THE WTC WAS DESIGNED TO SURVIVE THE IMPACT OF A BOEING 767, SO WHY DIDN'T IT?
by MAD MAX Saturday, Jul. 19, 2003 at 11:37 AM

In the early 1970's the World Trade Center's chief structural engineer, Leslie Robertson, calculated the effect of the impact of a Boeing 707 with the World Trade Center towers. His results were reported in the New York Times where it was claimed that Robertson's study proved the towers would withstand the impact of a Boeing 707 moving at 600 miles an hour. Little did he know that decades later two aircraft, almost identical to the Boeing 707, would impact the towers.

THE WTC WAS DESIGNED TO SURVIVE THE IMPACT OF A BOEING 767, SO WHY DIDN'T IT? Fact. The twin towers were designed to withstand a collision with a Boeing 707. In the early 1970's the World Trade Center's chief structural engineer, Leslie Robertson, calculated the effect of the impact of a Boeing 707 with the World Trade Center towers. His results were reported in the New York Times where it was claimed that Robertson's study proved the towers would withstand the impact of a Boeing 707 moving at 600 miles an hour. Little did he know that decades later two aircraft, almost identical to the Boeing 707, would impact the towers. Other engineers are on public record as saying that the World Trade Center would even survive an impact of the larger and faster Boeing 747. The maximum takeoff weight for a Boeing 707-320B is 336,000 pounds. The maximum takeoff weight for a Boeing 767-200ER is 395,000 pounds. The wingspan of a Boeing 707 is 146 feet. The wingspan of a Boeing 767 is 156 feet. The length of a Boeing 707 is 153 feet. The length of a Boeing 767 is 159 feet. The Boeing 707 could carry 23,000 gallons of fuel. The Boeing 767 could carry 23,980 gallons of fuel. However, the actual aircraft involved in the World Trade Center impacts were only flying from Boston to Los Angeles, and consequently, would have been nowhere near fully fueled on takeoff (the Boeing 767 has a maximum range of 7,600 miles (12,220 km)). The aircraft would have carried just enough fuel for the trip together with some safety factor. Remember, that carrying excess fuel means higher fuel bills and less paying passengers. The aircraft would have also burnt some fuel between Boston and New York. Government sources estimate that each of the Boeing 767's had approximately 10,000 gallons of unused fuel on board at the times of impact. To give you some idea how much jet fuel this is, an 11 foot by 11 foot by 11 foot tank contains 10,000 gallons (1 US gallon = 0.13368 cubic feet). So a novel way of destroying high-rise buildings is to load an 11 foot by 11 foot by 11 foot glass tank of jet fuel into a Ryder truck, drive it into the ground floor lobby, break the glass, set light to the fuel and walk away, the high-rise should collapse in about an hour (after all, 12,000 gallons of diesel was all it took to bring down WTC 7). Look mom, no explosives needed. Since, the Boeing 767 is much more fuel-efficient than the 707, a Boeing 707 traveling the same route would carry significantly more fuel and would therefore be a much greater danger from the perspective of a jet fuel fire. Thus the quantity of fuel that burnt on September 11 would have been envisaged by those who designed the towers. In fact, the towers were designed to survive much more serious fires than those of September 11. Over the years, a number of other high-rise buildings have suffered significantly more serious fires, but none have collapsed (not one). Before September 11, no steel framed skyscraper had ever collapsed due to fire. However, on September 11, it is claimed that three steel framed skyscrapers collapsed mainly, or totally, due to fire. See this article for proof that the jet fuel fires can be ruled out as the cause of the World Trade Center collapses. The cruise speed of a Boeing 707 is 607 mph = 890 ft/s, The cruise speed of a Boeing 767 is 530 mph = 777 ft/s. So, the Boeing 707 and 767 are very similar aircraft, with the main differences being that the 767 is slightly heavier and more fuel-efficient, and the 707 is faster. The thrust to weight ratio for a Boeing 707 is 4 x 18,000/336,000 = 0.214286. The thrust to weight ratio for a Boeing 767 is 2 x 31,500/395,000 = 0.159494. Since the Boeing 707 had a higher thrust to weight ratio, it would be traveling faster on take-off and on landing. And, since the Boeing 707 would have started from a faster cruise speed, it would be traveling faster in a dive. So in all the likely variations of an accidental impact with the WTC, the Boeing 707 would be traveling faster. In terms of impact damage, this higher speed would more than compensate for the slightly lower weight of the Boeing 707. To illustrate this point we calculate the energy that the planes would impart to the towers in any accidental collision