Regulate the boom, not the speed

In 1973 the FAA solved a noise problem by banning a speed. Civil aircraft could no longer cross Mach 1 over the United States — not because the speed itself was dangerous, but because breaking the sound barrier dragged a sonic boom across whatever sat below. This week the Department of Transportation moved to undo that, proposing to let aircraft go supersonic over land again, governed for the first time since the Nixon administration by how loud they are rather than how fast they fly.

A speed limit, retired. A noise limit, installed. The fifty years between those two sentences are a lesson in what goes wrong when a regulator writes down the proxy instead of the harm. Nobody in 1973 objected to Mach 1 as such. They objected to the boom. But speed is a number you can read off an instrument, and a boom's loudness bends with altitude, airframe, and weather — messy to define, expensive to police. So the rule banned the clean proxy and let it stand in for the messy thing anyone actually cared about.

Then the proxy hardened into law, and the law outlived its own reasoning. It forbade not just the window-rattling jets of the Concorde era but every design since that has learned to cross Mach 1 with a thump instead of a bang. NASA's X-59 was built precisely to make that thump — to turn the boom into something closer to a car door shutting down the block. Boom Supersonic is betting a commercial airliner on the same premise. Under the 1973 rule none of it was legal over land, no matter how quiet, because the rule never asked about noise. It asked about speed. That is the signature of a proxy that has escaped its purpose: it keeps binding the cases its author never meant to reach.

The honest objection is administrative. A speed limit is a bright line: one number, one radar reading, cheap to enforce and hard to argue with. A noise standard is a performance test — you have to define an acceptable boom, measure it across real conditions, and litigate the edges. Bright lines are cheaper to run, and there is a serious case that regulators should favor them where they can.

A cheap rule that bans the right thing is a bargain. A cheap rule that bans a proxy for it quietly taxes everything the proxy happens to catch.

Grant it — then look at what the bright line bought. Half a century of a blanket ban on an entire class of aircraft, the quiet ones included: the designs that would have passed the very test the rule refused to give them. That convenience was never free. It was paid by every airframe that could have cleared the real bar and never got the chance to try.

The airplane is the small story. The rule is the big one, because the shortcut repeats anywhere a regulator reaches for the measurable stand-in — ban the speed because the boom is hard, block the category because judging the instance is hard. Regulate the boom, and you free every design quiet enough to pass. Regulate the speed, and you ban the future for the crime of arriving early.