Article from today's New York Times, which links to above threads.
Investigators are pursuing a theory that excessive air speed -- potentially spurred by ice building up on electronic airspeed sensors -- contributed to the ocean crash of an Air France Airbus A330 amid heavy storms Monday, according to two industry officials familiar with the details.
The developments helped lead Airbus late Thursday to remind all airlines to follow certain backup procedures any time that pilots suspect their airspeed indicators are malfunctioning, according to the officials.
The Airbus announcement doesn't provide new details of the crash of Air France Flight 447. But it reflects the investigators' suspicion that the sensors -- also implicated in at least two other fatal airline crashes and numerous other incidents -- were involved, possibly as the first stage of a series of electrical and mechanical malfunctions aboard the jetliner. The reminder advises pilots to use backup devices including GPS systems to check their airspeed if readings from the primary indicators seem awry.
Investigators believe that the so-called pitot tubes may have iced up as the Air France plane with 228 people on board flew through a ferocious thunderstorm that could have included hail and violent updrafts, the two industry officials said.
Industry officials stressed it is too early to draw definitive conclusions from the scant data available, and theories of the crash could change in coming days. Investigators, for example, haven't ruled out the possibility of a fire or other electrical problems that could have led to the emergency. They also don't know what other actions the crew may have taken during roughly four minutes during which the plane apparently was going through a major storm.
The pitot devices have backup systems and are supposed to be heated to avoid icing. But tropical thunderstorms that develop in the area the plane was flying are full of ice at high altitudes, and air temperature at the plane's altitude is well below zero. A theory is that ice from the storm built up quickly on the tubes and could have led to the malfunction whether or not the heat was working properly.
If the tubes iced up, the pilots could have quickly seen sharp and rapid drops in their airspeed indicators, according to industry officials.
At this point, according to people familiar with the details, an international team of crash investigators as well as safety experts at Airbus are focused on a theory that malfunctioning airspeed indicators touched off a series of events that apparently made some flight controls, onboard computers and electrical systems go haywire.
According to people familiar with the thinking of the investigators, the potentially faulty readings could have prompted the crew of the Air France flight to mistakenly boost thrust from the plane's engines and increase speed as they went through what may have been extreme turbulence. As a result, the pilots may inadvertently have subjected the plane to increased structural stress.
It isn't known why other planes flying through such storms haven't suffered from such severe problems, but airline crashes often result from a chain of unusual events, not just a single trigger. Brazilian Air Force officials say three other jetliners flew in the general region around the same time; other airlines have reported no abnormalities in their planes' flights.
Investigators also are struggling to understand another big mystery: how the aircraft, equipped with its own weather-scanning radar, ended up engulfed in what is believed to be such extreme weather. The storm's exact force remains unclear, because the mid-Atlantic region isn't covered by precise ground-based weather radar.
Problems with pitot tubes have been implicated in many air accidents, and ice blockage wouldn't be unprecedented in commercial aviation.
Pitot-tube icing was suspected in the October 1997 crash of an Austral Lineas Aereas DC-9 in Uruguay that killed all 74 people onboard. Flight-data recorder readings showed anomalous airspeed readings and that the crew had adjusted settings in ways suggesting they thought they were flying much slower than the plane, built by McDonnell Douglas, was actually moving. Investigators concluded those settings caused the pilots to lose control of the plane, which plunged into swamps, according to the Aviation Safety Network, a crash database.
A Continental Airlines MD-82, also built by McDonnell-Douglas, skidded off the runway at New York's La Guardia Airport in March 1994 after the crew aborted their takeoff due to strange airspeed readings. Investigators from the National Transportation Safety Board later found the crew had failed to comply with checklist procedures to activate the pitot tub- heating system, allowing them to get clogged with ice or snow. Nobody was killed in the incident.
The NTSB cited similar issues with incidents during two flights of Boeing 717 jetliners in 2002 and 2005. Nobody was killed in those events, in which the planes encountered problems when the pitot tube heating system were temporarily inactive for reasons that were never determined.
In February 1996, a Boeing 757 crashed shortly after takeoff from the Dominican Republic, killing all 189 people onboard. Flight-data and cockpit recordings showed the crew got confused by conflicting speed readings and stalled the plane, which plunged into the ocean, according to Aviation Safety Network.
Investigators later concluded that wasps may have nested in the pitot tubes as the plane, operated by Turkish carrier Birgenair, sat grounded for several days. The tubes are supposed to be kept covered when a plane is parked, but a witness recalled seeing them exposed.
Wasp-nesting in pitot tubes was again cited in a March 2006 incident, where the crew of a Qantas Airways Ltd. Airbus A330 slammed on the brakes during takeoff from Brisbane, Australia. Nobody was injured, according to the Australian Transport Safety Board. Airbus is a unit of European Aeronautic Defence & Space Co.
The Air France jetliner was equipped with its own radar system, which normally suffices for letting pilots navigate through bad weather. But it doesn't always detect trouble, specialists say, or accurately depict the worst areas of turbulence. The signals can get absorbed by heavy rain or end up showing ground clutter, for example, preventing pilots from getting a clear picture of conditions in front of them.