Total = Delta S of the Particles plus Delta S of the surroundings.

Delta H Fusion water is +6 kJ/mol
Entropy of Liquid phase water is 63 Jk–1 mol –1
Entropy of Solid phase water is 41 Jk–1 mol –1


CW :)

A dead stick 'glider' approach?
 

Just before impact the aircraft was seen to have an unusaly high angle of attack.
Assuming the aircraft was in a dead stick 'glider' approach should the pilot not have lowered the nose to gain airspeed and would not the subsequent ground effect have extended his glide path?
Trying to maintain height and extend glides by subconsiously pulling back on the stick is an extremely popular way of taking pilots out, as we all know.
Has this pilot's perfomance at the time been evaluated as a part of the air accident investigation?

I think he may indeed have lowered the nose when the power faded, to keep the aeroplane flying and reach the field (I think the captain may also have removed the 'drag flap' as well?), then pitched the aeroplane up at the last minute, very close to the ground to dissipate as much energy as possible, resulting in a high-ish rate of descent at minimum forward speed for the last few feet?

Seems like a job well done to me.:ok:

No one was killed, barely anyone injured, the 777 missed the obstruction [lights/localizer?] on the approach by 15/20 feet. What more should the handling pilot have done? The undercarriage dug into wet ground and absorbed huge amounts of energy and everyone walked away.

Maybe if he had [being incredibly wise after the event!!!] extended the glide, would he have contacted the tarmac, causing sparks and maybe a huge conflagration, possibly engulfing the fuselage and exiting likely to have been much more hazardous.

Aeroplanes are not people and aeroplanes don't have grieving loved ones left behind.

Correction to Oluf's post #1456 - Boeing look at engine icing on the basis of TAT, not OAT, so although 038 may well have been in 'engine icing conditions' during the descent, that would have been due to TAT.

Also to say that some of us (pilots) are aware of engine icing risks above 'normal' altitudes. Boeing say one can ignore engine icing with a SAT of -40C or below unless throttled back. I have been somewhat more cautious than that for some time and I suspect that 'rule of thumb' may change.

Dated1...

I think you might have been wise to read the whole thread, even if it is over 1400 posts, before jumping in with such an ill advised comment.

Spot on Shaggy Sheep Driver, spot on.

None of it from the book and all in 50 or so seconds gets unequivocal respect from here.

And yes .......I've read every post.

CW

Blimey Chris.

If I'm ever corrected, it's nice to have someone who knows his stuff do it.

I 'knew' that water unfroze at 4C from childhood (ponds/fish/winter).

Thermo section of Naval Architecture degree course did not correct that. Water is wonderful stuff, but not as clever as I thought. Thanks for the lesson.

John

BA38s engines "Hesitated"
 

Dear BOAC,

I stand corrected, TAT it is! (What is the diff. at 250 kts in 10C. weather.?)

"A general lack of crew awareness and training concerning winter operations"

The above sentence was the End result of a study, at Cranfield Institute Of Technology, done by Javid Karim in 1995, over the subject:

"An Investigation Of Aircraft Accidents And Incidents Attributed To Icing, And Cold Weather Operations"

60 airlines participated, among them BA, SAS and Finnair, only the later had sufficient training and knowledge.

Oluf

I don't think OAT affects it and I also think it is around a 7 degree rise?

"I 'knew' that water unfroze at 4C from childhood (ponds/fish/winter)."
What's this fixation with +4°C?
I thought water was this weird stuff that reached max density at +4°C but froze at 0°C?
So, roughly speaking, when air temp went down below zero, the surface would freeze and the 'warmer' water underneath would sink to the bottom.

I think that due to something called "Latent Heat", it doesn't actually freeze or unfreeze at 0 degrees C even though the temperature of ice is 0 C. Water needs to go a bit below that temperature to solidify and a bit above that temperature to become liquid again.

