If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.
I think here is where you are not following me: in the part of the article I am objecting to, he is not specifically speaking about AF447. He has taken an aside to explain 'coffin corner' in general. And here is where he states that stall WILL occur due to lack of airspeed. Will occur, not might occur. And regardless of what aircraft or what set of exacerbating circumstances.
If the lift produced by the flow of air over the wings becomes less than the weight of the aircraft, you are no longer flying, but falling in a "stall".
That's "you" are "falling in a stall", not "they, or AF447, may have been falling in a stall".
This sort of misleading statement addressing air travel in general, coming from a respected scientific authority, is what breeds ignorance-based fear of flying in the general public. That is the problem I have with it.
If he was merely speculating that insufficient airspeed may have led AF447 to stall (leaving other factors as implied in the statement), I would not take issue with that, but here he is defining general aerodynamics incorrectly and thereby instilling fear in general. Do you follow me?
Yes, and I agree. I don't agre with your initial reply to that:
Untrue. You are still flying. You are descending. You are not stalled.
That's not necesarily the case. You might be stil flying, descending and not stalled, as you say. You can be stalled, as he sais. It al depends on how you got to that slow speed in the first place, and how you reacted to it (if you did).
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Well if you were entering convective activity where the speed could be expected to increase or decrease suddenly you could have things happen faster than you can react.
They were on the edge of known activity and depending on the maturity of the the thermals they could have encountered a sudden up or down draft.
Now dial that into the altitude and the numbers for the speeds flown.
How many times have passengers been tumbling while in cruise? In this case up or down drafts associated with the cells.
Time out here- I am thinking that before the crash, there was some evidence (perhaps injuries to the folks they recovered) indicating a largely vertical "pancaking" into the water.
And, now, at the bottom of the sea, we have a whole bunch of the airplane in a very small debris field (also indicates lesser horizontal speed).
Ok, I recognize that we are both using all sorts of relative comparrisons and wiggle words, but, I seem to recall a comment regarding compression fractures to spinal areas, that would suggest that the vertical speed was pretty high and the vertical deceleration when it hit the water was pretty severe.
I've worked on space capsules and aircraft ejection capsule;
Impact water at <25 fps vertical speed you survive (assuming adequate attenuation equipment)
Impact at >35 fps and you most likely die
Impact at >50 fps and you pick upthe pieces.
So I consider anything above 25 fps to be high
I'm sure that impact forces will be estimated from recovered debris and from bodies. It is quite possible the vertical accelerations varied thoughout the aircraft.
Yes, and I agree. I don't agre with your initial reply to that:
That's not necesarily the case. You might be stil flying, descending and not stalled, as you say. You can be stalled, as he sais. It al depends on how you got to that slow speed in the first place, and how you reacted to it (if you did).
Ok, now back to AF447 and coffin corner. Their last reported flight level was FL350. Their flight plan had intended a step climb to FL370 before that point, but they had not initiated it, probably due to turbulence and/or warmer than expected OAT. Going by the Opt Alt chart, FL370 would imply that they were anticipating an OAT of something around ISA +10. Even if the OAT had been ISA +20, the Airbus chart put their Opt Alt slightly above FL35. As I understand it, that still provides for a .3g margin. Would that not normally provide a fairly comfortable speed margin between Vs0 and Mmo? Is that really anywhere near coffin corner?
Now if there was a much more significant rise in OAT and/or turbulence and gust loading, then I can see the density altitude and weight factor increasing into a really dangerous situation, but then the issue to focus on is not really coffin corner anymore, but rather unexpected weather phenomena and why we need to detect and avoid it.
As I understand it, that still provides for a .3g margin. Would that not normally provide a fairly comfortable speed margin between Vs0 and Mmo? Is that really anywhere near coffin corner?
Short answer: Correct.
Long answer: Add an "I think" to every sentence here. I should go check the FARs to get the right numbers, but I'm too lazy today.
The plane is flying at a cruise speed. That cruise speed cannot be just anything. There are physical and legal limitations on both sides: low speed side and high speed side.
The low speed boundary is Vs. The plane just can't cruise at a speed below Vs. Then there is a legal margin above Vs. You say 1.3G. I don't know where that comes from. 1.3Gs provides for just a 1.14Vs. It looks too low to me. I think the requirement is the same as for Vref: 1.23Vs. That provides for 1.5G. So if the stall speed is say 200kts, the minimum speed would be 246 kts. That's a 46kts speed margin over Vs. Now, that speed is in EAS (equivalent speed, a sort of IAS/CAS corrected for compressibility). Take into account that the real airspeed (TAS) is about twice the CAS up there, so in fact in that example you'd need to loose 92 kts to stall at 1G.
