Air France Flight 447 crashed in the Atlantic 400-odd miles outbound from Brazil to Paris after its evening departure from Rio de Janeiro on May 31st, 2009. While the flight data recorder has not been recovered, 24 fault messages were relayed to the AF headquarters via satellite. From these messages, and from forensic evidence found floating in the area of the crash site, a picture of the event is beginning to emerge. Spiegel Online has published an analysis of the disaster based on what is presently known.

The evidence collected so far suggests that the aircraft impacted the water on its belly with a 5 degree nose up pitch attitude. The calculated impact force based on certain kinds of material strength data is 36 g.  The aircraft departed just under max gross takeoff weight with 70 tons of kerosene fuel on board.  Abnormalities did not begin to appear until the aircraft was ostensibly at cruising altitude of ca 35,000 ft. There was a suspicious uptick in the OAT reading (outside air temperature) of a few degrees. Investigators believe that this is an indication of icing on the OAT sensor and pitot tube.

The aircraft may have been attempting to penetrate an area of thunderstorms in the inter-tropical convergence zone. This is a band of atmosphere on either side of the equator where northward and southward flows from the respective hemispheres meet and produce vertical air movement. The convergence of these flows can result in moisture laden air being lifted. Together with the natural buoyancy of warm humid air, vigorous convection cells can be kick-started into severe thunderstorms. The cloud tops in this zone can be substantially higher than those at the mid latitudes. At altitude, storm cells commonly produce icing conditions.

Out in the midocean spaces at night, airline pilots have only on-board radar and the moonlight, and perhaps a few pilot reports by others who have just been in the area, to estimate the areas of high storm intensity ahead. Flight through the intertropical convergence zone can produce bumpy rides to the point of violent turbulence. What most passengers don’t understand is that passenger jets are build to absorb considerable abuse before a structural failure occurs due to turbulence.

The upshot of the report is that the pitot tube that senses the airspeed of the aircraft failed due to icing.  This failure basically causes the computerized flight control system to shut down owing to lack of input of this key airspeed data. In flight control, airspeed is one of the very critical pieces of information necessary to sustain controlled flight.

Without airspeed information, and without computer assistance in the control of the various flight control surfaces, the modern passenger jet becomes very difficult to handle manually. The is especially true if the aircraft is under instrument conditions with low/no visibility and in high turbulence.

A complex and aerodynamically clean aircraft being jostled along all three axes at a high mach number presents a large workload for the pilots. At a mach number (o.85 or so) as typically attained in high altitude cruise, a sharp pitch down in the nose can lead to transonic flow over the control surfaces and in the engine inlet. This can lead to engine instability and loss of flight control. Sonic flows over ordinary flight surfaces can lead to flow separation and loss of control. This lesson was learned the hard way in the early days of high speed aviation. Pilots typically throttle back after penetrating turbulent air.

The investigators of AF 447 have all but concluded that the aircraft crashed owing to loss of critical airspeed information and subsequent departure from stable flight.  While the Spiegel article states that investigators are confident in this analysis, recovery of the flight data recorder will undoubtedly provide important details for refinement of the investitgation.