ADVERTISEMENT
  About the SA Blog Network













Guest Blog

Guest Blog


Commentary invited by editors of Scientific American
Guest Blog HomeAboutContact

Flying in the Coffin Corner–Air France Flight 447

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



When Air France Flight 447 disappeared in June 2009, it was in the middle of the tropical Atlantic and had likely entered a mesoscale convective complex, a system of strong thunderstorms thought to have been at least four storms deep along 447′s flight path.

Flight 447 was also flying at a cruise altitude of 35,000 feet, an altitude where the relationship between an aircraft’s stall speed and the speed of sound has gained the name "the coffin corner". The name does not come from "it’s deadly to fly there", but from the shape of a plot of stall velocity versus altitude when the velocity is expressed as Mach number, the speed relative to the speed of sound. The curve then resembles the tapered corner of a coffin as shown at the right.

It’s far easier to go fast when you don’t have a lot of air to push out of your way. That simple fact motivates modern jet aircraft to fly above 30,000 feet, where the air is thin and fuel economy is high.

Flying that high also has its disadvantages. The same thin air that reduces frictional drag also reduces engine power, lift, and the effectiveness of control surfaces. In a U.K. Times article about Flight 447, former British Airways pilot Roger Guiver is quoted about large, fuel-heavy planes wallowing at such altitudes.

Flight 447′s Airbus 330 is also a "fly-by-wire" aircraft, meaning that there is no mechanical connection between the pilot and the control surfaces and hence no physical feedback. Even in a direct-fly mode, the controls are entirely electronic and connected only by copper wire, as the name suggests.

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". The minimum safe operating velocity (Vmin) for an aircraft is set enough above the stall speed so that normal fluctuations in wind and aircraft altitude won’t cause an airplane to stall.

The lift, however, depends on both air density (kg/m³) and on the plane’s velocity, and air density decreases with altitude. So, the higher you go, the faster you have to fly to stay above the stall speed.

As you go higher, temperature also decreases, at least in the troposphere were commercial planes are flying. As the temperature decreases, so does the speed of sound.

A commercial aircraft like Flight 447′s Airbus 330 will often cruise at around 82% of the speed of sound. Beyond about 86% of the sound speed, the airflow begins to break away from the wings, becomes turbulent, and lift is reduced.

With stall speed increasing with altitude and sound speed decreasing, the velocity window in which an aircraft can operate becomes narrower and narrower. The figure to the right shows this convergence of the minimum and maximum operating velocities for temperature and pressure vertical profiles typical of the atmosphere in the tropics.

The blue curve shows the type of variation expected for the minimum operating velocity. The red curve shows the maximum operating velocity at 86% of the sound speed. The green curve shows a cruise velocity of 82% of the sound speed. The flat black line across the top shows the "window" of velocities within which the plane can fly.

Under normal conditions, flying in the coffin corner is standard operation. When it entered the mesoscale convective complex, however, Flight 447 left normal conditions, perhaps seeing only the first line of storms and going between them only to find itself in the middle of the complex itself.

Experiencing an airplane stalling is normally a memorable experience. At one time, I soloed in flying a glider. Learning to recognize and recover from a stall was one of the first lessons taught. You pull back slowly on the stick, letting the nose come up and the airspeed fall. The glider gives a slight shudder and the bottom drops out. Suddenly you are falling rather than flying.

To recover, you push the stick forward, letting the craft’s nose go down and the speed over the wings increase. You start flying again and can bring the stick back to a straight and (almost) level glide.

With a powered aircraft, recovery from a stall approach can be accomplished either by applying more power to increase the airspeed or by dropping the nose, but you have to know that you are stalling. That means having working air speed indicators or being able to sense the drop in airspeed and lift. Once the plane is actually stalled, dropping the nose is required to recover.

For Flight 447, neither of those conditions may have held. The model of pitot tubes on the aircraft were known to have problems with icing and had been scheduled for replacement. Automatic messages from the aircraft indicated failures just before it disappeared from contact.

For the pilot, sensing a change in flying performance without instruments and without direct feedback from the flight controls may not have been possible, not in the middle of visibility limiting clouds and in the presence of violent wind gusts, updrafts, and downdrafts.

The interim report (pdf) from the BEA, the French Agency investigating the crash of Flight 447 indicated that "Observations of the tail fin and on the parts from the passenger [cabin] (galley, toilet door, crew restmodule) showed that the airplane had likely struck the surface of the water in a straight line, with a high rate vertical acceleration. … Based on the elements recovered up to now, no evidence of fire or explosion has been brought to light."

