For an air crash in which the ‘black boxes’ or any trace of the 228 victims may never be recovered, much has been learned about what happened to AF447 the Air France A330-200 jet that crashed in the mid-Atlantic early on 1 June.

This is because of a series of automated status messages relayed via satellite from the flight to the airline’s operational base in Paris.

And it is the reason why Qantas and all other A330 operators received an old fashioned telex message overnight recommending strict adherence to Airbus’s recommendations for flights where the pilots get unreliable airspeed indications from the instruments.

The advisory says the recommended speeds and the attitude of the jet need to be constantly maintained in these situations.

The ACARS or Aircraft Communications Addressing & Reporting System messages sent from AF447 to Paris provide almost real time monitoring of aircraft performance for maintenance purposes.

This screen grab from france2 TV last night shows a discussion of part of the ACARS message log from AF447
This screen grab from france2 TV last night shows a discussion of part of the ACARS message log from AF447
Each line logs an ACARS message which itself is a string of code in letters and numbers not shown here. The messages do not necessarily arrive at the airline base in real time but are often stored for transmission in a package at regular intervals especially if a ground receiving station will come within range in a reasonable time. The last message of any type sent from AF447 is believed to be at the top of this list, and contained an advisory of vertical cabin speed, thought by the investigators to have been generated by depressurisation that occurred just before or after the aircraft broke apart in flight.
Each line logs an ACARS message which itself is a string of code in letters and numbers not shown here. The messages do not necessarily arrive at the airline base in real time but are often stored for transmission in a package at regular intervals especially if a ground receiving station will come within range in a reasonable time. The last message of any type sent from AF447 is believed to be at the top of this list, and contained an advisory of vertical cabin speed, thought by the investigators to have been generated by depressurisation that occurred just before or after the aircraft broke up.

That and similar systems are expected to evolve in the near future to something able to replace voice and flight data recorders by continually archiving that information at airllne operations bases.

But not yet, because the bandwidth, necessary satellites and other technical requirements aren’t quite ready and perhaps pilot associations aren’t either.

ACARS messages showed that AF447 flew at the wrong speed through severe turbulence until the jet broke apart, and then fell into the sea, leaving widely dispersed wreckage trails.

They show that as the flight began traversing the turbulent intertropical convergence zone, a belt of severe storms that extends from South America to Africa, the flight control mode used in normal Airbus operations was disconnected.

It has not been determined if the pilots made the disconnection, as would often be the case when manoeuvring between storm cells or if the disconnection was an automatic response to a failure in some of the flight control systems.

This change, from normal law or alternate law, as Airbus calls the main flight control modes, is something all A330 pilots are trained in managing. It is itself normal.

But then the dragons spring into view in the ACARS messages. A series of electrical faults begin to overwhelm the jet. They crucially include the ADIRU units which inform other systems that integrate data or move the control surfaces on the wings and tail as to how fast the jet is moving, how fast the air itself is moving up or down in turbulence, whether the jet is drifting, where it is pointed, and how level or inclined the nose of the jet is.

There are indications of ice forming over pitots that directly measure air flows.

Ice is not supposed to form at 35,000 feet. It is too cold there for water to exist, which means ice particles can’t stick to the surfaces. But it seems icing did occur.

AF447 was, suddenly, in all sorts of trouble, and the pilots, who never tried to use the radio in this final period, were getting inconsistent airspeed measurements according to the Airbus advisory.

It is not clear if this resulted in the jet flying too slowly, and falling prey to a high altitude stall, or too fast, and having critical control surfaces on the wing or tail broken by the severe updrafts and downdrafts it was encountering, which could rapidly lead to overstressing of the airframe and rupture.

(Nor is it known if a ground collision between this jet and another Air France plane at Paris in 2006 may have left an undetected weakness in one side of the wing.)

The very last message sent via ACARS is a vertical cabin speed alert, which was triggered by a cabin depressurisation that either happened immediately before or after the main body of the jet broke apart.

Search update: Despite false identification of debris and a fuel slick from shipping as having come from AF447 the search effort  remains focused on an area close  to the last known position of the jet before  it ceased transmitting ACARS messages.  Most of the wreckage  is now  assumed to have sunk  to  the sea bed.  Specialised submersible equipment  which  may  have  the best chance of detecting signals from the flight data and voice recorders  and  retrieving them  will not reach the  area until  next Friday.

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Peter Fray
Peter Fray
Editor-in-chief of Crikey
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