Move ANYTHING fast enough, and it will penetrate:
http://youtu.be/I9zBGgpzl0I
Penetration is HIGHLY dependent on velocity, a solid lead bullet has to travel 2-3000fps to shear plate steel(cold rolled).
But you are still talking supersonic speeds, and in the case of aluminum to penetrate steel it's hypersonic speeds ie 5x the speed of sound. And that is SOLID aluminum, not hollow, case in point the airliner FLOATING IN THE HUDSON RIVER. That brings it's specific gravity to about (point) .3sg. A commercial airliner can not exceed 500fps at sea level, the drag is too great due to it's high weight and large frontal area. It would create a wake vortex, which is not indicated in the video evidence people continue to believe.
As for strong aluminum, the higher strength come at the cost of making it harder and more brittle(hence the life cycle of an airframe). Even if that alloy was used, it's still only 4mm thick and the box columns are 10mm at that level (x2 b/c it has a back side, 20mm). This doesn't even take into account the 6"x.4" section for the side walls and modules of inertia nor 4" concrete floors which have great compression strength.
All one has to do is look at what really happens when an aircraft strikes a skyscraper, Ref Old John Feathermerchant's strike of the Empire st bld in 45'. The airframe fell OUTSIDE onto a lower roof, only penetrating the CONCRETE b/c it has ZERO tensile strength. No structural steel was sheared by the airframe. The occupants stated "the building swung like a pendulum for 30sec" running for cover was difficult "it was hard to keep your feet under you". NOBODY made such a statement at the WTC and such an aircraft weights TEN TIMES MORE, most were all intentionally killed by PORT AUTHORITY to prevent this from getting out. The communication system was cut in the north tower(1st to get hit), and the 1st responders who made the south tower strike area reported "only small pockets of fire" NO reports of an aircraft inside. Official's reported "No black boxes recovered from either tower", but Jesse Ventura promotes a lie to keep ppl believing the HOAX. Even the "truther" Alex Jones, will go ballistic when the subject is brought up, claiming "you discredit the truth movement".
All of this relates back to how the towers were taken down, UNDERGROUND NUCLEAR DEMOLITION, the very reason molten steel was found in the basement.
DIMITRI KHALEZOV explained the physics behind the fission reaction. And for those who want to shout radiation, plutonium decays via ALPHA particles, completely consistent lung cancer and the many heath problems associate w/ the clean up workers.