The upper boundary is, well, no one knows what it is because no one has ever destroyed an A330 from overspeed and recorded the value where the destruction (or the irrecoverable loss of control) began. But we know there is one speed Vd/Md, where the plane still can sustain the full 2.5G of design load plus it's 50% margin to get the ultimate load of 3.75Gs, and the plane remains controllable with normal pilot technique, and any vibration or flutter do not impair the readability of the instruments. There is an unknown margin between Vd/Md and destruction or irrecoverable loss of control, but the margin is big enough that the plane must be flown at that speed during certification, and they won't be flying the plane at the boundary of destruction. Then we have Vmo/Mmo. That's the max allowable cruise speed. Vmo/Mmo is low enough that a number of upsets must be introduced at that speed during test flights (including a descent/dive of 7.5° -that's a lot- for 20 seconds -that's a lot too-) and Vd/Md cannot be reached during those upsets and during the recoveries. I think that the absolute minimum margin between Vmo/Mmo and Vd/Md is 0.07 of Mach, but I suspect that that number number will fall short of other requirements (like the upset). Finally, while the plane can be legally flown at Vmo/Mmo, it's never done (maybe except in an emergency descent) because that would trigger the overspeed warning all the time.
So the "operative" coffin corner (where the lowest operative speed meets the max operative speed) is such that:
Vs < 1.23 Vs = a few knots or cents of Mach below Vmo/Mmo < Vmo/Mmo = at least (but probably more) than 0.07M below Md < Vd/Md < speed of destruction or loss of control
The first and last terms, marked in red, is the physical coffin corner. The bold underlined terms is the operative coffin corner. The difference between both is the "comfortable" margin that you mention. AND THIS AIRBUS WAS NOT EVEN OPERATING AT THE OPERATIVE COFFIN CORNER, let alone the real one.
(Note: "Operative coffin corner" is a term that, AFAIK, I've just made up as defined above)
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
The other part of this is that high-perormance aircraft tend to have crummy stall/spin behavior, and often go into uncontrolled high-speed death-dives, where they shed various parts, including assorted big, flat aerodynamic thingies.
Though airliners are designed to be inherently stable aircraft. And i suspect this is reflected in that 447 appears to have been in a deep stall but remained in a wings level attitude during its decent to the sea, rather than wildly spinning our of control shedding large flat aerodynamic thingies.
Jet Fighters are inherently unstable which enables far superior manuverablity.
But I don't quite understand the point MCM is making, because you don't fly the Airbus by feel, you fly it by instruments, and the HSI should give you your pitch response to sidestick inputs (and/or THS setting). However I do understand his earlier statement that the A330 is not designed for precise manual flight at cruise altitude, and to do so would be very difficult in severe turbulance.
I think that is the crux of the possible problem. In the worst case scenario of and aircraft in an upset state, which control is going to provide the pilot with an intuitive means of control?
The system that has a few springs but relies on a number of avionics to give you feed back. Or the system that feeds back directly from the the control surface itself.
Not being a pilot i don't really know the answer. But from experience flying a few simulators at work with both mechanical link and fly-by-wire, all of them prove quite a work out when operating the controls at their extreme limits. You really need to push these things to get full deflection, but on the same score because of the tactile feel of the control, you have a good natural feel for the amount of control input you applying to the yoke.
In a crazy confused scenario where to think you need maximum surface deflection, does the sidestick really transmit that feel that you have maximum deflection? Or alternately inadvertently applying maximum deflection with out realizing.
In the worst case scenario of and aircraft in an upset state, which control is going to provide the pilot with an intuitive means of control?
The system that has a few springs but relies on a number of avionics to give you feed back. Or the system that feeds back directly from the the control surface itself.
There is no such a thing in any Boeing or Airbus.
Boeing has an artificial feel system that "mimics" what you would feel if there was a direct mechanical link (like cables, crankbells and pulleys) between the yoke and the surface. That system is fed by several variables, one of them is airspeed. I don't know to what that system would revert when there is no reliable airspeed available, but it will have to revert to something, so there will be a change from the "usual" feel feedback.
Airbus has a simple spring-loaded stick.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
In both alternate law (1) and (2, I think this is what you refer to as "abnormal alternate law"), the pitch is still a load factor demand law, similar to normal law.
Yuck, this sucks! You are right. That's the worst of both worlds!
Load factor demand and no stall protection!!!
You just have this: A low speed stability function replaces the normal angle-of-attack protection
System introduces a progressive nose down command which attempts to prevent the speed from decaying further.
This command CAN be overridden by sidestick input.
The airplane CAN be stalled in Alternate Law.
An audio stall warning consisting of "crickets" and a "STALL" aural message is activated.
The Alpha Floor function is inoperative.
But then you have this: Certain failures cause the system to revert to Alternate Law without speed stability.
I wonder if an unreliable airspeed is such "certain failures"?
So leave it alone, slow down, the flight control computers will keep 1G until it stalls with no nose-up command from the pilots! (human or auto).
I want THIS feature removed NOW!!! Forget about the "gimme my frigging plane now" guarded switch.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Accident investigators over the weekend determined that internal portions of the recorders removed recently from the undersea wreckage of an Air France jetliner were largely dry and undamaged, increasing the likelihood that experts can retrieve important data about the 2009 crash, according to people familiar with the details.
