Environmental performance of emerging supersonic transport aircraft
Death, taxes, and supersonic aircraft? Considering what’s certain, and what isn’t, under the FAA Reauthorization Act
There’s an old saw, most famously attributed to Benjamin Franklin, that nothing in life is certain but death and taxes. More recently, some have argued that the return of commercial supersonic flight is “inevitable”. Indeed, the FAA Reauthorization Bill that President Trump signed into law last week includes language that aims to clarify how environmental standards will be set for emerging supersonic transport (SST) aircraft. So, what’s in the FAA bill, and what does it mean for the future of supersonic flight?
First, a bit of context. The current lack of SST environmental standards is an important barrier to would-be manufacturers. It’s difficult to justify substantial new investments to develop these aircraft without a clear idea of what noise, air pollution, and fuel efficiency targets they will be held to. According to our preliminary assessment, emerging commercial SSTs are likely to emit between five and seven times as much carbon dioxide (CO2) as comparable subsonic aircraft flying the same routes (figure below).
That study also concluded that commercial SSTs built around existing “derivative” engines are likely to fail existing environmental standards for subsonic aircraft. So, policymakers face the choice between establishing new standards specific to emerging SSTs that would allow them to pollute more than today’s new aircraft, or applying standards that may require more advanced SSTs based upon new “clean sheet” engines.
What does FAA bill have to say about all this? As you’d expect from its length (more than 1200 pages), the bill itself includes a hodgepodge of provisions ranging from the regulation of drones, setting minimum standards for seat width and pitch, mandatory crew break times, and even the use of cell phones on planes. To see the language related to supersonics you need to dig down into Section 181 of the bill here.
The most important provisions are in Section 181 subsections (e) and (f). Subsection (e), entitled “Long-term Regulatory Reform”, requires that FAA propose a new landing and takeoff (LTO) noise standard for supersonic aircraft by March 2020, to be finalized by September 2021. However, the bill is silent on how stringent to set that standard, a key point that we return to later. A related provision aims to modernize supersonic flight testing in the US. Mostly, this seems to be requiring FAA to do something its already started, as previously highlighted here.
Subsection (f) covers near-term SST certifications and how they would be performed before standards are put into effect. Specifically, the FAA would need to initiate a rulemaking on noise standards for new designs if applications for type certification are made before the noise standard above is finalized. Again, anyone looking for certainty will be disappointed – no required stringency level is indicated. Finally, the bill would require that FAA reassess, at two-year intervals starting in 2020, whether the existing ban on supersonic overland flights can be lifted and replaced by an en-route (sonic boom) noise standard.
So, taken all together, what does this mean for the “inevitability” of further SST development? It’s hard to say much with confidence given that the bill grants discretion over standard setting to FAA, which is itself constrained by the aviation industry’s preference for global, harmonized standard established through the UN’s International Civil Aviation Organization (ICAO). That organization is famously bureaucratic, risk averse, and unlikely to develop comprehensive standards to support supersonics before 2025. There’s also a significant rift between the US and Europe on aircraft noise that needs to be resolved. Any interim FAA certification that allows noisy SSTs to enter into service is likely to be viewed skeptically abroad. That would be a serious problem for aircraft that would be operated largely on international routes if successfully brought to market.
How this continued uncertainty is viewed by companies attempting to revive SSTs likely varies. Two manufacturers– Aerion and Spike –intend to comply with today’s LTO noise standards no matter what FAA and ICAO ultimately decide. The third, Boom, had been advocating for FAA bill language that would have directed FAA to grandfather new SSTs into older LTO noise standards and pre-emptively lift the overland flight ban by 2022. That language came under fire from environmental and community groups, not to mention anti-Concorde veterans, and was ultimately stripped from the final bill. Boom has struggled to identify an engine partner to meet its demanding performance requirements, a problem that seems likely to linger along with the uncertainty about what environmental standards that engine will need to meet.
The basic argument that SST travel is inevitable echoes back to the Concorde era, where the large development costs needed to enable supersonic flight were justified as crucial to sustain technological progress. In the end, Concorde showed itself to be a technological dead end in 2003, in large part because airlines chose to prioritize reduced fuel burn and ticket costs for the average consumer over finding new ways to shuttle the very rich around at higher and higher speeds. As this analysis shows, new commercial SSTs will struggle to achieve fuel burn parity with existing subsonic business class. This suggests that, absent a fundamental shift in the airlines’ business model, new commercial SSTs may share the same economic fate as Concorde.
Will this time be different? Nobody knows for sure. But as subsonic aircraft become increasingly efficient, and the limits of our carbon budget ever more clear, new SSTs will look less certain, not more.
Myself, I’ll take the odds on death and taxes. Come to think of it, maybe just death.