What is known about the crash of an Air France airbus on
1 June bears similarities with the little-noticed loss much earlier
of two computer-controlled passenger jets. Those two crashes raised
questions of whether the pilots or systems were really in
control.
Nobody knows what caused an Airbus A330-200 to enter the
Atlantic Ocean off the coast of Brazil on 1 June with the loss of
228 passengers.
If the black boxes of Flight AF447 are recovered – and a search
is still underway - it may be months or years before a probable
cause is known, if ever.
Some information is known already though, largely because of
data sent by the aircraft’s onboard systems to Airbus Industries in
France before the links went dead.
Airbus said this data showed that the pilots might have received
conflicting information about their speed. There was a “divergence
in airspeed measurement” by the onboard systems of the Air France
aircraft. This is one of the matters being investigated, said
Airbus.
Data to the onboard computers about air speed came from sensors
called pitot tubes, at least one of which was due for replacement.
French authorities have suggested that inconsistent air speed
readings are not dangerous.
France’s chief crash investigator told journalists at a briefing
near Paris that the failure of the air sensor to convey reliable
speed data might have kicked off a chain of events that led to the
deaths of all 228 people aboard.
The sensors had not been replaced with the improved units, said
Paul-Louis Arslanian, head of France’s air-accident investigation
agency, at a briefing outside Paris. “But that doesn’t mean that
without them the plane was dangerous.”
Whether conflicting messages to pilots from onboard systems is
potentially dangerous is a moot point.
Using evidence gathered by documentary film-makers, cockpit
voice recordings and various reports, Computer Weekly has
investigated two little-noticed air fatal crashes in which a
blocked Pitot-static system set off a chain of events, leading to
the pilots receiving bewildering and conflicting messages from the
onboard systems.
Two fatal crashes of passenger aircraft show that it is
dangerous for pilots to have conflicting information and warnings
in the cockpit.
The recovered black boxes from two computerized commercial
airliners that crashed in the sea in the 1990s show what it's like
in the cockpit when the Pitot-static system is blocked.
Investigators of the crash of an Air France Airbus in the sea off
Brazil say that blocked Pitot tubes could have been a factor in the
accident.
The two earlier crashes were
Birgenair
Flight 301 andAeroperú
Flight 603. Both planes were Boeing 757s which crashed into the
sea in 1996.
In each case the onboard systems told the pilots that they were
are flying too fast and too slow. Visual warnings were backed up by
audible warnings, and the shaking of their flying controls - the
stick shaker.
If pilots fly too slowly the aircraft may fall out of the sky;
if too fast the aircraft may break up.
Static systems include Pitot tubes and static ports which feed
data on air speed to the onboard systems which display information
to the pilot.
The auto-pilot also gets its information from the Pitot
system.
The pilots in the aircraft did not know that the Pitot-static
system was blocked. This was established only after the black boxes
- the cockpit and flight data recorders were recovered from the
sea.
Investigators of the loss of an Air France Airbus in the
Atlantic Ocean on 1 June 2009 say that the pilots might have
received
conflicting information on their air speed from the aircraft's
Pitot tubes.
Report from Sky News
There are similarities between the crash of Air France Airbus,
Flight AF 447, and the losses of Flights 301 and 603, all of which
claimed a total of 487 lives.
With the three crashed aircraft:
- There were concerns about the performance of the
Pitot system
- There were inconsistencies in the cockpit air speed data
- The auto-pilot was off (in the two earlier crashes this because
the auto-pilot switched itself off when reaching the limit of its
authority and then passing control to the pilots)
That said, it is impossible to say what caused the loss of 288
lives on Flight AF 447.
Air crashes are usually the result of a chain of events. If the
Pitot systems were blocked and caused conflicting air speed
information to the pilots of the Air France Airbus that might have
had nothing to do with the accident, or might have been only one
link in the chain.
In the losses of Flights 301 and 603, the blocked Pitot systems
were important factors, in part because of the confusion of the
pilots.
This is what happened in the cockpit when the pilots of Flight
301 and 603 were faced, unknowingly, with blocked Pitot
systems.
The information is taken mainly from the documentary series "Air
Crash Investigation" on National Geographic Channel, and from the
official accident reports, together with transcripts of the cockpit
voice recordings.
Related articles:
Pictures fromRex Features
.