Air France 447 - AF447 Data Suggests Faulty Speed Data
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Jens Flottau/Frankfurt jflottau@freenet.de
Data from Air France flight 447's automatic ACARS message indicates the pilots may not have had access to the correct speed information during the final minutes of the flight.
The list of fault messages sent to Air France's operations headquarters in Paris includes a sequence of notes that hints at the three Air Data Inertial Reference Unit (ADIRU) computers to show diverging information. ADIRU compiles data delivered by the pitot tubes and process it to the aircraft's other computers.
The list, obtained by AviationWeek among others, gives an insight into the sequence of computer and system failures on the Airbus A330-200. During the last four minutes the situation worsened. At 2:10 a.m. zulu, the autopilot was either switched off by the pilots or automatically. The function is switched off automatically if speed drops by some margin below a previously defined minimum. The aircraft subsequently flew in "alternate law" conditions that do not provide full automatic envelope protection. Other functions, such as the Traffic Alert and Collision Avoidance System (TCAS) or the rudder travel limiter failed at the same time. Shortly after the ADIRU notices (2:11 and 2:12 a.m.), the ACARS alerts show a "F/CTL PRIM 1 FAULT" and a "F/CTL SEC 1 FAULT" indicating malfunctions of the first primary and secondary flight control computers. Shortly thereafter, the string of messages ends.
In an "Accident Information Telex" to operators, Airbus writes that the ACARS content indicates that "there was inconsistency between the different measured airspeeds. Therefore and without prejudging the final outcome of the investigation, the data available leads Airbus to remind operators what are the applicable operational recommendations in case of unreliable airspeed information."
The diverging speed inputs could come from icing of the pitot tubes, industry sources indicate - a phenomenon not uncommon when flying in weather conditions that prevailed at the time of the accident. In its telex, Airbus points out that "the aircraft was crossing a tropical multicell convective area at the time of the accident."
The ACARS content only shows what systems and computers malfunctioned, but there's no information yet available about what actually happened to important parameters such as airspeed, altitude or pitch angle during the last few minutes of the flight. It also is still unclear if the loss of control happened as a result of a multiple system failure and subsequent stall or because of the severe turbulence that is likely to have hit the aircraft during its 75 mile (or 12 min.) travel through the storm front.
Brazil's Defence Ministry had to admit on Friday that wreckage found floating on the Atlantic on Tuesday does not belong to the aircraft. A senior French government official pointed out that French search troops have not found a single piece of the aircraft yet. While there appears to be no hope for survivors among the 228 on board, finding the accident site quickly could turn out to be crucial to find out more about what caused AF448 to crash. The black box's batteries last for around 30 days, and the French air accident investigation branch BEA has indicated it is not optimistic it can find the piece in deep sea.
Separately, a Spanish newspaper quotes the crew of an Iberia Airbus A340 that flew seven minutes behind AF447 on the same track. According to the crew, air traffic control failed to contact the Air France jet after 1:33 a.m. zulu in spite of trying several times. The Iberia pilots - who deviated 30 miles east from the track to circumnavigate thick clowds - then tried to get in touch with their French colleagues, too, but did not succeed either. The pilot of a Lufthansa Boeing 747-400 that had passed the region 30 minutes earlier said that he had to fly several detours to avoid heavy weather, but otherwise described the flight as routine.
Photo credit: Air France
