Aircraft are basically flying fuel tube tanks. Engines, landing gear and a some other structures are the few robust parts of an airliner. The aircraft that struck the wtc towers were basically crushed on impact. Even the claimed velocities are in question. The NIST had to trump up the speed, angle and weight of the airliner for the impact to bring the towers down in the computer simulations. It would be very difficult for a large commercial aircraft at 1000ft or less to fly near max speed.