Airbus Focuses More Closely on Flight 447 Pitot Tube Problems

Information has surfacedregarding a problem with the sensors used to provide air-speed indications tothe crew of doomed Air France Flight 447. The devices, called pitot tubes, havea history of not performing properly in icing conditions and their replacementwas recommended by Airbus months before the crash of the Air France airplane.

The pitot tubes have heatingelements included in their design and normally these elements are sufficient tokeep the inlets of the tubes free of ice. However, the pitot tube used on theAirbus A330-200 may have a design flaw that allows it to become encrusted withice during encounters with heavy freezing precipitation.

A British newspaper, TheTelegraph, reported in its June 6 edition that Air France had not followed theAirbus recommendation to replace air-speed sensors on the doomed Flight 447 airplane,according to French crash investigators. Degraded air-speed indications from thesensors are therefore an area of very high interest as the investigationunfolds.

In another development, a telemetrytranscript was leaked to the BBC. The unnamed source claimed it was a copy of theautomated reports received from Flight 447 just before all communications werelost. The unverified information indicates the first fault report sentduring the minutes prior to loss of all telemetry indicated an autopilotdisconnect. The A/P disconnect could have been a manual action taken by thepilots or it could have been commanded by the computers. On automated airliners,computer-generated A/P disconnects occur when something is at variance with expectedinputs from sensors, or if there's a conflict between two sensors deliveringthe same or similar data. The list showed the pilot's and co-pilot'sairspeed indications disagreed.

Data that followed indicateda stream of cautions and warnings culminating in a loss of cabin pressurizationthat is common at the start of an in-flight breakup.

Because of the additionalmessages regarding failures and a degraded flight control system, there's alsointerest in looking at the consequences of a lightning discharge entering theaircraft via the radome.

An airliner has todemonstrate protection of the occupants, electronics and fuel tanks fromlightning strikes in order to be certified for flight by both European and U.S.authorities. Investigators are concerned about what would happen if lightningbypassed this protection by striking the aircraft on the radome. An aircraft isvulnerable in this area because the radar signal would be attenuated if protectivemeasures were included for the radome.

If a lightning strikeoccurred at the radome, the energy would search for an exit path and electricalcables eventually leading to devices outside the cabin and cockpit area mayprovide the path. The damage done to electrical buses and computers by a strikecannot be modeled well because there are so many variables. It's generallyconceded however that once the enormous energy of a lightning strike is insidean airliner, bad things are likely to happen.

In another development,debris and bodies have been found. The debris included seats that were tracedto Flight 447 via their serial numbers. Now that the crash site has beenlocated, 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 waterdepth at the crash site would put the black box's underwater sonar pingers atthe extreme limits of their range.

ContributingEditor John Loughmiller is an Electronics Engineer specializing in SingleChannel Per Carrier communications systems and control logic system design forautomated communications devices. He's also a 4,500 hour commercial pilot,flight instructor, aircraft owner and is a Lead Safety Team Representative forthe FederalAviation Administration.    
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