Investigators are pursuing a theory that excessive air speed -- potentially spurred by ice building up on electronic speed 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 airlines with any Airbus planes that their pilots should check backup systems including GPS any time they suspect their airspeed indicators are malfunctioning, according to the officials.
The Airbus announcement provides scant 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.
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 thunderstorm that could have included heavy rain 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 in which the plane apparently was going through a major storm.
The pitot devices -- the name, pronounced PEA-toe, refers to the sensor and its housing -- are supposed to be heated to avoid icing. But tropical thunderstorms that develop in the area where the plane was flying often form tiny particles of ice at high altitudes, and air temperature at the plane's altitude is below zero.
A theory is that ice from the storm built up unusually 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.
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.
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 possibly extreme turbulence, according to people familiar with investigators' thinking. As a result, the pilots may inadvertently have subjected the plane to increased structural stress.
The sequence of messages automatically sent by the plane to Air France maintenance in the flight's last minutes -- from autopilot disconnect through flight-monitoring system failures, then flight-control failures and depressurization -- has helped fuel the investigators' theory.
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; the other airlines have reported no abnormalities.
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.
The potential for pitot tubes to be blocked by ice, and the confusion it can cause pilots, is well known. A brochure from one manufacturer that provides the devices to Airbus points out it has improved their power and drainage "in order to meet the Airbus extreme icing conditions specification."
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. The flight-data recorder showed odd 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 built by McDonnell Douglas skidded off the runway at New York's La Guardia Airport in March 1994 after the crew aborted takeoff due to strange airspeed readings. Investigators from the National Transportation Safety Board later found the crew failed to activate the pitot tubes' heating, allowing them to get clogged with ice or snow. Nobody was killed.
The NTSB cited similar issues with incidents during two flights of Boeing 717 jetliners in 2002 and 2005, in which the pitot-tube heating system was temporarily inactive for unknown reasons.
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.
Investigators concluded that wasps may have nested in the pitot tubes as the plane, operated by Turkey's 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.
The Air France jetliner was equipped with its own radar system, which normally suffices for pilots to 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, for example, preventing pilots from getting a clear picture of conditions.
In Rio de Janeiro, hundreds of people attended a service for victims, and the local governor attending warned of "serious civil responsibilities" for the crash on the part of Air France and Airbus. Some victims' families said they would form a commission to monitor the crash investigation.
—Antonio Regalado contributed to this article
No comments:
Post a Comment