Back to the Old Thermo Text
 

Well, I didn't really understand Entropy in Thermodynamics 201 so now, 50 years later, I must break out that old text book and go through it, and the Calculus that supports it, again. Maybe I'll get a better handle on it this time. Maybe more of us should do the same. The more brains thinking about it the better. This really is a complex problem which probably does not have a simple answer.

Dead stick approach
 

You are right in that I did 'jump in' before reading the entire thread but I have just put that right by spending the last four hours looking for something regarding airmanship and aerodynamics as apart from the well argued technical aspect of the initial failure of the aircraft.
I began this reply by 'cherry picking' a number of posts that reflect my own thoughts but elected not repeat them as you have undoubtedly read them anyway.
However, to the point. Accepting that the aircraft was effectivly on a dead stick approach...the nose steadily rising as the speed falls...the 'picture' of the world outside starts to look wrong...asi?...108?...ooops...which way do I move the 'stick'....let me think....
Why did the FP, seeing that the AP was maintaining the glideslope at the expence of airspeed, choose not to disconnect the AP?
Stress, fatigue, workload?
All I ask is that the perfomance of the pilots at the time is evaluated.
Is that so very unreasonable?

Dated1

I just think that you should accept that many experienced pilots here, including those with much experience on type, have come to the conclusion that the actions of the two pilots in the very limited time that was available to act, performed a series of actions including the odd one outside the book, that resulted in the aircraft arriving on the grass short of the runway, with no fatalities. It is generally considered here that with actions other than those performed, the aircraft would have finished up the wrong side of the fence having demolished a number of obstacles and vehicles with great loss of life.

Most pilots here think that they did a great job :)

Dead stick approach
 

Well...with the greatest respect, and after having read the entire thread this very morning, I feel that 'most pilots' were not so sure that a great job had been achieved from 750' there is more than a 'very limited amount of time' available.
Although the aircraft 'cannot stall' the landing was so hard that an undercarriage leg went up through the wing. Stalled or not that aircraft virtualy fell the last few feet.
Of course, I wasn't there and it is easy to speculate from the lhs of my sofa these days but allow me to offer a sample of my mornings 'cherry picking' and again simply express the hope that the eventual enquiry will publish the results of a pilot perfomance evaluation.

Happy reading...

I would expect that the natural reaction would be to take manual control, knowing that the autopilot would attempt to maintain the glideslope to the detriment of the airspeed. The way I read this report (and I may well have got the wrong end of the stick of course) is that the aircraft must have stalled (read 'descended rapidly') after the autopilot 'flew' the aircraft to 175' and 108kts and disconnected itself.
--
As far as the kudos for the "wonderful" job the Autopilot did...
If I were handflying this approach, and lost thrust, would I sacrifice altitude for airspeed ( to prevent a stall ), damm fckng right I would.
And YES, with a loss of thrust, I KNOW I could FLY it to the ground a WHOLE lot better than the A/P. This is airmanship 101 guys. The only question I have is at what point do I realize I had no thrust and then act.
--
It seems apparant that the A/P remained engaged down to 175 feet. Although the A/P is a pilot`s best friend in most emergency situations, perhaps it was not the case in this instance.

The speed at 750 feet must have been about 140 Kts.
The speed at 200 feet was 108 Kts.

As the the A/P was engaged, the AFDS was trying to maintain the glideslope, with a probable linear speed decay. The speed loss was approximately 32 kts in 500 feet.

Every kt of speed below VRef 30 would result in a worse Lift/Drag ratio, with the result that the aeroplane could not eek out as much distance as it potentially could if it was flying at it`s best L/D ratio speed (approx VRef). At 108 Kts, the L/D ratio would be significantly reduced.
---
The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.

The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.

There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.

I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.

In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.

With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.

In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.



