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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