Both digital chips and memory cards inside the so-called black boxes, or flight-data and cockpit-voice recorders, were found to be in good condition, these people said. There was scant evidence of salt water corrosion or other types of obvious damage, they said.
Boeing has an artificial feel system that "mimics" what you would feel if there was a direct mechanical link (like cables, crankbells and pulleys) between the yoke and the surface. That system is fed by several variables, one of them is airspeed. I don't know to what that system would revert when there is no reliable airspeed available, but it will have to revert to something, so there will be a change from the "usual" feel feedback.
Airbus has a simple spring-loaded stick.
The 737 feel computers are purely mechanical, thought the Rudder does have pitot inputs on the tail, but these are completely independent of the pilots pitots and purely provide the feel computer with press diff from one side of the tail to the other. I don't think the flight controls feel computers don't have any avionic input at all.
joe, you are wrong. a full bottle of lemonade will deform because of dissolved gases but the full bottle of water will not? does lemonade have gases we don't know about? a sealed container completely filled with water at the surface will not deform when brought to depth since water does not deform water. the only reason our bodies deform at depth is because of the space we have inside that does get compressed under pressure. and i'm not so sure that human blood vessels contract after death. while there may be dissolved gases in them they will not "contract." what evidence do you have of the state of the bodies, aside from a journalist's description? and we all know how accurate those can be. the blood is no longer liquid? how so? frozen in un-frozen water? solidified while surrounded by liquid under pressure?
too many assumptions i think...
we could continue to argue about physics but you are not going to win since there is no proof of what actually happened. however, i'm reasonably certain that no one survived the descent long enough to make a difference in the relative pressure of dissolved gases in the tissues. the references you made to cns trauma are not relevant given the massive trauma of water impact. oh and what does the fact that the seats were still attached have to do with anything?
my qualifications? absolutely none. i on the other hand, don't speak in the authoritative tone and absolute terms you tend to use about everything in the few weeks you have been here.
so let's get this straight, you are a pilot and general aviation expert, scuba instructor, an expert in gas theory and physics in general. can't wait to see what comes next.
perhaps if you spoke in less absolute terms, people would not question your CV as much. but if you must and you don't like being questioned here, you can always leave. i don't need to have any qualifications to question yours. simply a matter of wanting to know if i should consider your opinion as anything other than that--opinion.
you are not the only one to be questioned and the reason for it is the types and tone of the statements made. i doubt there are too many true "experts" here, although there is no doubt whatsoever that there are at least a few folks on this board that have demonstrated the basis for their opinions. and even they get questioned when they make statements that others don't like.
grow some skin...
Lol
Ok, we will await the autopsy reports... Then I will await a virtual humble pie from you.. As you NOW say you are qualified in nothing, yet question others ? roflamo !. so that is a good start. authorative is an opinion you have, contradictory without any supporting evidence is also authorative dude !!! .... I appear to be wasting my time explaining to you, in the nicest way. - agree to disagree until the final reports are made.
MMMMMMMMMMMMMMM ! Mom's Apple Humble Pie... I look forward to it..
I won't even GO to your post about 'yanks' not leaving their 'military' behind as a matter of 'pride' .. hahahaha, but then it is not appropriate for others to want closure etc ??? Jeeez...... My god / Allah etc or whatever you believe, it is no wonder the US is under constant threat and the world perception against you is based on people like YOU, NOT the general US populace... but, again, that is another story..
Have a nice day, would you like fries with that ?
So, if the pilot can compensate for loss of pitot readings etc, then the aircraft goes into alternative law, and Colgan would not have happened with this protection etc etc..
Then, ? how come it smashed into the sea........ ? (AF447)
The Russian Airbus that was the focus of the Pilots son having hands on before they went to spiral dive etc, apparantly could have been recovered by a hands off action, instead of fighting, so, in THAT case, the pilot(s) killed the aircraft. ?
Surely it is possible to take these factors (loss of Pitot) into account and allow the computers to 'fly' ? or is it not... ?
What about ground proximity etc ?
Again, it seems like a case where either they had a total catastrophic failure, or, they fought against the flight envelope portection, or, the protection was not functioning. ? . If so, and a pilot was capable of flying but perhaps did it wrongly, then the computers are at fault.... ?? - becuase surely the computers can amke the same decisions, and have often made far better ones that were over ridden by the PIC....
We process personal data about users of our site, through the use of cookies and other technologies, to deliver our services, personalize advertising, and to analyze site activity. We may share certain information about our users with our advertising and analytics partners. For additional details, refer to our Privacy Policy.
By clicking "I AGREE" below, you agree to our Privacy Policy and our personal data processing and cookie practices as described therein. You also acknowledge that this forum may be hosted outside your country and you consent to the collection, storage, and processing of your data in the country where this forum is hosted.
Tweet
Comment