It was an interesting decision by the crew to leave the autopilot engaged as the speed decayed.Perhaps there was a good reason for this. Perhaps they were understandably so preoccupied with trying to work out why there was no response from the thrust levers, that the speed decay went unnoticed.However, the reason why the autopilot allowed the speed to decay was not because it was attempting to fly at the best speed for the situation presented to the crew. It was because it was attempting to maintain a glideslope that it was commanded to follow. Unfortunately, a decaying speed from about 140 knots at 750 feet to 108 knots at 200 feet, resulted in a severely degraded flight path angle. If the autopilot/authorottle is not performing what it is commanded to do, (in this case maintaining the commanded speed), then it is best to disconnect, and correct the situation manually.In this instance, as the speed started to decay, an autopilot disconnect followed by flying at a speed of between VREF and VREF minus 10 would, quite probably, have resulted in a different outcome, and a more controlled landing.



This is true...to an extent. However, in my company the following is beaten into us with a metaphorical big stick (and rightly so) at every recurrent: Aviate, Navigate, Communicate.

Someone has to be flying the aircraft at all times, either manually or through the automatics. With a loss of thrust at that height, it would not do to have both crew trying to troubleshoot the problem. One of them HAD to be flying the aircraft. With that in mind, personally I find it suprising that the PF would choose, seeing that the AP was trying to maintain the glideslope thereby rising the nose causing the speed to bleed off, to leave the autopilot in to the point that it disconnected itself at 108kts.

The above is a kind of subjective (eyes of the beholder) support for your argument in my view.

To my knowledge nobody has published the vertical g rates for the landing suggesting that it "fell" out of the sky. The track of the landing gear suggests that it was a lateral force that broke it as designed, from ploughing through the soft earth along the flight path.

Of course there is plenty of room for discussion here (CRM) once the facts are released.:)

Dated - you are now stirring a dark hornet's nest. You rightly describe 'the lhs of your sofa' and that is where we are both looking at this from. Whether the crew did behave to the best of their ability is not for us to judge. Nor is it really for us to even expect that they would have. Your figures actually indicate that leaving the a/p in with a similar failure at, say 550' would have been a 'perfect' solution. Put yourself now in the cockpit at 750'. Insufficient power. A windscreen full of a busy trunk road with factory buildings before it. It is going to take significant foresight to lower the nose and 'dive' at the road, lorries and cars, plus the buildings and fence, expecting the float and airspeed bleed to carry you over it. At what height do you start the 'flare'? Will the tail clear that high lorry? - this, of course, is only true of the OVERALL FPA - the ACTUAL, from 750' to 200' was seductively 'normal' at 3 degrees. Think about that.

To decide whether to raise flaps or not, as I have said earlier, is an unknown to me. Yes, leaving the a/p in until it disconnected courted disaster. I do NOT know how I would have reacted and nor, I suspect, do you?

I would query your 'statements of fact' regarding best L/D and I don't actually think Vref has ANYTHING to do with L/D? As for 'eeking out as much distance as it potentially could" - we all know that using the last few knots of airspeed down to just above stall CAN be more beneficial than maintaining best L/D in 'stretching that glide'. The unknown variable is from what height does that work better. Neither of us know for sure.

There will be an analysis of the crew actions, certainly inside BA, and probably some lessons will be learnt. I would not presume to make that analysis. I do not think any benefit can come with these 'ifs and buts' here.

dated1
 

er no, it didn't actually. It penetrated the tertiary structure aft of the wing and sizeable pieces of structure poking upwards in the photo are the gear leg and the beam which supports the pivot or trunnion. Look carefully at the intact wing forward of the gear oleo. If I recall there was no fuel leak except a small one from the centre tank?

http://i317.photobucket.com/albums/m...ndgearbeam.jpg

Spot On Christiaan J
 

Having done a"thesis'(of sorts) at Uni. on the physical properties of water,I can confirm that water(pure water that is) is at it's densest at +4degreesC. Hence,during countless ice ages of the earth,this layer of warmer water under the ice allowed micro-organisms th survive.What this has to do with this thread ,I've no idea!!

Flaps increase lift and drag, and of course the flaps on the 777 and other airliners also increase the wing area. But the later stages of flap are almost all drag-producing, and provide little if any lift. I don't know the 777, but I wouldn't be surprised if the flap setting normally used on a powered approach would be far too draggy if the power fails and there is a sudden need to stretch the glide.