Was this a plane intact up to the moment of impact? Did the plane stall in the "coffin corner" yet systems failures, turbulence, and lack of visibility caused the pilots to believe it was in level flight until too late?

Theoretically, as a plane descends out of the coffin corner, recovery from a stall becomes easier as the window of possible flight velocities becomes wider, engines become stronger, and control surfaces more effective. It’s unknown whether damage or lack of information undermined such recovery for Flight 447. Only now, after two years and with the retrieval of the black boxes will the answers start to be known.

About the Author: Keith Eric Grant is a freelance writer, physicist, and massage therapy instructor. He holds a PhD in Applied Science from University of California, Davis, and has worked as an atmospheric scientist and computational physicist at Lawrence Livermore National Laboratory. Keith is a member of the National Association of Science writers, American Geophysical Union, Society for Industrial & Applied Mathematics, and Associated Bodywork & Massage Professionals. He also writes the "Ramblemuse" column for the trade magazine Massage Today. You can follow Keith on Twitter as @ramblemuse.

The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 11 Comments

Add Comment
  1. 1. kegrant 5:37 pm 05/12/2011

    Twitter comments on my post included a link to a <a href="http://disc.sci.gsfc.nasa.gov/YOTC/additional/science-focus/air-france-447-gesdisc.pdf">NASA Goddard paper</a> on Visualization of the state of the atmosphere during the AF447 event. A postmortem on the weather in AF447′s flight path via satellite observations.

    There’s also a lot more background in the NY Times Magazine <a href="http://www.nytimes.com/2011/05/08/magazine/mag-08Plane-t.html">article</a&gt; "What Happened to Air France Flight 447?"

    Link to this
  2. 2. kevin_sinkay 8:00 pm 05/12/2011

    Mr. Grant, I don’t think it is accurate to call FL35 ‘coffin corner’ in this case. At this altitude, AF447 should have had a fairly comfortable margin between Vs0 and Mmo. A330 FCOM charts put max altitude at around 35,700 for its estimated weight of 205t and SAT of ISA +13, and there is still a margin of safety above that for G-loading. The actual point where these two reference speeds converge would probably have been in excess of 40,000ft. By applying the unreliable airspeed memory items of 3-5° pitch and CLIMB power settings, remaining within a 50-60kt speed margin should not have been difficult at that flight level.

    Furthermore, your definition of stall is incorrect. Under the condition you describe, where lift is diminished by reduced airspeed, the aircraft is still flying in a controlled descent. Stall is a function of angle of attack alone, and only relates to airspeed if the pilot (or autopilot, which was not engaged here) attempts to hold altitude at that speed by increasing pitch beyond critical angle-of-attack. Stall is a condition defined by airflow separation over the wing chord, not loss of lift due to reduced airspeed/air density. The result of stall is not just loss of altitude, but loss of flight control stability.

    That’s not to say that AF447 did not initially stall, but if it did, it was due to excessive pitch input, not insufficient airspeed.

    Link to this
  3. 3. denysYeo 9:53 pm 05/12/2011

    Now that the black boxes may be recovered, and assuming they still contain readable data, it would be interesting to match the "models" of what people think could have happened to actual data. If the match is high it would provide a level of confidence that people (with the right skills) are quite good at modelling accident situations without actual black box data. It the match is low, then maybe back to the drawing board!

    Link to this
  4. 4. ennui 5:36 pm 05/13/2011

    I just wonder:
    Would these people that are in these Flying Saucers know about that?
    Maybe they did not and that was the reason for Rosswell
    and other crashes.

    Link to this
  5. 5. mccamey 12:16 pm 05/14/2011

    Seems to me you are both right about the definition of stall. While it is technically true that stall can only be caused by angle of attack, once you fall below the aircraft’s stall speed, then level flight becomes an angle of attack at which stall occurs.

    Link to this
  6. 6. Grumpyoleman 10:20 am 05/21/2011

    The first thing I learned as a student naval aviator many, many years ago was that power = altitude and attitude = airspeed. An aircraft at a constant power setting gains airspeed when the pilot lowers the nose. An aircraft at a fixed attitude, defined by constant angle of attack, gains altitude when power is increased. Thus the statement in the article that adding power increases airspeed is incorrect. Unless the nose attitude is decreased, that is, lowered, the aircraft will simply climb higher. Control of an aircraft in almost all flight regimes is controlling the interplay of power and attitude.