at their cruise speed. The kinetic energy released by the impact of a Boeing 707 at cruise speed is = 0.5 x 336,000 x (890)2/32.174 = 4.136 billion ft lbs force (5,607,720 Kilojoules). The kinetic energy released by the impact of a Boeing 767 at cruise speed is = 0.5 x 395,000 x (777)2/32.174 = 3.706 billion ft lbs force (5,024,650 Kilojoules). From this, we see that at cruise speed, a Boeing 707 would smash into the WTC with about 10 percent more energy than would the slightly heavier Boeing 767. That is, under normal flying conditions, a Boeing 707 would do more damage than a Boeing 767. In conclusion we can say that if the towers were designed to survive the impact of a Boeing 707, then they were necessarily designed to survive the impact of a Boeing 767. So what can be said about the actual impacts? The speed of impact of AA Flight 11 has been estimated to be 470 mph = 689 ft/s. The speed of impact of UA Flight 175 has been estimated to be 590 mph = 865 ft/s. The kinetic energy released by the impact of AA Flight 11 was = 0.5 x 395,000 x (689)2/32.174 = 2.914 billion ft lbs force (3,950,950 Kilojoules). This is well within limits that the towers were built to survive. So why did the North tower fall? The kinetic energy released by the impact of UA Flight 175 was = 0.5 x 395,000 x (865)2/32.174 = 4.593 billion ft lbs force (6,227,270 Kilojoules). This is within 10 percent of the energy released by the impact of a Boeing 707 at cruise speed. So, it is also a surprise that the 767 impact caused the South tower to fall. Note that the speed of a projectile determines whether the impact damage is localized or spread across a large area. The faster the projectile, the more localized the damage. Common examples illustrating this effect are, the driving of a nail through a piece of wood, and the firing a bullet through a fencepost. Both are done at speed and thus do only local damage. In both of these examples, the wood just a centimeter or two from the impact point, is essentially undamaged. Similarly, the aircraft impacts were at great speed and the damage localized. This effect is illustrated in the above graphic from the simulation of the crash of a Boeing 747 (maximum takeoff weight 875,000 lb, unloaded weight 670,200 lb, fuel capacity 57,285 gallons) with a steel framed building. We are told that the "hijackers" wanted to cause maximum death and destruction, then why didn't they hijack Boeing 747s? Boeing 747s weigh more than twice as much, they can carry more than twice the fuel and travel faster than the Boeing 767. Consequently, Boeing 747s would have caused much more death and destruction than the 767s. Also, why did the hijackers choose to hijack aircraft leaving Boston, when they could have just as easily hijacked aircraft from one of the New York city airports (LaGuardia, Newark or JFK). Hijacking aircraft from Boston, meant that they had to deviate from their designated routes, while still a long way from Manhattan. Of course, as is usual, all sorts of alarm bells would be set off as soon as the aircraft deviated substantially from their prescribed routes. Not only that, the US Air Force specialist quick response unit, the Air National Guard, would almost certainly intercept them before they reached their target (and would have assuredly shoot down the second 767, after seeing what happened to the first). It is often claimed that the WTC was designed only to withstand the collision of a Boeing 707 that was seeking to land at one of the nearby airports, and that since such aircraft would be low on fuel, only small jet fuel fires were envisaged. However, this is an obvious lie. Why is it an obvious lie? Well, because if you take into consideration planes that are landing at an airport, then you must consider planes that are taking off, and such planes are potentially fully laden with fuel. Since the WTC towers were designed to handle extreme wind loading (140 mph hurricane force winds) they would survive the impact of a Boeing 707 (even one that was traveling at full speed) without adding any extra features to the design (above those already necessary to handle the wind loading). All that the designers would have to consider, is effect of a jet fuel fire from a fully fueled jet that crashed into one of the towers shortly after taking off from one of the local airports. Overall, it comes as a great surprise that the impact of a Boeing 767 bought down either tower. Indeed, many experts are on record as saying that the towers would survive the impact of the much larger and faster Boeing 747. In this regard, see professor Astaneh-Asl's simulation of the crash of the much, much larger and heavier Boeing 747 with the World Trade Center. Professor Astaneh-Asl teaches at the University of California, Berkeley.

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