Reducing flap to get rid of the drag, but not reducing it to a point that significant lift starts to be lost as well, will not result in any sink and will be benficial to an aeroplane in the situation that 777 found itself in.

SSD

Beating a Dead Horse
 

Dated1, you're beating a dead horse. :ugh: The crew did a commendable job. The task now is to find out why the engines didn't respond to a requested increase in thrust. Nothing more.

While I am not the world's greatest typist, the time it has taken to type THIS sentence is approximately the time available for the entire process on BA038, from recognition of problem to arrival.

And that's without the time for some minor corrections after the fact - an opportunity the flight crew did not have.

They took and passed with honours one of the shorter courses in test piloting in the whole history of aviation.

It seems a very satisfactory result, all considered. Job well done.

Smilen Ed

No doubt the crew did a commendable job under the circumstances.
I don't think many of us could claim to have been able to do much better.

What we are doing here now is attempting to suss out the optimum actions that would have 'stretched' the inadequately powered glide and perhaps have resulted in the gear reaching the hard stuff.

This is a continuing process for the skilful operators who seek to learn from opportunities such as this particular set of complex circumstances. I guess you can call it learning the hard way!

Any evaluation of the crews performance will be contained in the AAIB's concluding report. It will be up to the various regulating authorities to make recommendations, regarding operating procedures for the B777, as necessary.
My own thoughts at the moment, are that pushing the nose down to increase speed, might have led to a large hole in the ground to the east of the A30! -but at least the a/c would have hit the ground at the right speed:rolleyes:. Perhaps Right1 can share his/her knowledge of the B777's ground effect? Stretching the glide was quite obviously a successful manoeuvre, since all on board lived to tell the tale.:D

Approach Flaps setting
 

While configured with "landing Flaps", and flying at Vref, you have 30% margin against the stall (or minimum flyable airspeed, wahatever the terminology). Il, at that time, you retract flaps to "approach flaps setting", you are left with at least 20% margin above the stall ... and much les drag.

Retracting the flaps to the "aproach flaps setting" is one of those required immediate actions if you start a go-around initialy in "landing flaps config" and get an engine failure at the same time.

Any-one with a good training is familiar with this flaps retraction, so I can understand that the crew might have reduced the flaps from 30 to 25 without taking a chance of immediate stall.

Now, when I read that so many members consider that letting the auto-pilot fly the aircraft in such a situation -and loosing more than 30 knots in the process - is "doing a good job" ... I wonder about their degree of basic piloting skill and about their airmanship ... if any.

It was stated somewhere that replaying the situation in the simulator with more "creativity" resulted in an almost normal touchdown at the very beginning of the tarmac. No wonder ...

Analysis the crew performance will indeed give usefull information about the effectiveness of their previous training (flying skill, airmanship, crew co-operation), and as a matter of consequence, of modern, minimum training in general. "System operation" training ...

Actions of pilots
 

Dated 1,

We obedientally take it that the AAIB always leave 'no stone unturned' in their quest to provide ALL of the factors in any serious accident; especially unsolved accidents after many months of digging for clues?

I wrote to them a few months ago asking if the AAIB had checked the pilots blood and fat either at the time or subsequently. I then wrote again about another serious accident at EMA / BHX, with much the same reply.

Apparently as 'It is not the AAIB's policy to routinely subject a surviving crew member to blood/fat tests unless of course, the individual themselves complains of feeling unwell at the time of the occurence'.

***CRASH!*** AAIB "How do you feel?" Surviving pilots "Oh great, thanks!"

Anybody who has experienced the cumulative / acute effects of contaminated air will know the disastrous results it can have on ones performance. Indeed, the AAIB have helpfully published the effects in pilots own words over the years in many other incident reports. The majority obviously go unreported. Here are some 'officially published' descriptive words.

