The plan was to take off at 8:15 in the morning, roughly the minute United 175 left Boston Logan, and to be over United's Flight Training Center at 9:03, the exact minute that aircraft struck the South Tower.

A Boeing 777 in United colours, escorted by four F-16 Fighting Falcons of the Colorado Air National Guard's 120th Fighter Squadron. Low, over Denver, on the twenty-fifth anniversary. Nobody involved doubted the intent. On Wednesday afternoon, United called it off anyway.

The reason is uncomfortable and entirely reasonable: the city could not be certain that people watching from below would read it as a tribute rather than as the thing itself happening again.

A tribute built out of the timeline itself

What made the Denver plan unusual was not the flypast. American cities get flypasts. It was that the flight was choreographed against the clock of the attack itself.

United has a particular claim on that morning. Two of the four hijacked aircraft were its own: Flight 175, which left Boston and hit the South Tower, and Flight 93, which left Newark and came down in a Pennsylvania field after the passengers fought back. Denver is United's largest hub and the home of its Flight Training Center, the building where the airline teaches people to fly its aeroplanes. Marking the anniversary there, in front of employees, is not a publicity stunt. It is a company remembering its own crews.

United's spokesman Russell Carlton told 9NEWS that the flyover was not a public event at all, but part of private commemorations at the airline's hub and training centre. That distinction turned out to be the problem. A 777 at low altitude with four fighters on its wing is not a private event to the three million people who live under it.

Denver found out, rather than being asked

The city's position was blunt. A spokesman for Mayor Mike Johnston's office said Denver had been informed that United was doing this, not asked whether it could. Denver's Office of Emergency Management then began working out how to warn roughly a million residents in advance, largely so that the 911 and 311 lines would not collapse under calls from people who had just seen a widebody airliner being chased across their skyline by fighter jets.

About two hours after 9NEWS published its account of those preparations on Wednesday, United sent an update. The commemoration was cancelled.

Why a fighter escort reads differently now

There is a specific reason this particular formation carries weight, and it is not squeamishness. On the morning of 11 September 2001, fighters scrambling to intercept airliners was the defining image of American airspace going wrong. The escort was the emergency. Two and a half decades later, an airliner boxed in by armed fighters over a major city is still the visual shorthand for a hijacking in progress.

Aviation enthusiasts would have recognised the formation instantly and understood exactly what it meant. Most people are not aviation enthusiasts. They would have seen a large aeroplane, low, with fighters alongside, on the anniversary, and reached for their phones.

Flyovers work because they surprise you. That is also why this one could not be allowed to.

Sources: 9NEWS; KDVR/FOX31 Denver; Simple Flying; DVIDS.