As an aerospace engineer, as a control system engineer, as a private pilot, and as a French man, I am especially mourning the loss of all passengers and crew of AF447. All my condolences go to the families and friends of the victims.

This week, like many, I followed the news about AF447 in the media and on twitter. The first obvious element of explanation is weather. Cumulo Nimbus clouds are known to be the nightmare of every pilot. Airplanes are not really supposed to enter one. This includes airliners that are equipped with a weather radar to avoid entering most severe weather zones. So one can wonder if the weather radar really worked. It could have been damaged or victim of shadowing effect (see in the comments). Inside a CuNimb you will find severe turbulences, lightnings, heavy precipitation, hail and severe icing.
An excellent analysis of the weather involved in AF447 tragedy has been made by a former US Air Force meteorologist. Conclusion is “There is a definite correlation of weather with the crash.” It looks much like weather is a main factor but not the only factor. Weather conditions opened many ways of attacking the structure of the airplane and also its sensors and actuators.

As you read my previous posts about flight control systems, you know how fundamental the sensors are. And it happens that Airbus was not so sure of the speed sensors as there was “an ongoing program at the carrier to renew its planes’ speed sensors”. I do not know what Airbus was thinking about these pitot tubes made by Thales but they might have been thought not robust enough for severe weather conditions, the kind of weather conditions AF447 encountered. And indeed the speed sensors seem incriminated by ACARS messages. That is why Airbus published a communication recalling the procedures “in case of unreliable airspeed indication”. Airbus also added they were not blaming the pilots. Fortunately, because it is quite a desperate situation to be in a storm without AP and without speed sensors. Wrong airspeed could lead to a stall. For instance if the indicated air speed is overestimated, the pilot will want to slow down the aircraft to maintain the aimed speed. Notice that the turbulent condition, making the aircraft to jump and pitch could not allow the pilot to simply keep a pitch attitude and then a fixed airspeed. So the actual airspeed would have then reached a value below the stall velocity leading the aircraft to stall and fall like a stone. Then during the fall the aircraft could have dislocated leading to the scene seen by the Spanish pilot in the vicinity at the time reported: “Suddenly we saw in the distance a strong and intense flash of white light, followed by a downward, vertical trajectory which broke up into six segments”. Here I do not really understand how another pilot could see anything if the weather was so bad and thus leading to a low visibility.

One more word about flight planning, if I were the BEA (French Bureau of Enquiry and Analysis for Civil Aviation Safety), I would also investigate around the Air France (and airlines in general) flight planning procedures to make sure they take enough margin to avoid the most severe weather and that they do not privilege the fuel cost at the risk of compromising flight safety.

Conclusion

Severe weather and not robust enough speed sensors might be the causes of the crash. Let’s hope the official investigation will be quick enough to find the causes in terms of weather, flight planning, hardware or human errors involved so that they won’t be repeated. It won’t be an easy task but even without the DFDR (Digital Flight Data Recorder) or CVR (Cockpit Voice Recorder) (that is to say the black boxes, not so black as you can see on the image at the beginning of this article), thanks to the ACARS messages and the visual testimony (if it is indeed related…) a detailed analysis can be done. AF447 has unfortunately become the Titanic of the XXIst century with all his mysteries. Hopefully, it won’t take 73 years to localize the wreck.