An airliner crossing Kansas at Mach 0.85 is doing nothing remarkable. The same aeroplane at Mach 1.01 would be breaking federal law.

The line is not about safety, or engineering, or fuel. It is about a noise, and about an American town in 1964 that was used as a test subject and did not enjoy it.

That rule is now, finally, being taken apart. But it is still in force today, and it is worth understanding what it actually says before the headlines start claiming otherwise.

The Test That Poisoned the Well

In 1964 NASA and the FAA decided to find out how much sonic boom a city would tolerate. They picked Oklahoma City and flew eight booms a day over it for six months.

The answer turned out to be: not this much. The tests produced around 15,000 complaints, a pile of damage claims and a class action that the federal government eventually lost on appeal in 1969.

Four years later the FAA made it a rule.

What the Rule Actually Says

Section 91.817 does not ban supersonic aircraft. It bans supersonic flight over the United States, which is a different and much more awkward thing. An aircraft can be supersonic-capable, certified and in service, and still be required to fly the whole American leg of its route below Mach 1.

The FAA has been clear about why it wrote the rule that way.

In 1978 the rule was extended. It is not enough to be outside American airspace: an aircraft flying to or from a US airport may not let its boom reach the surface inside the United States either. That closed the coastal loophole, and it is the paragraph that shaped Concorde’s entire route network.

A Boom Is Not an Event. It Is a Wake.

The single most common misunderstanding about sonic booms is that they happen once, at the moment an aircraft passes Mach 1, like a balloon popping.

They do not. A supersonic aircraft trails a cone of shock waves behind it continuously, and wherever that cone meets the ground, people hear a boom. The aircraft drags that band along underneath itself for as long as it stays supersonic.

The rule of thumb is about one statute mile of boom carpet for every thousand feet of altitude, which makes the band roughly five times as wide as the aircraft is high. An airliner at 30,000 feet would lay down a strip of boom about thirty miles across, continuously, from coast to coast.

That is why the ban is written the way it is. There is no such thing as a supersonic route that only inconveniences the people directly underneath it.

What Concorde Had to Do About It

Concorde is usually described as having been killed by economics, and that is not wrong, but the economics were shaped by this rule.

Concorde went supersonic over water and subsonic over land. That worked for the North Atlantic, where almost the whole route is ocean. It did not work for anything that crossed a continent, which is why there was never a New York to Los Angeles Concorde, and why the aircraft ended its life serving essentially one route profile.

An aircraft that can only use its defining capability over the sea is an aircraft with a very small market.

The Rule Is Now Being Repealed

In June 2025 Executive Order 14304, Leading the World in Supersonic Flight, instructed the FAA to replace the prohibition. On 2 July 2026 the agency published its proposed rule.

The replacement is not “go as fast as you like”. It is a performance standard, and the number is specific: the aircraft must be operated so that sonic boom overpressure at the surface does not exceed 0.11 pounds per square foot, counting both the primary and any secondary boom.

How Quiet Is 0.11 psf?

Very. A supersonic aircraft under normal conditions produces a peak overpressure somewhere between about 1 and 10 pounds per square foot. Buildings in good condition are generally reckoned to be safe from damage up to around 11 psf.

So the proposed limit is roughly one hundredth of the damage threshold, and about a tenth of the quietest end of what an ordinary supersonic aircraft already does. It is not a rule that a conventional supersonic design can meet by flying carefully. It is a rule that requires a shape built specifically to stop its shock waves coalescing into a bang — which is exactly what NASA has spent the last decade testing.

So When Can You Book It?

Not yet, and not soon. What exists today is a proposal. The comment period closed on 17 August 2026, the FAA has to work through the responses, and a final rule has to be written and published before anything changes. Until that happens, 91.817 is the law and Mach 1 over land is illegal.

Then somebody has to build an aircraft that meets 0.11 psf, certify it, and find enough passengers willing to pay for the hours it saves. The regulation was never the only obstacle. It was just the one that made all the others academic.

Fifty-three years is a long time to be told to slow down over Kansas because of what happened to Oklahoma City.

Sources: Federal Register, Enabling Supersonic Overland Flight, 2 July 2026 (docket FAA-2026-6935); Executive Order 14304, 6 June 2025; Wikipedia (Sonic boom).