After the Air India Flight 171 Boeing Dreamliner 787-8 crashed into a medical hostel on 12 June, many wondered how the building could withstand the impact while the twin towers of the World Trade Center couldn’t when they were hit by planes flown allegedly by terrorists, which made the towers cave in on 9 September 2001. Is the comparison fair? No.

Earlier, in the comment sections of related social media posts by some friends, the question had been addressed simplistically, stating that the World Trade Center (WTC) towers were designed to collapse at impact so that adjacent buildings could be saved. This was somewhat like the deliberate caving-in design of the much smaller—but big by Indian standards—Supertech twin towers in Noida that were blown up using explosives on 28 August 2022.

It was explained that urban America has seen frequent changes in plans, with one generation of planners often finding flaws in the plans of their predecessors. US cities have a history of being built and razed. Considering such an eventuality, the office buildings were made in a way that made demolitions less risky, keeping people in the surroundings out of harm’s way. While this is true, other factors explain why the WTC towers could not withstand the impact of one aeroplane flying into each, which the hostel building in Ahmedabad could. The following is a deep dive into all the physical factors (factors of physics).

The aircraft for Air India Flight 171 was a Boeing 787-8 Dreamliner, a twin-engine, wide-body jet with a maximum takeoff weight of approximately 2,28,000 kg and a fuel capacity of about 1,26,000 litres. It carried 242 passengers and crew and was fully fuelled for a long-haul flight to London. The crash occurred shortly after takeoff, at low altitude (around 625 feet) and likely below 250 km/h, close to stall speed, which limited its kinetic energy upon impact.

In the WTC attacks, the planes were Boeing 767-200ERs (American Airlines Flight 11 and United Airlines Flight 175), slightly smaller than the 787, with maximum takeoff weights around 1,79,000 kg. They were travelling at high speeds—approximately 750 km/h (Flight 11) and 950 km/h (Flight 175)—with high kinetic energy. Both planes were also fuelled for transcontinental flights, contributing to large fuel explosions.

The factor of kinetic energy calls for further research. The Air India 787-8 crashed at a low speed, as it struggled to gain altitude post-takeoff. Low speed reduces kinetic energy (proportional to the square of velocity) considerably, resulting in less destructive force upon impact.

The 767s that hit the WTC were travelling at 4-5 times that speed, delivering exponentially greater kinetic energy. Take a demonstrative example: A plane at 965 km/h has 16 times the kinetic energy of one at 250 km/h, assuming similar mass. This high-energy impact caused extensive structural damage to the WTC towers.

The article next explores the construction of the buildings under study in further detail. The hostel at BJ Medical College was a reinforced concrete (RCC) structure, common in India, with thick concrete walls and iron reinforcement.........

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