But some Air France pilots aren't waiting for a definitive answer. With investigators looking at the possibility that external speed monitors iced over and gave dangerously false readings to cockpit computers in a thunderstorm, a union is urging pilots to refuse to fly Airbus A330 and A340 planes unless the monitors - known as Pitot tubes - are replaced.
An internal memo sent to Air France pilots Monday and obtained by The Associated Press urges them to refuse to fly unless at least two of the three Pitot sensors on each planes have been replaced.
The instruments have drawn attention because of other incidents in which the monitors have iced over at high altitudes.
The leader of another pilots' union, however, said Monday that Pitot troubles probably didn't cause the Flight 447 disaster.
Searchers must move quickly to find answers in the cockpit voice and data recorders, because acoustic pingers on the boxes begin to fade 30 days after crashes.
While large pieces of plane debris - along with 16 bodies - has helped narrow the search, it remains a daunting task in waters up to 1.5 miles (2.5 kilometers) deep and an ocean floor marked by rugged mountains.
"Finding the debris helps because you can eliminate a large part of the ocean," said U.S. Air Force Col. Willie Berges, chief of the U.S. military liaison office in Brazil and commander of the American military forces supporting the search operation.
But ocean currents over the eight days since the disaster have pushed floating wreckage far and wide, complicating the search, Berges said. "In the sense that as the debris drifts away, you're not sure exactly where the black boxes or other parts of the aircraft are on the bottom of the ocean."
The U.S. Navy has helped locate black boxes in difficult situations before: pings from an Adam Air jet that crashed Jan. 1, 2007, off Indonesia's coast were picked up 25 days later by a navy team.
The two towed pinger locators the U.S. is sending are expected to arrive in Brazil late Monday and will be dropped into the ocean near the debris field by Thursday, Berges said. The search is focusing on several hundred square miles (square kilometers) roughly 400 miles (640 kilometers) northeast of the Fernando de Noronha islands off Brazil's northern coast.
The listening devices themselves are five-feet long and weigh 70 pounds. One will be towed by a Brazilian ship, the other by a French vessel, slowly trawling in a grid pattern across the search area. The devices can detect emergency beacons to a depth of 20,000 feet (6,100 meters).
Cables attached to the devices lead to on-board computers, enabling a 10-person team that accompanies each device to listen for pings and to visually see them on a screen, like a radar spotting objects in air.
The French nuclear attack submarine Emeraude, arriving later this week, also will try to find the acoustic pings, military spokesman Christophe Prazuck said.
If the pings are located, French deep-water unmanned subs aboard the oceanographic survey ship Pourquoi Pas will attempt to retrieve the boxes from the ocean floor.
This area of the Atlantic Ocean is littered with floating garbage, vexing the initial search effort. Days after the plane went down, the weather let up and bodies began to surface, giving searchers more to go on.
Sixteen bodies were recovered Saturday and Sunday about 45 miles (70 kilometers) from where the jet was last heard from. Brazilian military officials said their earlier announcement of 17 was incorrect.
Searchers also spotted two airplane seats and debris with Air France's logo, and recovered dozens of structural components from the plane. They had already recovered jet wing fragments, and said hundreds of personal items believed to from passengers were plucked from the water.
President Luiz Inacio Lula da Silva said Brazil's military would do all it can to recover bodies for grieving families. "During this painful time it's not going to resolve the problem, but it is an immense comfort to know they can bury their loved ones," he said on his national radio program Monday.
France is leading the investigation into the cause, while Brazil focuses on the recovery of bodies and wreckage. The Ventose, a French military frigate now operating under Brazilian command, has brought aboard seven of the 16 bodies and about 30 pieces of debris that "most probably come from the plane," Prazuck said.
Brazil says the search area lies southeast of the jet's last transmission - automatic messages signaling catastrophic electrical failure and loss of cabin pressure. The messages mean Flight 447 likely broke apart in turbulent weather while flying from Rio de Janeiro to Paris. The location of the wreckage could mean the pilot was trying to turn around in mid-flight.
The L-shaped metal Pitot tubes jut from the wing or fuselage of a plane, and are heated to prevent icing. The pressure of air entering the tubes lets sensors measure the speed and angle of flight. An iced-over, blocked or malfunctioning Pitot tube could cause an airspeed sensor to fail, and lead the computer controlling the plane to accelerate or decelerate in a potentially dangerous fashion.
Air France said it began replacing the Pitot tubes on the Airbus A330 model on April 27 after an improved version became available, and will finish the work in the "coming weeks." The monitors had not yet been replaced on the plane that crashed.
An official with the Alter union, speaking on condition of anonymity because the memo was not publicly released, said there is a "strong presumption" among their pilot members that a Pitot problem precipitated the crash. The memo says the airline should have immobilized all A330 and A340 jets pending the replacement, and warns of a "real risk of loss of control" due to Pitot problems.
France's investigating agency said the messages suggest the plane received inconsistent airspeed readings from different instruments as it struggled in a violent thunderstorm.
But the secretary general of another French pilots' union, SNPL, said Monday the tubes were not likely the cause of the crash. Pitots are "a possible contributing factor," Julien Gourguechon said, but even without them, "we can make the plane fly."
Brazilian military officials declined to comment on the condition of the bodies or identify hundreds of recovered personal items, after some relatives said they felt devastated by Saturday's announcement that a laptop computer and briefcase containing a plane ticket had been found.
"We don't want to cause them more suffering," Air Force Col. Henry Munhoz said.
Recovered bodies and wreckage are being taken first to a military staging area at the Fernando de Noronha islands, and then to the northeastern coastal city of Recife for identification.
The Pentagon has said there are no signs that terrorism was involved. French officials have also said there are no signs, but that terrorism has not been ruled out. Brazil's defense minister said the possibility wasn't considered.
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