The September 11 attacks, often referred to as 9/11, were a series of four coordinated terrorist attacks by the militant Islamist terrorist group Al-Qaeda against the United States on September 11, 2001.

United Airlines 93, 175 and American Airlines Flights 11 and 77 were the four aircraft that crashed in various locations in the United States.

However, it is unclear as to how the hijackers gained access to the cockpit of American 11. Reports from flight attendant Betty Ong speculated they “jammed their way in.”

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While passengers on United 93 revolted against the hijackers and managed to deny the terrorists from reaching their intended target in Washington D.C., the other planes struck the two World Trade Center towers in New York and the Pentagon.

These attacks left thousands dead and many more injured or suffering complications from quickly responding to the disaster.

After the attack, then-President George W. Bush signed the Aviation and Transportation Security Act in November 2001, which aimed to prevent such an event ever happening again.

An important part of the legislation was the airlines would have to comply to reinforce their cockpit doors, specifically to: “fortify cockpit doors to deny access from the cabin to the pilots in the cockpit.”

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This means those outside the cockpit would be denied access when the locking mechanism was engaged.

It is also said that by fortifying the cockpit doors, the pilots would remain safe while they deny or delay access to hijackers from entering the cockpit.

Meanwhile, it should be noted that in March that year, Germanwings flight 9525 took off from Spain’s Barcelona El Prat Airport for a routine journey to Dusseldorf Airport, in Germany. This flight was a low-cost airline and was owned by Germany’s biggest airline, Lufthansa.

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However, the plane would never complete its journey. It was deliberately brought down by co-pilot Andreas Lubitz after he locked the cockpit door when he was alone.

It was reported that all 144 passengers and six crew members were killed the moment the aircraft smashed into the French Alps.

An investigation, led by French and German investigation into the crash was concluded in January 2017. The investigation revealed that Lubitz was solely responsible for the crash and had previously considered suicide, although he was later declared healthy and allowed to fly.

The report stated that “The reinforced structure of the cockpit doors, designed for security reasons to resist penetration, could not be broken from outside to enable somebody to enter before the aircraft impacted the terrain in the French Alps.”

Later, several countries – including Australia and Canada – required two authorized personnel to be present in the cockpit at all times, in order to avoid a similar disaster.

However, in countries of the European Union — where the Germanwings flight crashed — a single pilot being present in the cockpit is still allowed.