chris weston

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

Dated1

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?

Shaggy Sheep Driver

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.

aviate1138

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.

BOAC

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.

petermcleland

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.

chris weston

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

boguing

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

Oluf Husted

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

BOAC

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

ChristiaanJ

"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.

petermcleland

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.

Smilin_Ed

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.

Dated1

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?

petermcleland

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

Dated1

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.

lomapaseo

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.

BOAC

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.

Starbear

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

trident-too

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!!