The first sign of trouble was a glitch that appeared in the air speed sensors.
Inside the sleek cockpit of Air France Flight 447, according to aviation experts, the crew would within minutes be confronted with a cascade of mysterious system failures.
The atmosphere of a routine international flight would vanish. Warning lights would be flashing and alarms would sound as one high-technology system after another of the highly automated jetliner began going off line.
At the same time, the Airbus A-330 was flying through turbulence caused by a tropical storm rising from the equator 35,000 feet below that was capable of jostling the pilots so strongly that they may have had difficulty reading the cockpit instruments.
Were the wings level? What speed were they traveling? Why were the computers reducing their ability to move the plane's control surfaces? As the pilots frantically worked to understand what was happening during the chaotic final minutes before the plane crashed into the Atlantic Ocean on May 31, the sky was illuminated by sporadic flashes of lightning.
"It would tax a really experienced pilot," said Robert Ditchey, a pilot, aeronautical engineer and former U.S. airline executive. "All hell is breaking loose."
French authorities investigating the accident that claimed the lives of 216 passengers and 12 crew members are still at the early stages of determining the cause. But preliminary evidence has already made it clear that the flight crew was battling a multitude of problems during the final four minutes.
The jetliner sent out 24 automated messages about abnormalities to Air France's maintenance department in the minutes before the crash. The airline has not released that data, but some information has been leaked to outside aviation experts, and ultimately to the news media.
The initial event, based on what is known so far, was the simultaneous failure of three sensors that determine airspeed. The sensors are called pitot tubes, a decades-old technology that pilots still depend upon.
Pilots rely on several instruments for speed information. A system of laser gyroscopes and a separate GPS system can tell pilots their ground speed. But the pitot system provides data about the speed of air flowing across the wings, crucial to maintaining lift.
If all that information disappears, as apparently happened, Air France provided pilots with an emergency procedure checklist.
"It's a very long checklist," said Guillaume Pollard, an Air France pilot and a member of one of the three unions representing the airline's pilots. "You have to take the book out, look for the right page, be available to read what is written and understand what is written. So you have to have practically nothing else to do but to read this list."Airbus officials warned against reading too much into the failure of the airspeed sensors.
"We know there has been an indication of unreliable airspeed, but we don't know how it is linked to other events," said Airbus spokesman Stefan Schaffrath. "What we know for sure is that data transmitted, including the unreliable airspeed, do not explain the accident of AF447."
Louis Jobard, the president of one of the Air France pilots unions, said the loss of the airspeed data "doesn't justify an accident." Nonetheless, the union obtained a pledge from Air France that it would immediately replace at least two of the three pitot tubes on each of the airline's jets.
When the pitot tubes failed, Flight 447's autopilot disengaged, forcing the crew to fly the plane "by hand." Pilots would need to keep the wings and nose of the plane at their proper attitude and maintain proper lift, tasks made difficult if airspeed is uncertain and turbulence is buffeting the plane.
"One of the toughest things for a pilot to do if you have multiple emergencies is trying to determine which is the most critical one. That is the one you deal with first," said Amos Kardos, a veteran U.S. commercial pilot. "It is tough to stay on top of it if there is a multiple failure scenario."
Kardos and others say flying a plane manually should not be difficult in normal circumstances, but Air France 447 was apparently in a tough fix.
After the autopilot disengaged, at least two other systems went off line.
The system of laser gyroscopes that provides crucial information about the pitch and roll of the plane failed, meaning that the pilots may have lost their primary electronic image of the horizon.
"Not only did they lose their speed readings, but they may have lost their artificial horizon," Pollard said. "And I can tell you that you might as well give up. In the day, it's fine, when you have the horizon in front of you."
The Airbus A-330, like most modern jets, has a backup system that would have provided the information. Whether it was available or also compromised is unknown.
After the laser gyroscope system failed, at least two additional problems materialized.
The stabilization system for the rear control surfaces went off line. As that system failed, the plane's computers automatically reduced the pilots' ability to move the plane's control surfaces -- technically known as going to "alternate flight law."
Ditchey said alternate flight law is meant to protect the plane by reducing the ability of pilots to make a mistake, but ultimately it also may limit the ability of pilots to save the plane.
The flight laws that Airbus programs into its computer embrace a fundamentally different approach to flying than those used by Boeing, which gives humans far greater authority to fly the plane, according to a wide range of pilots interviewed by The Times.
"Airbus is a little more autonomous," Kardos said. "Airplanes are designed sometimes by engineers for engineers."
One critical piece of the accident puzzle involves whether the flight control system also had some role in causing the plane to break apart before it actually hit the water. The vertical stabilizer, or tail, was recovered intact, similar to how the tail of an American Airlines A-300 that crashed in New York in 2001 was found.
The concern is that the flight control system may have applied excessive aerodynamic forces on the tail assembly, ripping it off the plane. That is essentially what occurred in the earlier accident.
"It doesn't surprise me that the plane might have broken up before hitting the water, said Vincent Fave, a former investigator with the French agency that studies air disasters. "I thought that before seeing the photos, because if we were in a scenario of having control difficulties in very high altitude, there's little chance of arriving to the water whole."