The Air France Crash: More Questions Than Answers

The crash of Air France flight 447 from Rio de Janeiro to Paris on the last day of May is bad news for a number of reasons. The deaths of all 228 people on board make it the worst air disaster since 2001. And while advancing technology has enabled investigators to recover a limited amount of flight data through a remote data link that was operating at the time of the crash, the deep waters where the plane went down may prevent the recovery of the "black box" containing voice and detailed data recordings.

What do we know today, eight days after the crash? There were thunderstorms in the area that night, and early speculation centered on the possibility of a lightning strike to the plane. Although lightning hits planes hundreds of times a year, relatively little damage usually occurs and most modern aircraft can be considered essentially (though not totally) lightning-proof. Evidently, there was a satellite or other radio-mediated data link which was continually feeding certain types of flight data to the ground. Examination of this data shows that the flight speeds during the last minute or so of the flight became "incoherent," followed by a loss of cabin pressure and failure of electrical systems. While this information will be helpful in deciphering what went wrong, it apparently lacks the detail that flight data recorders can preserve. One can imagine a day when such radio links will take the place of, or at least duplicate, the capabilities of flight data recorders so that mechanical recovery of the black box will no longer be so urgent. The black box is designed to emit a sonar signal for 30 days after the crash, so the underwater recovery crews gearing up to find it are operating under tremendous time pressure, not to mention the water pressure at depths exceeding 20,000 feet. The box may never be found.

Recent news reports have focused on the fact that the plane's Pitot tube, the device that measures airspeed, had not yet been replaced with a newer model as the plane's manufacturer Airbus recommended. A Pitot tube is a small tube that faces directly into the airstream. The difference in pressure between the air inside the tube (which is blocked off and registers what is called "stagnation pressure") and the ambient or "static" air pressure, is an indication of airspeed, which is the most important kind of speed to know about when you are trying to fly a plane. These days, when most parts of a flight except for landing and takeoff are under automatic control, the airspeed data from the Pitot tube forms part of an elaborate computer-controlled feedback loop that maintains constant speed, altitude, and other flight characteristics.

The old saw about computers regarding "garbage in, garbage out" goes double when a feedback loop is involved. If enough ice forms on a Pitot tube to plug up the entrance, the indicated airspeed goes way below what is actually the case, and either the automatic pilot guns the engines inappropriately or the real pilots may take incorrect action based on faulty airspeed data. This is exactly what happened to an Argentine DC-9 flight in 1999, which resulted in a spectacular crash, killing all aboard.

Although I have no independent information on this, I hope that modern aircraft such as the Air France A330 that crashed have more than one means of measuring speed: either a second Pitot tube (which of course would be just as likely to ice up as the first one), or other means such as radar altimeter and speed measurements or GPS-based airspeed indicators. But whether the autopilot takes all these other inputs into consideration, and whether the real pilots do too, I don't know. Clearly, if the Pitot tube was involved in this crash, the right thing to do in the circumstances wasn't done.

All flight-critical Pitot tubes have heaters to prevent icing, but evidently the one on Flight 447 was deficient or non-optimal in some way, or else Airbus wouldn't have recommended replacing it. Of course replacements can be recommended for all kinds of reasons, some of which have nothing to do with safety. All that will come out in the investigation report, which will take months or more to complete.

Despite this crash, the general trend in air safety has been a positive one. More people fly every year, and so the safety record per passenger mile is even better than the raw statistics on crashes would indicate. But this record can be maintained only through the painstaking work of investigators, engineers, regulators, inspectors, and the pilots and crews who actually do the work. Most of the time the system works well, and the silver lining in every accident is the fact that it carries with it potential answers to problems that need to be addressed to improve safety even further. I just hope they are able to recover the flight data recorders in order to develop a complete picture of what went wrong, and to teach us how similar situations can be avoided in the future.

We will revisit this accident when more information is available, and in the meantime, our sympathy is with the relatives and friends of those who lost loved ones in this tragedy.

Sources: I drew on reports from Fox News at http://www.foxnews.com/story/0,2933,525117,00.html and an Associated Press report obtained from Yahoo News at http://news.yahoo.com/s/ap/brazil_plane, as well as the Wikipedia article "Pitot tube."