What the big deal about pitots is


I'm getting tired of reading news stories that don't explain all this:

The reason the errors in the speed input matter in the crash of AF 447 is this:

Airplanes have to go a certain speed to stay in the air; anything slower, and they stall. That speed is determined by a range of factors, and varies depending on many of those same factors (weight and altitude, say).

You also have the possibility of a high speed stall (or buffeting forces that literally tear you apart) on the other end of the spectrum, by going too fast.

Generally, those two speeds are fairly far apart -- say, too slow is 120 knots and too fast is eighty or eighty-five percent of the speed of sound. But as conditions change, they get closer and closer together. As it happens, one critical place those conditions change is at high altitude. All of this varies according to the design of the aircraft (and its weight, the temperature of the air, etc.).

There is a point at which the difference between too slow and too fast is extremely narrow. Pilots call this "coffin corner." At some theoretical point, there is literally nothing the pilot can do to prevent a stall.

And let me mention that stalling at high altitude and (relatively) high speed - though not absolutely and necessarily fatal - is not very pretty. Even in good weather.

The theory that's being advanced behind the scenes in AF 447 is that the crew and plane found themselves in a coffin corner, and that something subsequently occurred to make them lose control of the aircraft.

That's why the fuss about the pitot tubes, which record the aircraft's speed. Getting the wrong speed first makes it impossible to use the automatic pilot, and then adds considerably to the pilot's difficulty level. (There are backup methods for judging your speed, which would surely have been used or at least tried in this case.)

There's a lot more to the theories, but frankly most of it is simply speculation. Until there's more information, not even the most educated guess can tell us what happened. We don't know for a fact that they were in that sort of flight condition, let alone why and how they got there, or what happened next.

One thing does seem unlikely at this point - given what we know about the weather and the maintenance messages that came from the aircraft right before it disappeared make it unlikely that a bomb went off. Unlikely, though admittedly not 100 percent impossible.