The Transportation Security Administration was founded two months after the attack and, according to PBS NewsHour, this included "stricter guidelines on passenger and luggage screening" and "an ever-changing array of machinery and procedures were introduced to scan for weapons and destructive items". Also according to PBS NewsHour, airplanes fortified their cockpit doors, and some airliners dropped first-class cabin curtains. Overall, these changes, among others such as the PATRIOT Act helped to improve air transportation and national security.
May the families of the victims be blessed with love and peace in these times of grief and despair. It's hard to lose loved ones, and we grieve over the loss of them - that is normal, and human.
As a nation, we will never forget the tragic events of 9/11. As George W. Bush said, "We will not tire, we will not falter, we will not fail."