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.
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.
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.
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.
Mark Stephens, a retired Delta Air Lines pilot who flew for the airline for 30 years, told Business Insider that pilots were previously trained to accommodate hijackers' demands and take them where they wanted to go in order to buy time. That changed once the 9/11 terrorist attacks showed how planes themselves could be used as a weapon.
"Up until September 11th, using the airplane as a kamikaze-type thing was never, ever, ever discussed," he said.
After 9/11, pilots received additional defensive training focused on thwarting potential cockpit invasions and fending off attackers while seated at the controls. The Federal Flight Deck Officer program, launched in 2003, also authorized qualified pilots to carry firearms on board.
More bomb-sniffing dogs were deployed to airports.
Bomb-sniffing dogs had been used for security sweeps before 9/11, but the TSA took over and expanded the program in 2002, training dogs to detect explosives in and around the airport, including on persons and luggage.
In 2025, the TSA's Canine Training Center deployed 1,076 transportation security specialist canine teams and traditional explosives detection canine teams at more than 100 US airports.
By the end of 2002, all checked bags were screened for explosives.
The Aviation and Transportation Security Act mandated the screening of all checked bags for explosives, weapons, and other dangerous items — a requirement that had not previously been required or widely practiced.
The TSA said it's mainly looking for weapons and explosives.
The TSA's website said its systems are designed to "detect potential threats to aviation and passengers," and its "security officers do not search for illegal drugs" but will refer any found to law enforcement.
Only ticketed passengers were allowed past security.
People who were not traveling themselves used to be able to go through airport security to drop off or meet ticketed passengers at the gate. After 9/11, with few exceptions, only ticketed passengers were allowed past security checkpoints.
This security standard is beginning to change. In September, the TSA launched a Gateside pass program allowing TSA PreCheck members and those with a Known Traveler Number from other trusted travel programs who register in advance to go through security without a boarding pass at 13 US airports.
Orlando International Airport has a similar program in coordination with the TSA.
In 2006, carry-on liquids were restricted to 3.4 ounces as a result of a foiled terrorist plot involving liquid explosives.
In 2006, UK authorities uncovered a planned terrorist attack involving liquid explosives hidden in bottles of soft drinks that would have been detonated on planes traveling to the US and Canada.
As a result, the US temporarily banned all liquids and gels in carry-on luggage, with exceptions for items like baby formula, breast milk, and prescription medications. The restriction was then adjusted to any liquids and gels larger than 3.4 ounces, a rule that remains in place at TSA checkpoints.
Full-body scans were added to TSA screenings after an attempted bombing in 2009.
After an Al Qaeda terrorist attempted to detonate explosives hidden in his underwear on a Northwest Airlines flight on Christmas Day in 2009, the TSA implemented full-body scans, known as advanced imaging technology, into screening.
The machines use millimeter wave technology to detect both metallic and non-metallic items.
TSA Precheck was introduced in 2011 to expedite security lines for vetted travelers.
Travelers who apply for TSA PreCheck undergo a background check and, if approved, pay up to $85 to receive a Known Traveler Number. TSA PreCheck allows passengers to wait in separate, faster-moving security lines with fewer restrictions.
Post-9/11, Arad said intelligence agencies worldwide worked more closely together on aviation security.
Beyond the airport gates, Arad told Business Insider that intelligence agencies across countries started sharing information more openly to flag potential threats, a change he witnessed firsthand as head of security at Ben Gurion Airport.
"We share information, and it helps, because if there is a bad guy in Germany and he's planning on coming to the US, people are aware of it and they're waiting for him," Arad said. "I'm telling you, I know it. I've been there."
