In This Issue
Summer Bridge Issue on Aeronautics
June 26, 2020 Volume 50 Issue 2
The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come.

Guest Editors' Introduction

Friday, June 26, 2020

Author: Alton D. Romig Jr. and John J. Tracy

Aeronautics: Back to the Future

It has been 16 years since The Bridge last focused on aviation (fall 2004). Since then aviation has witnessed significant advances in propulsion, structure, and guidance, navigation, and control, to name just a few areas. There is even renewed interest and considerable momentum toward supersonic and hypersonic vehicle design concepts.

It should be noted, though, that all of this work is being accomplished during a period when 30–40 percent of the aviation/aerospace workforce is eligible to retire in the next 5 years, creating a huge need to inspire and train the next generation of engineers (Hedden and Sands 2019).


Commercial aviation is responsible for between 5 and 8 percent of the world’s gross domestic product. The aviation ecosystem employees millions of people around the world (FAA 2016). And aviation continues to grow in technical sophistication and popularity as average incomes rise and international trade touches every ­corner of the planet.

Furthermore, aviation remains by far the safest form of transportation in terms of passenger-miles traveled (Ferro 2018). When there is an accident, however, it is dramatic and raises concerns. Two deadly crashes of the Boeing 737 MAX in 2018 and 2019 garnered much attention and brought the topic of aviation safety to the fore in discussions throughout the world. The impacts and lessons learned continue to reverberate.

In addition, public concern about climate change now regularly includes the role of aviation. Manufacturers and operators are heeding the call for more environmentally sustainable designs and fuels.

In This Issue

Since the beginning of the jet age in the mid-1950s, fuel efficiency has increased by over 70 percent and in general continues to improve about 1 percent a year. Aerodynamics, structures, materials, and propulsion all play into this improvement, but none more than advances in gas turbine engines. Movement toward higher bypass ratio engines has driven the performance of the industry to levels that were previously unimagined. Alan Epstein reviews advances in aerodynamics, thermodynamics, design, materials, and manufacturing that are enhancing engine performance, life, safety, and efficiency. He also discusses propulsion challenges related to super­sonic flight, electrification, and sustainability.

Motivated by the need to improve even further in the areas of safety and the environment, exciting work is being conducted in hybrid and fully electric propulsion. Both of these areas offer great promise over longer time horizons and various use cases. As with any new technologies, there have been many more discussions of the potential benefits than there have been of the technical and cost constraints. Jean Botti lays out the benefits and the challenges for hybrid propulsion approaches, considering battery energy density, durability, charging, safety, and pilot training.

All-electric propulsion has received a great deal of attention, catalyzed by the growth in electric automobiles. The general public has seen numerous popular accounts of flight-based “ride-sharing services” using all-electric propulsion. John Langford and David Hall explore the benefits and challenges of electrification in an analytical assessment of specific all-electric configurations. They also look at some particularly interesting distributed propulsion approaches not readily available for gas turbines as well as required battery energy density, fuel cells, and solar power.

Despite substantial reductions in fuel burn and, in turn, CO2 production, aviation is still a measurable contributor (~2 percent) to greenhouse gases. The desire to return to supersonic air travel is dampened by concerns about its environmental impact and noise. Raymond Russell, Lourdes Maurice, and Rachel Devine discuss these concerns in the context of progress since the ­Concorde. They describe advances in aerodynamic design, engines and turbofans, materials, and sustainable fuels as well as operational changes, all of which improve fuel burn and reduce noise.

Like the environment, safety is an area of great interest to both the aviation specialist and the general public. Air transportation has actually seen orders of magnitude improvement in safety over the past half-century (­Allianz 2014) because every accident and near miss is investigated to its root cause in a joint effort between industry and government regulators. As aviation systems become more complex and have human, ground, and in-air components, the safety and risk analysis of these systems also become more complex. B. John Garrick and Ali Mosleh discuss quantitative risk–based approaches that take account of this complexity, data limitations, performance-influencing factors that affect pilot decision making, and the key enablers of hardware, software, and modeling. Specific applications discussed include collision avoidance, accompanied by analysis of a 2002 midair collision.

As automation increasingly permeates daily life, its impact on aviation safety requires thoughtful consideration. J-P. Clarke and Claire Tomlin provide baseline definitions of autonomy for aircraft applications and review safety aspects of autonomy, describing the use of human-machine teaming, neural networks for flight management, the loyal wingman program, and run-time assurance. They also touch on the highly anticipated possibility of urban air mobility.

The prospect of being able to fly from New York to London in 45 minutes is sure to get the attention of business travelers and the public. Hypersonic aviation offers the promise of such travel. But with that promise come some of the most significant technical challenges that the industry has ever faced. Kevin Bowcutt reviews the motivation for hypersonic flight, progress to date, and the challenges going forward in design, materials, fuels, costs, noise, regulation, and impacts on the environment.

The progress described in these articles resulted from the hard work of generations of engineers both in the United States and around the world. But the aero­nautics engineering workforce is now at a crossroads. In 2019, 30 percent of its employees were over 55 years of age. Over the next few years there is a very real risk that more people will voluntarily leave the industry for retirement than can be replaced with new graduates. The need is clear to improve ways to inspire and educate the future workforce. Darryll Pines surveys the history and importance of prize-based competitions in the critical success factors of imagination, invention, innovation, investment, and impact as related to advances in aerospace. His examples illustrate the attractiveness of such competitions to innovators of all ages.

Finally, we thank the following, who were asked to evaluate the accuracy, coverage, and substantiation of these articles: Juan Alonso, John D. Anderson, Karl Bilimoria, Marty Bradley, Gillian Bussey, Nicholas Cumpsty, Brian German, Edward Greitzer, Jonathon How, Keoki Jackson, Jeff Lenorovitz, Charles Leonard, Muni Majjigi,  Dimitri Mavris, Roger L. McCarthy, and Hamid Shirazi. 

The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come. We welcome your feedback on this aviation-focused issue.


Allianz. 2014. Global Aviation Safety Study. Munich: Allianz Global Corporate & Specialty SE.

FAA [Federal Aviation Administration]. 2016. The ­Economic Impact of Civil Aviation on the US Economy. Washington.

Ferro S. 2018. Fear of flying? Airplanes are, in fact, the safest way to travel. Mental Floss, May 9.

Hedden CR, Sands C, eds. 2019. 2019 Workforce Data Pinpoints Need for Tailored Recruiting as Job Requirements Boom. New York: Aviation Week Network.

About the Author:Al Romig (NAE) is NAE executive officer and former vice president and general manager of Lockheed Martin ­Aeronautics Company Advanced Development Programs, better known as the Skunk Works®. John Tracy (NAE) is retired chief technology officer and senior vice president of engineering for the ­Boeing Company.