Difficulty concentrating, feeling of fainting, odd pressure in the head, nasal itching and ear pain, felt discomfort and a feeling of ‘moon walking’, mild dizziness, very nauseous, markedly dizzy and groggy, difficulty focussing, light headed and hot, felt progressively worse, tingling feeling in fingertips and arms started shaking, “feeling dreadful”, white face and pupils dilated, hands trembling, double vision, light headed and tired, dazed, headache and eye irritation, blisters inside mouth, tight chest, sore throat for days, coughing, metallic taste in mouth, tingling sensation on lips, errors of judgement and garbled speech, verge of passing out, tunnel vision, loss of balance, loss of feeling in hands and lower arms.

Perhaps by coincidence, chemicals are also found in ones blood and fat - just like an athlete may be hauled over the coals for having specks of performance enhancing drugs in their bodies or drunk drivers are grounded for their known performance degrading effect. Even many affected pilots memories have also been professionally measured as being 'abnormal'.....

Dated 1, i'm with you. Some very non SOP things happened on both those days at LHR and EMA/BHX in extraordinary circumstances and I wouldn't bet that leaving the auto pilot in until the last second in BA 038 was not actual skill - but more like pure luck.

Good for them, all those pilots did a great job in the circumstances. But what were those circumstances. Exactly?

Who thinks that pilots should routinely have their blood / fat tested after such an incident? If only to rule out any question of pilot's being influenced by anything.

It STILL doesn't add up.

If you are one of the 70% who have never been affected by contaminated air then lucky you; but please have the courtesy of listening to the apparent minority who have experienced the devastating above effects whilst operating a public transport aircraft, often totally unaware of the likely reason.

It can all be very confusing but requires understanding - urgently.

DB :ugh:

- I think you are mis-reading most of the 'pilot' posts? The overall impresion we have is that the END result was a 'good job' ie no-one died. Most of us are happy to await the outcome of the various enquiries. Whether the actions were 'the best' is relevant but not 'big picture' stuff.

The post before yours by point8six certainly reflect my views, and, I'm sure, many others.

Well, if we take it that the pilots are part of the flight system, and all other parts of the system are examined, then why not medically examine the pilots. They may be the system component that has broken.

Dated1, you're not thinkng clearly. Consider this:

At 700ft, landing gear and flaps out, you suddently discover engines don't have the power to maintain glide slope, you quickly know you're going down. Point the nose, try to stretch the glide to the runway, retract the flaps a notch to lower drag (but only a notch, because you don't have the airspeed). Quickly you realize you don't have the energy to make the runway, and this is ALL about energy.

You then see you can make the infield grass, but how fast do you want to set down on it, 140kts to 150kts? The undercarriage will be ripped off anyway in the soft grass and soil, so point the nose in the air near the stall, and touchdown at 108kts instead. Sure it'll be a hard touchdown and the undercarriage will be crushed and ripped off (was going to happen anyway), but you won't slide very far with much less energy to disintegrate the airframe structure. Also think of the undercarriage as a beneath the airframe crumple zone.

As I said this is ALL about energy, especially when contacting unpredictable terra firma instead of a hard straight runway. Keep the energy levels down on impact, and everyone lives.

All in all, this was outstanding airmanship, the results speak for themselves.

OK, Dangerous to post on this thread in relation to the pilots, however one point I have not seen mentioned before is below.

First however I would like to, for the purpose of this post, to take it as given that in this situation these pilots made at least good decisions for this situation. Further, those decisions resulted in no major injuries and would not have been bettered by perhaps 99% of other pilots in similar circumstances. As such there is thus no room to personally criticize these pilots. Whether you agree or not with this is immaterial to this argument as it is taken as given.

Now, lets imagine this fuel/engine issue had occurred just a few seconds (2-3??) later. In such a situation, it could be argued that the pilots would have made exactly the same decisions, but the result could then have been a scrape on the runway, fire, and perhaps many deaths.

Alternately it could have happened a few seconds earlier and only that 1% of better pilots would have been able to even land it even on the grass.

