A man portable SAM, and most air to air missiles target heat sources. Specifically engine heat. A 747 gives off a tremendous amount of heat from it's engines, and at the time of that explosion, most commercial aircraft did not mount any form of missile countermeasure. A heat seaking missile would always hit a (relativly) slow moving airliner in one of the engines. The newer 2nd, third or fourth generation seaker heads which incorporate camera, laser and radar guidance are also programmed to aim for the engines, since that is invariably located close to the fuel cells and will usually explode with catestrophic force even in the event of a miss generated by pilot evasion. Electronic homeing anti-radar missiles would have hit the radar array in the nose or the cockpit avionics.
The cause of the aircraft crash was catestrophic structural failure resulting from an explosion in the central wing spar fuel tank. The surrounding metal clearly shows a traumatic force internal to that fuel cell, which radiated outward ripping metal open and out much like you would peel a bananna. The engines were all recovered remarkably intact and were not damaged by a primary explosion.
There is no way in hell that a missile hit that center tank with all the other jucy, more lucrative targets on the aircraft to choose from. Internal cargo hold bomb? maybe, but Boeing would have fought the blame if there was even a hint of it not being that faulty light. The fact that the fuel vapor ignition by faulty light tests boeing conducted after the fact resulted in ignition and detonation with a blast pattern almost identical to the recovered wreckage, and the fact that they spent hundreds of thousands of dollars per aircraft replacing those components out of their own pocket tells me that this is completely bogus. Also, it wasn't a @#%^ing cessna that was flown into the pentagon either. Argh!!!!!