50 years of silence
It's time to end the speed limit in US airspace
I remember the first time I got boomed. I had traveled with my colleague, Matt, to NASA Langley Research Center to get educated on sonic booms. The NASA aeronautics folks were, as ever, welcoming and generous with their time. They escorted us to a room that contained the sonic boom simulator.
The simulator is a small chamber made of 20-cm thick concrete blocks. From the outside of the door, which housed four subwoofers and four midrange speakers, protruded wires that supplied power and signal to the system. Inside the chamber there is room for only one person—sitting, not standing.
They have taken all possible measures to reduce acoustic resonance. A tiny window on the side is made of inch-thick plexiglass. The floor has thick carpet, the walls are covered with acoustical foam. Oh, and when the door closes, the chamber is airtight.
I am not very claustrophobic, but as I sat in the chamber, my mind alternated between musing about when I was going to run out of air and bracing myself for the boom. I gripped the arms of the chair. I had heard many times that sonic booms are not only loud, but startling.
When the boom finally came, I was most surprised by how mild it felt compared to the buildup. What they played for us was the sound of a boom emitted by an F-18 at cruise altitude. It was probably 100 PLdB, or around 85 dB(A), perhaps five decibels quieter than a Concorde’s boom. They also played a softened boom, of the kind future low-boom aircraft might produce, and it was indeed wholly inoffensive.
We used a different simulator as well, and saw some other acoustic equipment, and chatted with the NASA scientists. Then Matt and I said our thanks and goodbyes, walked into the parking lot, got into the rental car. Finally, we could speak privately. Even the loudest boom we heard, we both thought, was not that big of a deal.
Fifty years ago today, on March 23, 1973, Alexander P. Butterfield, the Administrator of the Federal Aviation Administration, issued a rule that remains one of the most destructive acts of industrial vandalism in history.
“No person may operate a civil aircraft at a true flight mach number greater than 1 except in compliance with conditions and limitations in an authorization to exceed mach 1 issued to the operator under Appendix B of this part.”
This text was slightly modified in 1989 and again in 2021, but the upshot remains the same. The rule imposed a speed limit on US airspace. Not a noise standard, which would make sense. A speed limit.
This speed limit has naturally distorted the development of civil aircraft. For fifty years, the aviation industry has worked to improve subsonic aviation. Commercial passenger aircraft are safer and more economical today than they were in 1973, but they are no faster.
The global airline industry brings in around $800 billion in revenue per year, but only a tiny fraction of the funds sloshing around the industry has been invested in supersonics, and only a minuscule sliver of those private investment dollars have gone toward low-boom designs that could make supersonics go mainstream.
If we had propagated the rate of growth in commercial transatlantic aircraft speeds that existed from 1939 to the mid-1970s, we would have Mach-4 airliners by now. But the overland ban put an end to all that. It made small supersonic aircraft, which need to fly shorter overland routes, essentially illegal, closing off the iteration cycle that could drive progress in the industry.
A sonic boom is caused by a shockwave that is similar to the wake of a boat. When a boat moves very slowly in the water, it doesn’t make a wake. As its speed increases, the disturbances it creates in the water pile up on each other, creating a shockwave. This shockwave propagates much farther than an ordinary ripple in the water. This difference between shockwaves and ordinary disturbances explains why you can hear a plane’s sonic boom at cruise altitude, but not its engine noise.
The analogy also corrects a misconception some people have about sonic boom, that it only occurs when an aircraft crosses the sound barrier. Like the wake of a boat, a shockwave is generated the entire time the aircraft is flying supersonic. However, an observer on the ground only hears the boom once, just as someone swimming in a lake only experiences the wake of a boat once as the boat passes by.
Unlike a boat, an airplane has wings and an empennage. Each appendage of the aircraft initially emits its own small shockwave. As the aggregate shockwave propagates from the aircraft towards the ground, all the overpressure bits of the shockwave migrate to the front of the waveform, and all the underpressure bits migrate to the back. This creates an N-like waveform that we experience like a rapid-fire double boom.