Would it be fair to criticize these pilots in either of the other two situations? I would suggest not, even though they may have resulted in many deaths. The simple fact is that the lack of injuries seems to be a result of pilots making reasonable decisions in a window in which those reasonable decisions were able to have a mostly positive outcome.

Going back to the question about pilots posted above, I do think it is reasonable to question the pilots actions, but ONLY from the perspective of seeing if it would be possible to somehow increase the chance in the future that the right decisions are made within the right window. In reality, I don't think this is possible, but asking if it is possible does seem important.

TME

Dead stick approach
 

Let me refer back to my original post, 1464, and perhaps make it clearer that no critisism of the pilots is intended. I just wish to see a clear pilot evaluation carried out for the further education of those still fortunate enough to be on a flight deck. Milt, post 1486, got the point of my thinking as did Bis47, post 1488, along with several others. Point8six, post 1487, 'Right1'? Loved the humour. A valuable contribution to this thread.
Rightly or wrongly, and please forgive this, I have always felt that a pilot is better for substantial glider experience, preferably current and on flapped gliders. I warmed to any young man, or young lady, in the rhs that mentioned gliding, but perhaps word had got out...
Such pilots are high on 'stick and rudder' flying. Would my gliding experience have kicked in and reverted to 'flying', put the nose down and trade altitude for speed in a 777? No idea. In a 737? Absolutely.
Thank you however for all of your comments gentlemen. How wonderful this site is for communication with you. Any tips for how to get my old airframe and this sofa airborne would be so welcome.

- helium ballons and an air rifle?

If you trawl back into the long grass on this thread you will see unequivocal 'statements' (as yet unsubstantiated) by BA pilots that the flap WAS changed and that sim evaluation runs have shown this was beneficial by several vertical feet.

There is an ace gliding BA man, who when I last heard was in 777's RHS. As an ex 'glider' I know what you mean.

Three points I would like to throw into the chat –

The inertia of a large aircraft acts in the pilots favour when stretching a glide (the opposite is also true if you stuff the nose down in an attempt to trade height for speed).

Modern swept wing airliners will have a lower lift curve slope than a modern glider and so can be expected to have a more progressive, gradual and benign stall.

While there are several reasons why airliners are certificated to fly the approach at 1.3 times the stall speed it does mean that there is a bit of slack when it comes to stretching the glide.

Angle of Attack Indicator?
 

Do large jet transports like the 777 have an angle of attack (AOA) indicator? If so, there should be a published AOA for the best glide speed for each flap setting. AOA takes the guess work out of it.

No they don't, at least not on any Boeing production aircraft which I have ever flown.

The question would have to be asked how useful it would be given that the instances of jet transport aeroplanes gliding are mercifully rare!

It is an option on the 777, but I do not know how many operators display the information. Regardless, the AoA gauge information is translated into various other more suitable readouts, and we are not dealing with a FJ operating at AoA limits in normal flight.

Dated1 - at least you still have a sense of humour in your old age!
I think John Farley - as ever - has a very valid point when considering the inertia of large aircraft. I have not flown the 737, but have flown a British jet with a similar wingspan, however, it had all the gliding properties of a 'brick-built out-house' especially in ground effect (it was unaffectionately known as The Ground-Gripper). I still think that instinctively most pilots (non-gliders) would trade speed for height to make the threshold.
As for getting your old frame and sofa airborne - why not follow your own advice - lower the nose, build up airspeed and wait for the ground effect - ouch!:ooh:

AOA Usefulness
 

M. Mouse wrote: "The question would have to be asked how useful it would be given that the instances of jet transport aeroplanes gliding are mercifully rare!"

I too hope that using AOA to stretch a glide would not happen often, but it would also be useful for ordinary landings because it automatically provides the proper speed for the actual weight of the aircraft without any calculations, especially if your calculations (or fuel quantity readings) are wrong.

AoA Usefulness
 

Goodness knows how I recall this, but I read this article a number of years ago regarding AoA readout installation in Boeing Aero:

Full article: Aero 12 - Angle of Attack