Posted by: harrisonjones | May 26, 2011

Air France Flight 447

Two years ago (July 1, 2009), Air France 447 crashed into the Atlantic with 228 souls on board. The A330 was flying through the ITCZ (Inter Tropical Convergence Zone), an area  near the equator known for massive thunderstorms. I would never presume to draw  conclusions about an accident until all the facts are known, but I have been reading the speculation in the press and on various blogs. Neither of which I consider an unimpeachable source. Some facts do seem to be clear and I think will be the focus of the investigation now that the FDR and CVR have been recovered. The pitot tubes that provide airspeed info seem to be the target. Previous known problems with the tubes icing over and giving false readings lead to the speculation. The rudder system is also being scrutinized. I’ll try to keep this simple for my civilian readers and still offer some insight for my aviation readers. 

The pitot tube simply collects ram air as the airplane moves forward and the more air that is packed into the tube, the higher the airspeed needle moves along the instrument. In a small airplane the system is as simple as that and as long as a dirt dobber don’t cover the opening it works fine. If the pilot does lose the airspeed indication, he simply reverts to attitude flying. He sets an attitude and a power setting that he knows will produce a given airspeed. The main concern is not to get too slow. 

It’s a little more complicated in an airliner. First of all, there are three pitot systems. One  for the captain’s instruments, one for the first officer’s instruments and an auxiliary system. Each pilot has a selector to switch his instruments to the opposite pitot system if his fails. Another big difference is that the pitot tube does not feed the airspeed indicator. Its input is routed to an air data computer (ADC) which in turn computes info and feeds the airspeed indicator, the mach meter, the over speed warning system and the stall warning system. It also has angle of attack input. The abnormal checklist for erroneous airspeed indications can get rather long and complex. During abnormal procedures the two pilots are designated the pf and the pnf. Pilot flying and pilot not flying. It’s important that the pf concentrates on flying the airplane and not get too involved in the abnormal procedure that the pnf is working through. Much easier said than done and many examples will attest to that. Some fatal. 

Attitude flying in an airliner is a little more involved also. At altitude the jet actually operates  in a speed envelope. If you go to fast, you approach mach 1 and the airplane will mach tuck. The nose will drop as the shock wave travels further back on the wing. If you go too slow, you will obviously stall. The envelope grows very small at high altitudes and large gross weights. This is called coffin corner and it’s possible to get yourself in a situation where you can’t speed up and  you can’t slow down. AF447 was at 35,000 feet and carrying 228 people with enough fuel to reach Paris. Attitude flying at lower altitudes is a little different also as the gross weight of an airliner can vary several hundred thousand pounds. Some aircraft manuals actually have charts that equate attitude and throttle setting to airspeed. 

I mentioned the rudder earlier and I want to explain that. The size of the rudder on an  airliner has to be big enough to maintain directional control if you lose an engine or two on one wing during takeoff. It might take full rudder to do that until you build up some speed to make it more effective. However, if you jam that big rudder to the stops at .85 mach, it will rip the tail right off the machine. The engineers, God bless them, solve that problem by installing what is  called a rudder limiter. On the older jets it was a mechanical device that operated in conjunction with the flaps. When the flaps were up, the rudder was mechanically restricted to about one third of its travel. Now I will finally get to the point. On the newer jets, the rudder is restricted by airspeed. The faster the airplane goes, the more the rudder is restricted. This is a function  of the ADC which is fed info by the (all together now) pitot tube. Blocked pitot tube and erroneously low airspeed input equals the availability of full rudder at .85 mach.  

Autopilots are wonderful things, but they are controlled by computers that operate in a binary  world of their own. One, zero. Yes, no. Black, white etc. They don’t have the advantage of some old flight instructor reminding them to take it easy on the rudder. Who knows what happened in the cockpit of AF447? What was the speed margin between high and low buffet? How bad was the turbulence? Were the pilots getting over speed and stall warnings at the same time? I’m glad you and I were not there to know. Somebody should write a book.     


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