Upon taking his first ride in an airplane, William E. Boeing set his feet firmly back on solid ground and immediately remarked, “I think we could build a better one.” A century after this bold claim was made, millions of people travel the world every day on Boeing airplanes—which are among the safest, fastest, and most efficient modes of transport ever built. It is with the same audacious spirit that airplane designers are now considering the rebirth of a supersonic commercial airliner.
The idea of a supersonic aircraft for commercial use is not a new one. Major aerospace companies invested significant resources in development of such an aircraft in the 1960s and ‘70s, resulting in production of the iconic Concorde. Offering the ability to cut transatlantic travel times in half, the Concorde was wildly popular among frequent long-distance travelers and businessmen. However, operation of the Concorde was riddled with a myriad of issues. The last Concorde was retired in 2003, just twenty years after its introduction, without any existing supersonic civil transport to take its place. New proposals for a high-speed civil transport call to mind the Concorde’s troubled past, requiring discussion of the ethical implications such a challenge poses. Consideration of noise, greenhouse gas emissions, safety, and this project’s potential to advance the aerospace industry show that the most ethical course of action is to proceed with the design of a new supersonic transport.
One of the most obvious obstacles to supersonic aircraft design is greenhouse gas emissions. As aircraft fly faster, more thrust is required to propel them to those speeds—which requires burning more fuel. Consequently, supersonic aircraft appear to be inherently more polluting than subsonic aircraft. However, deeper analysis reveals some interesting phenomena that have yet to be fully explored. Supersonic aircraft must fly at higher altitudes for efficiency and noise mitigation purposes. Because the characteristics of the atmosphere are different at higher altitudes, supersonic aircraft emissions have a markedly different effect on the planet than those at lower altitudes. For example, emissions of nitrogen oxides, a primary concern for global health, are predicted to be significantly lower for supersonic flights than subsonic.  However, the effect of contrails, the streams of condensed water vapor often seen trailing behind aircraft, is yet to be fully understood. Climate scientists hypothesize that the contrails emitted by supersonic aircraft could cause a warming effect nearly ten times greater than those emitted by subsonic aircraft.  Emitted contrails quickly fall to lower altitudes where they are entrained by large weather systems and cease to have an effect on the global climate. Because of their significantly higher cruising altitude, supersonic contrails would likely not be entrained in such a way. This gives rise to one of the most fascinating ethical dilemmas of our time—supersonic flight at high altitude has the potential to improve global health, but it may have unintended impacts on the global climate. This dilemma lends itself to the utility test, whereby good is maximized and harm is minimized. Research has been able to quantify the health effects of low-altitude subsonic flight, while the effect of supersonic flight on global climate is largely speculative. In order to mitigate the adverse health effects of which engineers are currently aware, the utility test favors the development of supersonic aircraft.
Since 1973, the FAA has prohibited supersonic flight over land due to noise and safety concerns. Further, noise emissions have been a primary focus of aircraft development in recent years. The FAA is now proposing a noise reduction requirement of 17db from its previous baseline , and the Boeing Company is touting noise reductions of 40% over the previous generation of single-aisle aircraft with its newest aircraft, the 737 MAX-8.  Such improvements have been facilitated by new engine and airframe designs, such as the chevron cutouts found on the latest commercial jet engine nacelles.  The 1973 restrictions were imposed after the FAA received thousands of complaints from civilians in areas where the military had been conducting tests to measure the effect of sonic booms on communities on the ground. This regulation is a classic example of the rights test, wherein the actions of those who benefit from flying on the airplane adversely affect those on the ground. Consequently, supersonic aircraft are restricted to operation almost exclusively over water. Clearly, the FAA is aware of these issues, and engineers working on this problem are already working within an existing ethical framework. The FAA has stated that in order to consider deregulating supersonic flight over land, designers must be able to prove they have developed technology that sufficiently mitigates the effects of sonic booms.  Noise reduction is a huge point of emphasis in the industry today, motivated by protecting the rights of those living near airports and under flight paths. These groups have long had their right to a peaceful living environment violated by increasing aviation activity. Therefore, it may seem irresponsible to focus on new aircraft that push us farther away from the ambitious noise reduction targets set by the FAA. However, the recent focus on acoustics signals a shift in aircraft design philosophy. Rather than focusing entirely on minimizing fuel burn, modern aerodynamic analysis now allows concurrent emphasis on noise reduction. Great advances have been made in the field of aeroacoustics, enabling some groups to develop supersonic aircraft designed entirely around mitigating sonic booms and passing the rights test in the process. 
