It seems likely that we will never know what failure of man or machine it was that sent a Paris-bound planeload of terrified souls plunging to their deaths in the Atlantic off the coast of Brazil.</p><p> The definitive answer is contained in recording devices lying on the ocean&rsquo;s floor at a depth that makes their recovery improbable, perhaps impossible.</p><p> Speculation has been that external sensors on Air France Flight 447 may have fed wrong information to the craft&rsquo;s computers, causing the jetliner to fly at a speed that resulted in its breaking up in midair.</p><p> According to one report, replacement of the sensors had been recommended by the manufacturer and was scheduled, but had not yet been accomplished on that plane.</p><p> For now, all that is certain is that 228 lives were ended in a manner so horrifyingly sudden and mysterious that it is sure to send a chill of unease through many travelers who board flights during weeks or months to come.</p><p> Circumstances were much different in the February crash of the turboprop commuter plane that went down near Buffalo, N.Y., killing all 49 passengers and crew on board and one person on the ground.</p><p> Investigation of that tragedy has focused largely on human error rather than any mechanical problem.</p><p> In a restaurant the other evening I happened to meet and strike up a conversation with a man who identified himself as a former commercial pilot.</p><p> He&rsquo;d been flight-trained in the military, and after leaving the service had taken employment with a major U.S. carrier, retiring at age 60 as was mandatory at that time. His had been a rewarding career.</p><p> &ldquo;We were well paid,&rdquo; he said. &ldquo;We had reasonable work schedules, and good benefits.&rdquo;</p><p> But with the escalating cost of aviation fuel and the competition for passengers in this time of economic distress, much has changed.</p><p> &ldquo;I have a son who&rsquo;s a pilot,&rdquo; he said, and named the airline. &ldquo;It&rsquo;s harder for a young person getting in the business today. First there&rsquo;s college to pay for. Then it can cost as much as $100,000 more to get qualified to fly.</p><p> &ldquo;On what he&rsquo;s paid, $30,000 or $40,000, he has a hard time supporting a family.&rdquo;</p><p> I mentioned the Buffalo crash, and reports that the pilot had failed three previous proficiency tests. He had flown from Florida to Newark, N.J., the place of that day&rsquo;s departure. His co-pilot, a 24-year-old earning just $24,000 a year, had made an all-night commute from her parents&rsquo; home in Seattle.</p><p>Perhaps due to their fatigue, or distracted by conversation as they entered the approach to Buffalo, they failed to react properly to the signal of a grave problem.</p><p> When the plane was at risk of stalling, the yoke, or &ldquo;stick,&rdquo; automatically shook in the pilot&rsquo;s hands. In that critical moment, the only saving response would have been to lower the plane&rsquo;s nose to increase air speed and restore lift. </p><p> &ldquo;They weren&rsquo;t paying attention. And he did exactly the wrong thing,&rdquo; said the man in the restaurant, pantomiming the action. &ldquo;He pulled the stick back.&rdquo;</p><p> From that moment, the plane and all in it were irrevocably doomed.</p><p> In the Air France tragedy, deferral of the recommended sensor replacement may have been a contributing factor. In the Buffalo crash, inexperience, inadequate training or lack of cockpit discipline would appear to be implicated.</p><p> Several years ago, I was at Chicago O&rsquo;Hare International Airport, booked on a connecting flight home. The hour was late. The scheduled boarding time passed. Passengers were getting impatient.