Reddit reviews City in the Sky: The Rise and Fall of the World Trade Center

They were both 767's that hit the towers on 9/11 and contrary to widely promoted misconceptions, the Boeing 767-200s used on 9/11/01 were only slightly larger than 707s and DC 8s, the types of jetliners whose impacts the World Trade Center's designers anticipated

FEMA's Report shows the sizes of a 707 and a 767 relative to the footprint of a WTC tower. Flight 11 and Flight 175 were Boeing 767-200s. Although a 767-200 has a slightly wider body than a 707, the two models are very similar in overall size, weight and fuel capacity

property| Boeing 707-320| Boeing 767-200
---|---|----
fuel capacity| 23,000 gallons| 23,980 gallons
max takeoff weight| 328,060 lbs| 395,000 lbs
empty weight| 137,562 lbs| 179,080 lbs
wingspan| 145.75 ft| 156.08 ft
wing area| 3010 ft^2| 3050 ft^2
length| 152.92 ft| 159.17 ft
cruise speed| 607 mph| 530 mph

Given the differences in cruise speeds, a 707 in normal flight would actually have more kinetic energy than a 767, despite the slightly smaller size. Note the similar fuel capacities of both aircraft. The 767s used on September 11th were estimated to be carrying about 10,000 gallons of fuel each at the time of impact, only about 40% of the capacity of a 707



Statements by Engineers

John Skilling was the head structural engineer for the World Trade Center. In a 1993 interview, Skilling stated that the Towers were designed to withstand the impact and fires resulting from the collision of a large jetliner such as Boeing 707 or Douglas DC-8.

>Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed, ... The building structure would still be there

A white paper released on February 3, 1964 states that the Towers could have withstood impacts of jetliners travelling 600 mph -- a speed greater than the impact speed of either jetliner used on 9/11

>The buildings have been investigated and found to be safe in an assumed collision with a large jet airliner (Boeing 707—DC 8) traveling at 600 miles per hour. Analysis indicates that such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact, page 131



One aspect of engineering that is not widely understood is that structures are over-engineered as a matter of standard practice. Steel structures like bridges and buildings are typically designed to withstand five times anticipated static loads and 3 times anticipated dynamic loads.