25 years later, 9/11 still shapes the way we fly. Here's how air travel changed after the attacks.

A TSA security checkpoint at John Wayne Airport in Santa Ana, California. Jeff Gritchen/MediaNews Group/Orange County Register via Getty Images Aviation security was completely overhauled after the terrorist attacks of September 11, 2001. The TSA, which federalized airport security, was established in November 2001. After 9/11, only ticketed flyers were allowed past security, and cockpit doors were bulletproofed. Flying has completely changed since Al Qaeda hijackers carried out the terrorist attacks of September 11, 2001, 25 years ago. The newly created Transportation Security Administration replaced the private screening checkpoints that flyers were used to and introduced a new normal at airports — one where water bottles are emptied, bodies are scanned, and goodbyes are said before reaching the gate. The planes were similarly hardened with bulletproof doors and the strictest protocols for cockpit access; pilots and flight attendants also received additional training to handle terrorists. Arik Arad, who advised the President's Commission on Aviation Security and Terrorism following the 1988 bombing of Pan Am Flight 103 and served as head of security at Israel's Ben Gurion Airport, said that these changes were part of a post-9/11 paradigm shift favoring a layered approach to security. "The principle is simple — if one layer fails, another should detect or stop the threat," Arad, who now operates an aviation cybersecurity company, told Business Insider. "Intelligence, passenger assessment, screening, baggage security, airport access control, aircraft protection, trained personnel, and law enforcement all became parts of an interconnected security system. That change in philosophy was, in my view, more important than any individual technology introduced after the attacks." Here are the biggest ways flying has changed since 9/11. After the September 11 attacks, airport security was federalized through the establishment of the Transportation Security Administration. CHICAGO - SEPTEMBER 10: A security worker with the Transportation Security Administration (TSA) screens a travelers belongings September 10, 2002 in Terminal 1 at O'Hare International Airport in Chicago, Illinois. Over 500 new federally trained security workers took over the screening work today at security checkpoints in the airport's largest terminal, replacing workers from Argenbright Security. Tim Boyle/Getty Images Before 9/11, airport security was handled by numerous private companies. In November 2001, the Aviation and Transportation Security Act established the Transportation Security Administration, a federal agency whose trained workers took over security screening at airport checkpoints by November 2002. There are still 20 US airports that use private contractors instead of federal employees through the TSA's Screening Partnership Program. In an effort to curb staffing shortages and long security lines resulting from missed paychecks during government shutdowns, the Trump administration is pushing for greater privatization. The agency has also come under scrutiny for its effectiveness. In 2015, an internal Department of Homeland Security investigation found that the TSA failed to detect weapons and fake explosives carried by undercover agents 95% of the time, leading the acting head of the Transportation Security Administration to be reassigned. Cockpit doors were reinforced. 396032 01: Chief pilot of JetBlue Airways Lanny McAndrew, stands behind one of the new bullet-force-resistant cockpit doors that have been installed on all JetBlue planes after the September 11 attacks, October 17, 2001 at JFK airport in the New York borough of Queens. The reinforced cockpit doors, which cost the company approximately $10,000 per aircraft, are the first such doors to be installed on a major carrier. Spencer Platt/Getty Images Flight attendants used to have keys to open the cockpit from the outside. After 9/11, the cockpit got reinforced, bullet-resistant doors and could only be opened from the inside once locked. More air marshals were placed on flights. A federal air marshal service instructor at the TSA training center in Atlantic City, New Jersey. Rachel Wisniewski/The Washington Post via Getty Images In the years following the September 11 terrorist attacks, thousands of new air marshals were added to the Federal Air Marshal Service, which previously had fewer than 50, according to the Government Accountability Office. Pilots and flight attendants received more training in defensive tactics. ARTESIA, N.M. - OCTOBER 16: Unidentified commercial airline pilots practice defensive tactics in a Boeing 727 at the Federal Law Enforcement Training Center October 16, 2003 in Artesia, New Mexico. U.S. commmercial airline pilots take this course and others during the 56 hour, six-day Federal Flight Deck Officer training program, implemented by the TSA. Pilots are instructed in the use of firearms and other defensive tactics on board airplanes