More than two decades after September 11, 2001, nearly every American knows the basic outline of what happened that morning. Nineteen al-Qaeda hijackers boarded four commercial airliners. American Airlines Flight 11 and United Airlines Flight 175 were flown into the World Trade Center. American Airlines Flight 77 struck the Pentagon. United Airlines Flight 93 crashed in Pennsylvania after passengers fought back. The attacks killed 2,977 people, not including the 19 hijackers, and changed the United States forever.
But amid the well-known story of those four aircraft is another aviation mystery that has continued to attract attention over the years.
It involves United Airlines Flight 23, a Boeing 767 that was scheduled to fly from John F. Kennedy International Airport in New York to Los Angeles on the morning of September 11. Unlike the four hijacked aircraft, Flight 23 never left the ground. Yet members of its crew later described a series of unusual events that made them wonder whether their aircraft could have been targeted.
At the time, however, no one knew how dramatically the morning was about to unfold.
The passengers and crew of Flight 23 arrived at JFK expecting an ordinary cross-country journey. The aircraft was preparing for departure, and nothing initially suggested that the flight would become part of one of the darkest days in American history.
Then the first reports began reaching the airport.
News spread that an aircraft had struck the World Trade Center. The situation was still confusing, and information was arriving from multiple sources. Airline employees and flight crews were trying to understand what was happening while continuing to follow procedures that had been designed for ordinary aviation emergencies.
Inside Flight 23, members of the crew began paying closer attention to their surroundings.
Some later described passengers whose behavior seemed unusual or concerning. One flight attendant reportedly noticed a passenger who appeared extremely nervous and was sweating heavily despite the cool temperature inside the aircraft. Other crew members remembered an uncomfortable interaction involving several passengers in first class.
At the time, however, these incidents did not necessarily indicate a terrorist plot.
That distinction is important.
Human beings are remarkably good at recognizing patterns after something terrible has happened. An innocent interaction that seemed meaningless in the morning can appear deeply suspicious when remembered after learning that a coordinated terrorist attack has taken place. This is one reason the story of Flight 23 has remained controversial.
The crew members were dealing with an unprecedented situation without knowing what would happen next.
Then came the warnings.
United Airlines dispatcher Ed Ballinger sent messages to the airline’s transcontinental flights warning crews about the attacks and instructing them to prevent cockpit intrusion. The warning was part of United’s response as information about the hijackings became clearer. The 9/11 Commission documented Ballinger’s decision to send defensive warnings to United’s aircraft at approximately 9:19 a.m.
Flight 23 was still on the ground.
That detail would become extremely important.
The aircraft had been delayed and had not yet taken off. As the crew learned more about the attacks, the possibility of becoming airborne suddenly seemed far more dangerous than it had only minutes earlier.
Eventually, the decision was made not to continue the flight.
Flight 23 remained at JFK as the entire American aviation system began shutting down.
For the crew and passengers, the decision meant that they had avoided entering an increasingly dangerous airspace. For people looking back years later, however, the story raised a haunting question.
What would have happened if Flight 23 had taken off?
The question is impossible to answer.
There is no confirmed evidence establishing that the aircraft was intended to be hijacked. The official 9/11 Commission report identifies four hijacked aircraft and does not establish Flight 23 as a fifth target.
Nevertheless, the story continued to attract attention because of what crew members remembered.
Years later, some former members of the Flight 23 crew spoke publicly about their experiences. Their accounts brought renewed interest to the possibility that something unusual may have been happening aboard the aircraft before it was grounded.
One of the most intriguing aspects of the story is how ordinary some of the suspicious behavior initially appeared.
A nervous passenger.
An uncomfortable conversation.
People who seemed out of place.
None of those things, by themselves, prove a terrorist plot.
But when placed against the backdrop of the attacks taking place around them, they naturally became more difficult for the crew to forget.
The story also illustrates how little people understood about the threat they were facing in the early minutes of September 11.
Before the attacks, airline crews had been trained primarily to treat hijackings as situations in which terrorists might attempt to negotiate. The possibility that hijackers would deliberately crash commercial aircraft into buildings was not part of the established expectation.
Everything changed that morning.
The four successful hijackings demonstrated that commercial airplanes could be transformed into weapons on a scale few people had previously imagined. Security procedures, cockpit access rules, passenger screening, and crew training would all be fundamentally changed in the years that followed.
For the crew of Flight 23, however, those changes came too late to provide the certainty that people now associate with modern aviation security.
They were living through the crisis as it happened.
They did not know which aircraft were safe.
They did not know how many hijackings might occur.
They did not know whether more attacks were coming.
And they certainly did not know whether their own aircraft might become another target.
That uncertainty is what makes the Flight 23 story so compelling.
It was an aircraft sitting on the ground at one of the world’s busiest airports while the country was experiencing an unprecedented attack. The crew had to make decisions with incomplete information, relying on warnings, instinct, and rapidly changing instructions.
Ultimately, the airplane did not become another hijacked aircraft.
It did not fly toward Los Angeles.
It did not become another weapon.
It remained on the ground.
But the questions surrounding it never completely disappeared.
In the years since September 11, the phrase “fifth plane” has sometimes been used to describe Flight 23. However, that phrase can be misleading if presented as established fact. There is a difference between saying that crew members later wondered whether their aircraft could have been targeted and saying that investigators proved it was part of the terrorist plan.
The first is supported by accounts from people connected to the flight.
The second has not been established.
That distinction matters, especially when discussing an event as consequential as September 11.
The official historical record remains centered on the four aircraft that were hijacked and the 19 al-Qaeda operatives who carried out those attacks. The 9/11 Commission’s report provides the documented account of those four flights and the broader failures that allowed the attacks to occur.
Flight 23 occupies a different category.
It is a story of uncertainty.
A story of an aircraft that was in the wrong place at the worst possible moment, surrounded by extraordinary circumstances and later memories that raised difficult questions.
Perhaps the most haunting part is that the crew did not know how close they might have been to danger.
They simply knew something was terribly wrong.
Today, Flight 23 remains an intriguing footnote to the larger history of September 11. It was not one of the four aircraft that were hijacked, and there is no established evidence proving that it was intended to become a fifth.
But its story provides another perspective on that chaotic morning.
While millions of people were still trying to understand what was happening, airline crews across the country were making split-second decisions with almost no reliable information.
Some aircraft were already in the sky.
Others were preparing to depart.
And some, like United