    Link to this
  7. 7. kegrant 12:48 pm 05/26/2011

    In my article, I apparently muffed setting up the link associated with the phrase "dropping the nose" (is required for recovery). I’d intended this to handle the technical correctness of discussion of velocity, angle of attack, critical angle and approach to stall. The presentation on stall recovery that I’d meant to include is at http://bit.ly/l05tbw

    That presentation notes a classical use of applying power to increase velocity on approach to stall and also notes that this can be problematic. The recommendation is to implement dropping the nose as the universal recovery procedure.

    It’s interesting that the article this week in der Spiegel quotes use of the phrase "deep stall".
    http://www.spiegel.de/international/europe/0,1518,764227,00.html

    If that term is applicable, there’s an extensive discussion on "deep stall" at http://www.rbogash.com/Safety/deep_stall.html

    Link to this
  8. 8. kegrant 3:56 pm 05/27/2011

    Bloomberg: Air France Probe Shows Jet [#AF447] Stalled, Near Freefall http://bloom.bg/kQGjVs Data suggests pilots unaware of stall. (article links to BEA findings)

    Link to this
  9. 9. lillybruce 2:53 am 03/17/2012

    I believe one simple thing would have saved that plane. If the two control sticks had been “slaved” together, so that when one moves, the other would also move, the second copilot would have known that the first copilot had the stick back. Unfortunately, the way Airbus chose to build the plane, when one pilot is flying, the only way the other pilot knows the position of the stick is if the first pilot tells him. That is insane.
    Air France USA Coupons

    Link to this
  10. 10. Thayabharan 3:15 pm 07/7/2012

    This Airbus A330-203 did not have multiple independent systems for detecting speed of the aircraft such as a GPS based system that would at least cross check the readings being given by the pitot tubes and then provide a cockpit warning that the airspeed could be wrong, or another safety mechanism whereby the pitot tubes are heated as long as this would not impact the reading so that ice could not occlude them.

    The accident was caused by the co-pilot induced Deep Stall condition and remained in that condition until impact.

    To recover from deep stall is to set engine to idle to reduce nose up side effect and try full nose down input. If no success roll the aircraft to above 60° bank angle and rudder input to lower the nose in a steep engaged turn.

    Pilots lack of familiarity and training along with system malfunction contributed to this terrible accident. Also the following contributed to the accident

    (1)the absence of proper immediate actions to correct the Deep Stall

    (2) Insufficient and inappropriate situation awareness disabling the co-pilots and the captain to become aware of what was happening regarding the performance and behaviour of the aircraft

    (3)lack of effective communication between the co-pilots and the captain which limited the decision making processes, the ability to choose appropriate alternatives and establish priorities in the actions to counter the Deep Stall

    During most of its long descent into the Atlantic Ocean, Airbus A330-203 was in a stalled glide. Far from a deep stall, this seems to have been a conventional stall in which the Airbus A330-203 displayed exemplary behavior. The aircraft responded to roll inputs, maintained the commanded pitch attitude, and neither departed nor spun. The only thing the Airbus A330-203 failed to do well was to make clear to its cockpit crew what was going on.Its pitch attitude was about 15 degrees nose up and its flight path was around 25 degrees downward, giving an angle of attack of 35 degrees or more. Its vertical speed was about 100 knots, and its true airspeed was about 250 knots. It remained in this unusual attitude not because it could not recover, but because the co-pilots did not comprehend, in darkness and turbulence and amid a tumult of conflicting warnings of mysterious system failures, the actual attitude of the aircraft. The co-pilots held the nose up. If the co-pilots had pushed the stick forward, held it there, and manually retrimmed the stabilizer, the airplane would have recovered from the stall and flown normally.

    Practicing recovery from “Loss of Control” situations and improve flight crew training for high altitude stalls (simulator training usually has low altitude stalls which are significantly different due to energy status of the aircraft) should become the mandatory part of recurrent training.

