As they work to unravel the mystery of Air France Flight 447, aviation analysts and pilots are now urging investigators to focus attention on the plane's tail fin, known as the vertical stabilizer, in addition to the design of the Airbus's computerized flight controls.
The vertical stabilizer is one of the largest intact pieces of the plane recovered so far, and the Times of London reported this week that "one of the 24 automatic messages sent from the plane minutes before it disappeared pointed to a problem in the 'rudder limiter,' a mechanism that limits how far the plane's rudder can move."
Aviation analysts note that several Airbus 300 series jets have had tail fin and rudder problems in the past. (The rudder is the flight control on the vertical stabilizer, or tail fin.)
The most deadly event was the 2001 crash of American Airlines Flight 587, in which 265 people died when the plane's vertical stabilizer tore off soon after takeoff. Investigators blamed that crash on "over use" of the rudder pedal by the co-pilot. But critics note that just prior to take off, that plane also had problems with a computer tied to the rudder. That computer was reset by a technician prior to takeoff.
Request to look again at Flight 587
In light of the circumstances surrounding the loss of AF447, some analysts and pilots are now calling for the National Transportation Safety Board (NTSB) to reopen the investigation of AA587 in light of potential similarities between the two crashes. They're also calling for a thorough review of all past vertical stabilizer, rudder, and computer incidents on Airbus planes.
"Absolutely the NTSB should reopen the investigation," says Lee Gaillard, an aviation analyst in Saranac Lake, N.Y. "Given the implications that seem to be surfacing in this Air France crash involving the rudder and potential computer problems, the whole [Airbus] computerized system needs to be taken a very close look at."
French investigators see progress
French investigators Wednesday said they're now developing "an image that is progressively less fuzzy" about what happened that stormy night June 1 over the Atlantic Ocean, when Flight 447 disappeared.
Judging from the wreckage and bodies recovered so far, and the few clues sent electronically in the last four minutes of the flight, investigators believe the Airbus 330-200 jet probably broke apart in flight, then scattered over several miles.
"We are in a situation that is a bit more favorable than the first days," Paul-Louis Arslanian, head of the French civil aviation safety agency told reporters at a press conference in Paris on Wednesday. "We can say there is a little less uncertainty, so there is a little more optimism ... [but] it is premature for the time being to say what happened."
He cautioned that the search is continuing for the flight data recorders, which could hold critical clues as to what happened.
Investigators so far have focused on the potential that speed sensors, called Pitot tubes, could have iced up and provided faulty information to the flight control computers. In the past week, Air France has replaced the Pitot tubes on all of its Airbus 330s.
That theory was gleaned from the burst of automated messages about mechanical events sent during the last four minutes of the flight. Most of the messages appear to be linked to "incoherent" speed readings, which then affected other systems of the plane, Mr. Arslanian said.
History of Flight 587 probe
But the report that one of those automated messages also indicated problems with the rudder limiter has renewed concerns first made public during the AA587 crash investigation in 2001.
At the time, a group of American Airlines pilots presented to the NTSB a 68-page dossier documenting incidences of uncommanded rudder movements in the A300 series jets.
The NTSB eventually concluded the cause of the crash was not a computer problem, but the co-pilot over-using the rudder pedal during some wake turbulence.
The animation in this NTSB simulation shows the pilots pushing the rudder pedals abruptly and sharply to the floor, which is what investigators believed caused the plane to lose its vertical stabilizer and crash.
But some pilots familiar with the A300 series jets still doubt that conclusion. They say that it would be physically very difficult for a pilot to make the kind of abrupt rudder pedal movements indicated in the simulation, particularly while going 250 knots, which the NTSB indicated was the plane's speed at the time.
"I just don't see the co-pilot making the kind of abrupt movement at that speed," says an A330 pilot with more than 20 years experience in military and commercial aviation. "At 250 knots I don't think you can move the rudder pedal that far. It's going full deflection [which means it would be extremely difficult to push down as far as the simulation asserts]."
This pilot suggests that a computer malfunction could also have caused the rudder to fluctuate wildly, particularly because of the past incidences of uncommanded rudder movements in some Airbus jets.
But NTSB investigators note that the investigation took almost three years, and they say potential computer problems were thoroughly investigated at the time.
"In that case, the flight recorder was the source of detailed information that indicated how rapidly and frequently the rudder was moved. Then it was just a matter of aerodynamic calculations to see [what caused the tail to tear off,]" says Richard Healing, who was a member of the NTSB at the time of the investigation. "We were totally convinced the pilot's feet were on the pedals and he was moving the controls manually."
But the A330 pilot and others note that Airbus's computerized flight controls are highly complex and have resulted in other uncommanded rudder and other component movements.
Two other unusual incidents
In addition to the rudder incidents documented by American Airlines pilots prior to the 2001 crash of AA587, last year two Qantas Airlines Airbus 330s experienced uncommanded pitches nose-downward.Nine months before that, in January 2008, an Air Canada Airbus 319 also "experienced a sudden upset when it rolled uncommanded 36 degrees right and then 57 degrees left and pitched nose-down," according to a report on file at the NTSB.
As a result, some pilots and analysts would like to see a more thorough investigation of whether a potential computer glitch may have played a part in the dramatic rudder movement during the AA587 crash. They believe that could hold a key to help understand whether a similar "uncommanded" movement could have played a part in the Air France plane suddenly breaking apart and losing its vertical stabilizer mid-air during a routine flight.
"Airbus has every single incentive to do whatever it takes to find out what could have gone wrong to be sure that information gets in the right hands to prevent further accidents," says Kevin Mitchell, chairman of The Business Travel Coalition in Radnor, Pa. "On the other side of the issue, if they have fundamental structural or design flaws and billions of dollars invested, then it doesn't get any worse in terms of strategic prospects. So organizations like the NTSB and the Federal Aviation Administration must be on the alert for potential conflicts of interests."
Airbus has not returned calls asking for comment on this story. But in a conversation earlier this week, an Airbus spokesman speaking on background cautioned against continued speculation about the cause of the accident.
"All we know at the moment is that, yes, there's a piece of the rudder that's been found and that we know that there were some maintenance messages sent from the aircraft, and one said there was inconsistency with air speed measurements. That's all we know, and it's not enough to build a picture of what happened to the aircraft, which is why it's so important to find the missing black boxes," he said.