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A lesson in Fluid Mechanics

Reader comment on item: Why Did American Airlines 587 Crash?
in response to reader comment: NO PILOT ERROR OR WAKE TURBULENCE HERE.

Submitted by anonymous (United States), Feb 10, 2007 at 18:43

Alright, I'm not going to debate over the structural stability of Airbus' aircraft. However, I do wish to say that before you begin making conjectures of "common sense" about what can physically happen or not happen from a technical standpoint, you should be an engineer, physicist, or FAA rated pilot. Fortunately for you, I'm all three.

Regardless of pointing fingers of whether terrorists are involved or not, a vertical stabilizer no matter how it is engineered or designed, when over-controlled at those speeds in a low level wind shear situation can, and most likely will, separate or fail to some degree. I don't presume to blame Airbus or AA for this, since I am not an aircraft mechanic. I don't presume to know about the gentleman's job that made a comment here earlier. I'm sure he can agree, however, that large aircraft at high airspeeds and low altitude are susceptible to structural fatigue and/or failure when in an over-controlled situation.

That's why the FAA teaches pilots about a little thing called maneuvering speed. This is defined as the speed at which one may encounter structural fatigue and/or failure if abrupt control inputs are made in turbulent situations (which this classifies as one). This isn't an issue of taking off close behind a 747. This is an issue of taking off too early before the wingtip vortices could dissipate, and flying right into them. Any large aircraft generates the most turbulence at slow airspeeds with flaps extended (quite common during takeoff and landing). When aircraft take off, they are vectored towards their course of departure. They don't all fly off in a straight line. All it takes is for this aircraft to lift off beyond the 747's lift off point, and turn off in the same direction below the 747's altitude during climb out (since wake turbulence descends over time).

Basically, the F/O flying the aircraft went on instinct instead of training and tried to correct for the initial deviation, and then ended up over correcting and then had to correct that as well, leading into an out of control situation. Over-controlling is actually a common occurrence, but not usually to this degree. So when there is no crash from a wake turbulence situation, we don't hear about it from the media, because there is no need for the airlines or NTSB to release information on minor incidents of turbulence in flight.


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