    Link to this
  11. 11. Thayabharan 8:12 am 08/1/2012

    Air France complain that the pilots did not have enough time to analyze the situation. Gravity does not allow timeouts, so a round table could be called together to thoroughly discuss the situation to find out what went wrong? The pilots missed the cardinal rule that first they must fly the airplane, and after start analyzing the situation, since a falling airplane is not going to wait for them. If they did not understand the instruments, then instead of pondering on it they should have come to the quick conclusion that they did not understand those instruments, and apply the unreliable airspeed procedure clearly prescribed for that situation, which is a blind, given thrust and pitch setting for the given configuration, and let the airplane fly itself, and only after get to analyzing what went wrong, and by the time they finished, the root-cause (pitot icing) would have probably cured itself. It was the safe solution to the problem, but not applied.
    Since then Air France changed the Thales pitots (which were already slated for change), its training, including cockpit resource management, as a tacit acknowledgement of its own fault. Further, the aircraft performed exactly as it was designed and described when the stall warning cut out at the end of valid values (extreme stall), except the pilots did not know it. Unfortunately, it happens too often with catastrophic results that pilots are not familiar with the systems of their own airplane, such as in the case of American Airlines 587 over Queens, which is clearly the airline’s fault. Of course, afterwards it is easy to make various arguments of how the situation could have been saved by others, but in case pilots do not or cannot fly by the book, the blame is solely theirs. Air France also argues that the stall warning system in the A330 is too “confusing”. Well, it must be realized, that an airplane is quite a confusing piece of machinery. It is full of buttons, levers, all kinds of red, yellow, green lights with buzzers, and a host of other miracles inside, which can look very confusing indeed, but it is the pilot’s duty to reign on them, or not to be pilot. You simply cannot be a pilot if you are only familiar with the fun part.
    With respect to the big confusion, the question is, was this stall warning device the straw that broke the back of the camel? In other words, if the pilots would not have had to remember just this one thing that the stall warning stops in extreme situations, then confusion would not have set in, and they could have perfectly saved the situation? Well, the A330 is a new generation, highly automated piece of equipment with drastically simplified controls, displays, and instrumentation compared to older models. Still, pilots with the same human capabilities as the ones on flight 447 could very well stay in full control in those planes, and many times acted heroically saving situations much graver than where the plight of 447 started, such as UA flight 232 at Sioux City, or Air Canada 143, the Gimli Glider. If those pilots could perform well in those older, much more complicated aircraft in tougher situations, then there is no excuse for the pilots of 447 to be confused in a generally much simpler and easier-to-fly aircraft.
    Some say the A330 is a “video-game” aircraft because of its side-stick control, which does not match up in real hard situations. But who can say that after the brilliant ditching of US Airways 1549 on the Hudson River? It was an A320 with the same side-stick control, and it matched up with the hardest situation very well, of course, with a seasoned pilot at the controls. The A330 is not a video-game aircraft, it is the airlines that make it a video-game by cutting corners, taking advantage of its superior automated capabilities thinking that it flies by itself, and no training and no knowledge of even the basics of the principles of flying is required in them for their pilots, as was demonstrated by the pilots of flight 447, who seemed to be incapable to react even on a basic level to the phenomenon of the aerodynamic stall. Evidently, it might not be what Airbus had on its mind designing the aircraft. They might have meant the best of the two, an airplane with superior controls, matched with seasoned pilots with superior education in the principles of flying and the handling of hard situations, best of the best, as airlines are prone to boast of their flying personnel, to represent quality improvement in flying safety by this pairing. Now, if this piece of equipment falls in the hands of the airlines who use it as a video game to save training costs, telling only their pilots that “if the red light on the right side blinks, just pull the stick back as hard as you can, and let the system do the rest”, they can get away with it as long as everything is normal, the airplane is good enough for that, but in unforeseeable situations, such as the one en-route to Paris on that night, without any independent knowledge of flying in general, the video-gaming with the aircraft may ultimately come to a fatal end.
    However, beyond the reasonings and explanations there is still some eeriness about the crash, taking in consideration that Air France is certainly no third-world airline, and still three of their pilots just sat there in daze squeezing the control stick, barely being able to do more than commenting on how the airplane was falling out of the sky until crashing into the Atlantic, the arrival of the captain in the cockpit not making much a difference either. The question might arise whether weren’t they in a mentally incapacitating state of shock and disbelief? Whether do (or can) airlines test their pilots of how well they can keep their mental stability under the duress of a catastrophic situation? Wasn’t it a twist of fate unknown to anyone that three pilots prone to loose their cool and judgement in life-threatening situations got together in one cockpit and got into this situation, as stipulated by Murphy’ law, a true scourge of aviation?
    None of it seems to be the fault of the airplane, which seems to need only matchingly good, trained pilots to give superior performance for the good of the flying public.

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Holiday Sale

Black Friday/Cyber Monday Blow-Out Sale

Enter code:
HOLIDAY 2014
at checkout

Get 20% off now! >

X

Email this Article

X