We know from research in sonic boom simulators and from dive maneuver studies that human response to sonic boom is driven almost entirely by the sharpness of the N-wave. If you could round the corners off the N-wave, slowing the rise time of the overpressure on the bow shock, you would have an extremely unobjectionable boom that just sounds like a regular noise.
NASA will soon be testing an experimental low-boom aircraft called the X-59. First flight is due this year. The aircraft is designed to have a cruise sonic boom in standard atmosphere of 75 PLdB or less across the boom carpet. With design margin, it may turn out to be even quieter than that.
Once the aircraft checks out, NASA will use it to start booming American cities again. The purpose of the program will be to gather data on human response to sonic boom. After two or three years of collection, the data will be analyzed and turned over to FAA and foreign regulators who participate in ICAO’s Committee on Aviation Environmental Protection, ostensibly to help design a sonic boom standard.
While I admire NASA’s work on X-59, we already know what the studies will say. 75 PLdB is certainly acceptable. We know this because, as NASA titled one report, there have been Six Decades of Research on sonic boom. In fact, since the report came out in 2014, there have now been Seven Decades of Research.
Some of the research was glorious, if ill-advised. In 1964, over a period of six months, the FAA dropped 1,253 sonic booms over Oklahoma City in a study known as, and I’m not joking here, Operation Bongo II. These were not baby booms. They were full-sized, unabated N-waves.
1964 was a different time, and the government was extremely bullish on supersonics. The US was soliciting designs for an American supersonic airliner to rival Concorde, with Boeing eventually winning the contract on January 1, 1967.
FAA, ever eager to bolster supersonic aviation, put a gloss on the findings from Operation Bongo II. “The overwhelming majority,” their contractor cheerfully wrote, “felt they could learn to live with the numbers and kinds of booms experienced during the six month study.”
This was true. About 73 percent of respondents said they could live indefinitely with the cumulative level of sonic boom experienced in the study. But about 3 percent of the population was vehemently opposed to the point of filing complaints.
Most recently, in 2018, NASA dropped 52 shaped booms on Galveston, TX over the course of 9 days of testing. They did this not with a low-boom aircraft like the X-59, but with an ordinary fighter jet using a nifty maneuver. The pilot would go some miles offshore and climb to an altitude around 50,000 feet. Then he or she would roll the aircraft into an inverted position and pull “up” into a vertical dive, pulling out to recover around 30,000 feet. The aircraft goes supersonic during the vertical dive, generating a sonic boom.
NASA had microphones stationed around the city, recording the waveform in several neighborhoods. I was in Galveston for one day of the tests, and on the day I was there, the measurements at different sites ranged from 65.5 to 89.7 PLdB. One of the booms, which we knew was coming due to NASA’s comms and me watching carefully on the FlightAware app on my phone, was completely inaudible. For one of the louder booms, I watched a fisherman on the pier. He was briefly startled but went back to fishing in under one second.
NASA surveyed Galveston residents based on both individual event exposure and cumulative daily exposure. For cumulative exposure, 17 out of the 2,041 daily responses reported being highly annoyed, which is less than one percent. Given that some people are annoyed by anything, you’re not going to do better than that.
So let’s say that NASA has robust data from X-59 on human response to sonic boom, more robust than what we have managed to collect over the last 70 years. They bring it to ICAO, specifically CAEP’s Working Group 1, which devises noise standards, in around 2025 or 2026. What happens then?
Most likely, nothing happens then. European companies like Dassault and Airbus don’t want to do supersonics and have no interest in competing with American companies who do. Regulators like France’s DGAC are completely in these companies’ pockets. Europe generally acts like a bloc within ICAO, and standards are made by consensus, so it will be like running into a stone wall.