Experimental aircraft, or “X-planes,” have been used throughout history to reach aviation landmarks such as breaking the sound barrier, investigating stability characteristics of unusual designs, and achieving fully autonomous flight.  The next generation of X-planes, which is being funded by Congress through NASA, is focused on making strides in green aviation technology through full-scale tests of new technologies that have, until now, existed only on paper. In order to demonstrate new low-boom capabilities, it has been proposed that one of the X-planes be a supersonic aircraft. Lockheed Martin’s Advanced Development Programs, commonly known as “Skunk Works,” has finalized a preliminary design for a Low Boom Flight Demonstrator (LBFD) for this purpose.  The LBFD works by understanding the shape of the pressure distribution across the entire airframe. When an aircraft goes supersonic, a pair of booms are produced—one when the pressure peaks, and another when it returns to normal. Shaping the lifting surfaces to force smaller, weaker shockwaves to diverge into smaller shocks, rather than converge into a large, strong shock, drastically reduces the noise signature of the aircraft. Rather than a loud boom, such aircraft would produce a soft “heartbeat” sound.
With profits in the aerospace industry stagnating and focus shifting almost exclusively to cutting costs, there is little incentive in the industry today to push boundaries and continue to develop cleaner, quieter, and safer aircraft. However, there is ample evidence that advances in high-tech sectors trickle down to more commonplace modes of transportation. One example of this is the automatic dependent surveillance broadcast system (ADS-B). ADS-B is a flight deck situational awareness tool that allows aircraft to relay their positions to one another in order to avoid collisions. Originally developed for commercial aircraft, the FAA mandated in 2011 that all general-aviation aircraft begin being equipped with ADS-B transponders in order to fly in certain airspaces.  While the safety record of commercial aviation has steadily improved over the last several decades to a point where incidents are now exceptionally rare, general aviation has continued to lag far behind. Since the introduction of the ADS-B mandate, the rate of general aviation incidents has begun to steadily decline. A market for commercial supersonic aircraft could have a similar effect on the subsonic transport market, leading to aircraft that are cleaner, quieter, and safer in all areas of the industry.
In addition to environmental and safety concerns, engineers must consider the impact their designs will have on their companies’ reputations and finances in order to make ethically-sound decisions. Such factors were key in the decision to permanently retire the Concorde, as costs of fuel rose and public perception declined due to a couple of high-profile fatal crashes. Applying the virtue test, engineers may be justified in refusing to pursue development of a high-speed civil transport due to safety and financial concerns. However, many have argued that the Concorde was not an inherently infeasible design, but rather was plagued by excessive regulation and used technology that was not yet fully mature. As discussed previously, new technologies in development have the potential to persuade the FAA to roll back regulations of supersonic flight over land, potentially making a supersonic commercial aircraft an attractive option once again.  It may be difficult to see how such an ambitious development program may be successful, both financially and in adequately considering the rights and viewpoints of a variety of affected audiences, but the history of aviation has been defined by problems such as these. With the aerospace industry’s recent emphasis on noise reduction, ongoing research into the effects of high-altitude emissions on the global climate, and the potential for supersonic aircraft development to benefit the entire industry, aircraft designers are ethically compelled to pursue development of a new supersonic commercial transport.
By Addison Salzman, Viterbi School of Engineering, University of Southern California
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