Computer simulation and mathematical analysis of the UA175 impact by MIT, University of Purdue and others indicate that upon impact the wings of the Boeing 767-200 would have shattered and the fuel ignited outside the towers facade, the aircraft would have lost about 25% percent of its kinetic energy on impact and that the tail fin would have sheared off due to torsional forces.
In layman's terms this means that the aeroplane would have decelerated sharply, crumpled up and exploded against the tower's wall with only heavy objects like the engines and undercarriage perhaps puncturing the facade. The entire airframe would not have glided through the outer wall and would not have left a large hole roughly the same shape and size of a Boeing 767-200.
If a Boeing 767-200 had hit the tower it would have exploded externally and bent the facade inward noticeably while depositing pieces of fuselage, wings, tail fins...etc in the streets below. There would have been some column damage but it would have been virtually impossible for any of the lighter airframe sections to pass completely through the tower.
The 911 researcher Geoff King (aka "PlaguePuppy") illustrated these points in a radio interview with Webster Tarpley in early 2006. King's report, which describes in detail what the WTC2 videos should have shown if a real Boeing 767-200 had hit WTC2, can be downloaded here.
The airframe of a modern aircraft is not as strong as you might think; they tend to be manufactured as light as possible in the interests of economy. For example, the aircrafts cabin is not pressurised to sea level. Instead it is pressurised to about 8000ft above sea level, the purpose of which is to save weight by not strengthening the structure to withstand internal sea level cabin pressure while flying at cruise altitudes.
http://www.911research.dsl.pipex.com/ggua175/structural/
