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Airbus Focuses More Closely on Flight 447 Pitot Tube Problems
Questions arise regarding device's design and Air France's maintenance procedures
John Loughmiller, Contributing Editor -- Design News, June 8, 2009
Information has surfaced regarding a problem with the sensors used to provide air-speed indications to the crew of doomed Air France Flight 447. The devices, called pitot tubes, have a history of not performing properly in icing conditions and their replacement was recommended by Airbus months before the crash of the Air France airplane.
The pitot tubes have heating elements included in their design and normally these elements are sufficient to keep the inlets of the tubes free of ice. However, the pitot tube used on the Airbus A330-200 may have a design flaw that allows it to become encrusted with ice during encounters with heavy freezing precipitation.
A British newspaper, The Telegraph, reported in its June 6 edition that Air France had not followed the Airbus recommendation to replace air-speed sensors on the doomed Flight 447 airplane, according to French crash investigators. Degraded air-speed indications from the sensors are therefore an area of very high interest as the investigation unfolds.
In another development, a telemetry transcript was leaked to the BBC. The unnamed source claimed it was a copy of the automated reports received from Flight 447 just before all communications were lost. The unverified information indicates the first fault report sent during the minutes prior to loss of all telemetry indicated an autopilot disconnect. The A/P disconnect could have been a manual action taken by the pilots or it could have been commanded by the computers. On automated airliners, computer-generated A/P disconnects occur when something is at variance with expected inputs from sensors, or if there's a conflict between two sensors delivering the same or similar data. The list showed the pilot's and co-pilot's airspeed indications disagreed.
Data that followed indicated a stream of cautions and warnings culminating in a loss of cabin pressurization that is common at the start of an in-flight breakup.
Because of the additional messages regarding failures and a degraded flight control system, there's also interest in looking at the consequences of a lightning discharge entering the aircraft via the radome.
An airliner has to demonstrate protection of the occupants, electronics and fuel tanks from lightning strikes in order to be certified for flight by both European and U.S. authorities. Investigators are concerned about what would happen if lightning bypassed this protection by striking the aircraft on the radome. An aircraft is vulnerable in this area because the radar signal would be attenuated if protective measures were included for the radome.
If a lightning strike occurred at the radome, the energy would search for an exit path and electrical cables eventually leading to devices outside the cabin and cockpit area may provide the path. The damage done to electrical buses and computers by a strike cannot be modeled well because there are so many variables. It's generally conceded however that once the enormous energy of a lightning strike is inside an airliner, bad things are likely to happen.
In another development, debris and bodies have been found. The debris included seats that were traced to Flight 447 via their serial numbers. Now that the crash site has been located, there's at least a small chance that the cockpit voice recorder and flight data recorder may be found. It will be dicey however, since the water depth at the crash site would put the black box's underwater sonar pingers at the extreme limits of their range.
Contributing
Editor John Loughmiller is an Electronics Engineer specializing in Single
Channel Per Carrier communications systems and control logic system design for
automated communications devices. He's also a 4,500 hour commercial pilot,
flight instructor, aircraft owner and is a Lead Safety Team Representative for
the Federal
Aviation Administration.
Read additional in-depth Air France crash coverage at Flightglobal:
Crashed A330's Vertical Fin among Recovered Wreckage
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I think the vertical stabilizer snapped off first, causing the plane to go into a flat spin or a dutch roll, either of which would cause a disparity in the pressure on the pitot tubes on either side of the plane. This would explain the difference among the speed readings. There seems to be a problem in the connection between the tail fin and the fuselage on all Airbuses. Re-designing and retro-fitting the whole fleet of Airbuses w/ new tails, is much more expensive than their insurance premiums going up because of the crash. Replacing the pitot tubes they were going to replace anyway, is much cheaper, but they should make the tail more robust, and not rely on a software controlled rudder limiter. Maybe use two or three short vertical stabilizers instead of one long one, so there's less leverage against each one. France may drag their feet about finding the black boxes. Socialism creates conflicts of interest. Think of the US government looking the other way if GM made an unsafe auto, now that they help run GM. Remember Air France flight 296 in the '88 Paris air show? It came out 10 years later, the black boxes were switched to make it seem like the pilots fault!
Joe Perkins - 2009-28-6 19:03:51 EDT -
'Air France had not followed the Airbus recommendation to replace air-speed sensors on the doomed Flight 447 airplane' ----Engineering decision or ? decision -Very SAD....passemgers & crew did not desrve this fate
RAJ SEELA RAJ - 2009-10-6 00:29:07 EDT