ICAO formally decides new environmental standards at the full CAEP meeting, which occurs every three years. If data arrive in 2025 or 2026, Europe can easily stonewall for two to three years to prevent a new standard from being developed by the CAEP meeting in 2028. They can do this either by arguing for a standard so stringent that nobody can meet it, or by correctly noting that a standard should normally be developed not only based on human response data, but also on verified commercial aircraft performance data, which do not exist yet.
Suppose a standard is developed at ICAO by 2031. Great, now every country can take it home and implement it. Before the FAA can formally adopt the new ICAO standard, it will need to do an Environmental Impact Statement under NEPA. EISs can be challenging in any circumstance, but for supersonics it will be especially hard because aviation will be one of the last sectors to decarbonize. Even if companies are using synthetic, net-zero fuel, the aircraft will still be burning liquid hydrocarbons, which raises emissions relative to creating synthetic fuels and, say, storing them underground. Environmental groups will inevitably sue, potentially delaying the standard beyond the EIS preparation time.
If we’re lucky, we’ll have a sonic boom standard implemented in the United States by the late 2030s.
There is a better way. Congress could repeal the supersonic ban this year in the FAA reauthorization act. I have proposed text along these lines:
Until such time as the Administrator creates standards that allow routine civil supersonic operations in the United States, civil supersonic operations shall be allowed in the United States provided that the aircraft's empirically determined or analytically predicted mean cruise sonic boom on the surface directly beneath the flight track is less than 90 PLdB for daytime operations or 80 PLdB for nighttime operations.
I think this proposal is very clever, if I do say so myself. It would change nothing overnight, because no aircraft that can do a cruise boom less than 90 PLdB exists.
What it would do is signal to the aviation industry that America is open for business. It’s time to build new low-boom aircraft. Manufacturers would start working on new designs, knowing that when they are ready to be certified there won’t be any further obstacles.
Furthermore, although my proposed interim standard is fairly permissive, airframers would likely aim for a boom lower than 90 PLdB. Reducing the boom through aircraft shaping causes increased drag and thus increased fuel burn, but the effect is nonlinear. I expect that airframers would set their sonic boom levels to be around the kink in the curve, perhaps, for a medium-sized airliner, around the low 80s in PLdB.
Ultimately, FAA would replace this interim standard, and it would do so based on the aircraft in operation. If it turns out that a lot of aircraft are operating at around 82 PLdB, that’s probably where FAA will draw the line. The first non-interim standard could also be more complex, using Mach number or aircraft weight as a correlating parameter. Today, heavier aircraft are allowed to be louder on takeoff and landing, and it makes sense for correlating parameters to be used in sonic boom standards as well.
I’m struck by the fact that American economic growth went off the rails in 1973, the same year the overland ban on supersonic flight came into force. The speed limit cannot be responsible for the entirety of the Great Stagnation, of course. The cumulative amount of missing growth is comparable to the entire economy, not to the size of the aviation industry. The numbers don’t add up.
Yet, the ban is not unrelated to economic stagnation. To borrow a term from Ross Douthat, there is something decadent about putting a complete halt to the development of a key technology simply because a few otherwise harmless sonic booms might annoy a vocal minority. With boom-shaping technology we know is possible, a tiny vocal minority. The cultural forces that led to and sustain the ban have certainly halted other progress.
We need to get back to doing great things. In an FAA-sponsored paper that came out in 1972, perhaps a last gasp of virtue before decadence set in, Prof. Ronald Kohl evaluated how to reduce sonic boom using focused laser beams to create a “phantom body.” He modeled a Mach-2.7, 600,000 pound aircraft and concluded it would take a 420 MW laser. If only we could have such nice things.
Fifty years of boomless skies is more than enough. If we want growth—if we want greatness—it’s time to make America boom again.
Great history I somehow did not know despite years in Aerospace. We should free up the limit. However, the value vs cost of higher airspeed is a bit wobbly. Even point to point ATM has been hard to justify. The limit however should be modified to let entrepreneurs make their business case.
I sort of like the occasional sound of sonic boom.