What Caused the AirAsia Crash? The Likeliest Theory

On February 20, as crews were still struggling to raise the fuselage AirAsia flight QZ8501, Indonesia's top transportation official said the aircraft climbed at an extreme rate just before it crashed. The accident investigators have had time to study the flight recorder; we can assume they now know what went wrong on the doomed Airbus A320.

After being initially candid, why have Indonesian officials now decided that the preliminary report, due by the end of the January, will not be made public?

It can be claimed that release of preliminary findings could cause problems later if further analysis leads to a different conclusion. But there is a more likely reason. As time passes, public interest wanes. The media moves on to other matters. Whatever the fault is, when revealed after a lengthy delay, the public will react far less.

Similarity to the crash of Air France 447 is striking. In that crash, the ice-blocked pitot tube (a crucial pressure measurement instrument; example from an A380 pictured below) sent incorrect speed information to the plane's computers and to its pilots. Though the pilots could be blamed for not recognizing the speed information was bad, every pilot is trained to trust what the instruments say.

In my view, it is much too much to ask pilots to deal with bad speed information. The Air France pilots' union was of the same opinion when they told Air France that they would not fly the A330 unless the problematic Thales pitot tubes were replaced by reliable Goodyrich tubes.

The Thales tubes that caused the AF 447 crash were quickly replaced with Goodyear tubes on the A330s. Thales tubes were. however, allowed to remain on the Airbus A320. Though there had been a number of problems with the Thales tubes on the A330, there had be relatively few on the A320.

But a tube is a tube is a tube. If it causes a crash on an A330, doesn't it seem feasible that, even if it caused problems less frequently on the A320, that sooner or later, the tube will cause an A320 crash?

In my search for info about the pitot tubes, I ran across a remarkably interesting discussion on the Nolan Law Group web site. See: http://nolan-law.com/latest-air-france-crash-update-bereft-of-analy...

It suggests that, in the case of AF 447, as the pitot tubes began to block, the lower pressure in the pitot system signaled the fly-by-wire computer (and informed the pilots in the cockpit) that the plane was flying too slow. Thus the fly-by-wire system added power to speed the plane up, causing the plane to fly faster than it should have.

As the blockage increased, plane's speed continued to appear too slow. The fly-by-wire system added still more power. This, the discussion suggests, continued until the plane was flying much too fast. 

As a plane speeds up, more air flows across the wing. This additional airflow causes an increase in lift, which, if nothing is done, causes the plane to climb. But, if the autopilot is engaged, as the speed increases, the autopilot lowers to nose as necessary to keep the additional speed from resulting in a climb. If the speed continues to increase, the autopilot continues to lower the nose.

With a conventional autopilot, lowering of the nose by the autopilot is obvious to the pilots. But not on the autopilot of the fly-by-wire Airbus. At some point - according to this analysis - the autopilot's authority to push the nose down reached its limits, and it disconnected. No longer pushed down, the nose rose abruptly. Due to the speed buildup, the plane zoomed rapidly upward. But, with the nose up so high, the speed build-up quickly dissipated to a speed slower than was needed for flight, and the "angle of attack" - the angle at which the wing meets the oncoming air - became too extreme for the wing to work. With the wing producing little or no lift, the plane descended rapidly, even though the nose was elevated.

Indonesian investigators say the AirAsia plane zoomed up at an extremely high rate, and then stalled. An Indonesian weather official claimed the plane zoomed up due to an extreme updraft. An updraft would simply lift the plane, leaving its forward speed unchanged, and not cause a stall.

Airbus recommended back in 2006 that the original Thales pitot tubes (model - AA) be changed, either to a later version (model - BA). This was only a recommendation, not a directive. In retrospect, changing would have made little or no difference, as a later assessment by Airbus found the - BA tubes were no better than the - AA. The Goodrich tubes, however, were said by Airbus to perform better than the either the - AA or the - BA Thales tubes.

In 2009, following the AF 447 crash, the Air France pilots' union demanded the Thales pitot tubes be replaced, saying they would not fly the A330 unless the tubes were changed. Shortly thereafter, Airbus recommended all A330s and A340 have either three Goodrich tubes, or two Goodrich tubes and one Thales tube in a specified location. The Airbus recommendation did not extend to the A320. At about the same time, the FAA required tube replacement on all A330s and A340.

So, where do things stand now?

  • With the A330 and A340, the problematic Thales tubes have been replaced with Goodrich tubes.
  • Airbus has backup synthetic airspeed on the A380 and made it optional for other models
  • Boeing planes have had little trouble with pitot tubes. Even when speed troubles arise on a Boeing, natural feedback from the plane lets the pilot know what to do. Even so, Boeing has installed synthetic airspeed on the 787.
  • On the A320, the same Thales tubes have been problematic on the A330 have - at least until now - performed satisfactorily on the A320. On the routes, altitudes, and shorter flights the A320 flies, it may have less exposure to pitot tube icing conditions. This AirAsia crash may be the exception..
  • Accident rate stats show no difference in safety between the A320 and the Boeing 737.
  • My guess is that this will turn out to be a pitot tube accident and that a change of pitot tube will - at some point - be called for on the A320. But with no significant past pitot tube trouble with the A320, there is no reason not to fly it confidently in the meantime. If the accident turns out to be due to some other cause, again, the record of the A320 gives us reason to fly it with confidence. Emotion does not always follow logic, and if help is needed in feeling comfortable when flying, help is available at www.FearOfFlying.com

Tom Bunn, L.C.S.W., is a retired airline captain and licensed therapist who has specialised in the treatment of fear of flying for over thirty years. He is the author the bestselling book on flight phobia, SOAR: The Breakthrough Treatment for Fear of Flying. His company, SOAR, Inc., founded in 1982, has helped more than 7,000 clients control fear, panic, and claustrophobia.

image: Wikipedia/Albion, Wikipedia/David Monniaux

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