On the morning of June 1, 2009, Air France Flight 447 from Rio de Janeiro in Brazil to Paris failed to make any contact with Air Traffic Control after about 0200Z (“Zulu” time is UTC, so two hours behind Paris time). The aircraft had been flying in the region of a series of significant convective storms in the Intertropical Convergence Zone (ITCZ), southwest of the Cape Verde islands.
The airline said it had received 24 ACARS messages announcing various faults with the avionics, timestamped between 0210Z and 0214Z. This was the last known communication with the aircraft. ACARS is a digital data service, in which alphanumeric data is passed between airline personnel or electronic on the ground and the aircraft avionics and crew. These messages would have been transmitted by SATCOM, satellite communications, since the aircraft was presumed out of range of VHF radio communication at the time it was lost.
The ACARS messages between these two times were typical of failure and warning messages that would be displayed to the crew on the display used for that purpose (acronym is ECAM) and logged by the Central Maintenance Computer and available to maintenance personnel and Flight Operations Quality Assurance (FOQA) personnel after the flight has landed. There was another ACARS message, presumed to have been initiated by the pilots, of some significant turbulence timestamped about ten minutes before this sequence. It is common for pilots to send reports on actual weather conditions (called PIREPS) so that, amongst other things, following flights can be informed of those conditions.
There was no other information about AF447 at all, until wreckage and some bodies were found a week after the disappearance.
The French state television channel France 2 ran a program on Thursday 4 June, in which it displayed a print-out of the ACARS messages in cryptic terminology and explained to viewers to some degree what people thought they meant. The program was posted on the TV station WWW site for a while.
This event started a flurry of interpretation on, for example, the professional pilots’ forum PPRuNe. By the weekend, Sunday 7th June, there seemed to be consensus amongst aviation experts that, if the ACARS messages were veridical and their interpretation correct, there had been some issues with the air data systems, specifically the pitot systems which measure ram-air pressure and compare it with the static air pressure measured by the static systems, in order to determine the “indicated air speed”, IAS, which is displayed to the pilots (IAS is not the same as true air speed, for it is dependent upon the density of the air through which one is flying, and it is lower than true air speed when the density is low. It is, however, that indication of speed which is directly correlated with the aerodynamics of the airplane. For example, the speed at which the airplane will stall in level flight in a particular configuration is constant when expressed as IAS, although varying with altitude when expressed as true air speed). It was suggested that maybe the aircraft had encountered icing conditions which had overwhelmed the pitot heating and iced up the pitot tubes, and that maybe this had happened while the aircraft was in severe turbulence. This could have happened had the aircraft flown into a powerful convective storm cell, and indeed there were such cells, towering up to 50,000 ft, penetrating into the stratosphere. Had the crew indeed lost reliable air speed indications, which the ACARS messages hinted at, then they would have been trying to fly the aircraft on “pitch and power”: the engine thrust is set to a given level, and the pilot flying tries to keep the nose of the aircraft pointed at a particular angle to the horizon, in this case 5° up. It can be very hard to maintain such an aircraft attitude, especially in severe turbulence, and it was supposed that the crew had finally lost control of the aircraft.
A news conference had been convened on Saturday 6 June, at which I believe the French Minister of Transport, the director of the French air accident investigation agency BEA, and a spokesperson for Air France were all present, and the information they gave substantiated this interpretation. The BEA director, M. Arslanian, did indicate that without the flight data recorder and cockpit voice recorders (FDR, CVR, the so-called “black boxes” although they are not black but, rather, dayglo orange with stripes) he was pessimistic about establishing facts definitively.
By Sunday, 7th June, there seemed to be a remarkable consensus on what was likely to have happened. The French government, BEA, Air France, and numerous professional pilots on pilot forums, all seemed to agree. And all without a shred of physical evidence. Indeed, inferring what had happened from just 24 electronic messages, whose partial interpretation had been made quasi-public. And all this within a week. Usually, even in accidents in which there is a plethora of information, such as the crash-landing of British Airways Flight 38 just short of the runway at London Heathrow airport in January 2008, arguments and discussions and differing views abound for weeks and months and sometimes years about what happened and why. Indeed, it has been nearly one and a half years since BA038 flopped onto the grass and it is still not known why. It is not unusual to wait two to four years for an accident report to be finalised.
In contrast, there seem only to be two big questions remaining about AF447 after a week: is this consensus interpretation anywhere close to the truth, and how could we possibly tell?