Air Traffic Control in the Upper Reaches

Historically, not many aircraft have flown above 60,000 ft. For the most part, the upper reaches of the atmosphere belonged to a few high-performance military aircraft, along with transiting spacecraft. Airspace that high, where the sky turns black and the horizon bends with the curvature of the earth, needed little control.

That’s about to change, and regulators in the United States and Europe are trying to prepare.

In the coming years, more aircraft will operate above 60,000 ft, in what’s officially known as “Upper Class E airspace.” Some of these aircraft will be slow or stationary. In the United States, airship operators such as the New Mexico-based Sceye hope to use what they call high-altitude platform stations (HAPS) for internet service and earth imagery.

Unmanned free balloons, used mainly for scientific purposes, also can reach altitudes well in excess of 60,000 ft. 

Other aircraft fly those altitudes at supersonic speeds. The now-retired Lockheed SR-71 Blackbird, first flown in 1964, reached altitudes up to 85,000 ft. . More modern supersonic aircraft, such as the Lockheed Martin X-59, are now under development, and supersonic airliners may also return to the skies. Earlier this year, Boom Supersonic announced a successful flight of the XB-1, an experimental aircraft designed as a test bed for a supersonic transport dubbed Overture. The company hopes Overture will become the first civil supersonic transport since the retirement of the Concorde two decades ago.

This mix of fast-movers and slow-movers, some crewed and some not, in airspace that will only get busier, presents a challenge.

The U.S. National Aeronautics and Space Administration (NASA) and the U.S. Federal Aviation Administration (FAA) are working on a project to manage that airspace. The concept for Upper Class E Traffic Management, or ETM, calls for a collaborative approach. Operators will coordinate with each other regarding separation of aircraft in Upper Class E airspace, and air traffic control (ATC) will coordinate separation as required during the ascent and descent phases of flight. The ETM Concept of Operations describes how operators will accomplish three main tasks:

• Plan flights to Upper Class E airspace;
• Interact with ATC during transit to and from Upper Class E airspace; and,
• Manage contingency events.

Based on an article on the NASA website, a typical day of these procedures might go something like this: Before departure, the crew of a commercial supersonic airliner checks the Upper Class E airspace they plan to use, and they see that a stray balloon has drifted into the area. The pilots create a route that avoids the balloon, and they share that information with other users. The airliner takes off and flies with normal ATC clearances and procedures until it climbs above 60,000 ft. Then it flies the planned Class E route, which is familiar to other high-altitude operators. The next day, the operators of a high-altitude long endurance (HALE) weather drone note the stray balloon at 68,000 ft, thanks to the same information-sharing program. They direct their aircraft to alter course to avoid a conflict. When the HALE aircraft descends for landing, it comes under ATC control below 60,000 ft.

How to make all this happen smoothly remains a topic of study. A paper presented by NASA researchers this year for the American Institute of Aeronautics and Astronautics (AIAA) outlined two systems needed for vehicle operators and controllers to avoid airspace conflicts. One was described as an “integrated visualization tool” to display flight information and airspace designations. The other was an “integrated digital communication tool” for exchanging information about vehicle positions and  coordinating airspace approvals.

Other research makes clear the need for a good picture of airspace traffic. Another AIAA paper noted the huge differences in speeds and mission times for various high-altitude vehicles. They range from HALE airships that move from 8 kt to 100 kts depending on winds, with a months-long mission time, to supersonic aircraft that are in the airspace for only hours.

An added complexity is the maneuverability difference between various aircraft at these altitudes. Part of the ongoing research has considered problems such as the dynamics of a high-altitude balloon, with limited maneuverability, drifting at the edge of space. 

European countries also are working on how to manage what they term higher airspace operations (HAO). The European Concept for Higher Airspace Operation (ECHO) envisions a collaborative approach similar to the NASA/FAA plan. ECHO describes how ATC would control traffic transiting through lower altitudes. Then, when the vehicle climbs into higher airspace, the operator becomes free to navigate and monitor by its own means. “However, it is assumed that since (Flight Information Service) may be applicable,” the ECHO document says, “communicating position and flight intentions regularly by the operator . . . will be possible.” 

The ECHO concept includes provisions for managing what might go wrong. Contingency plans describe responsibilities for vehicle operators, controllers, government regulators, and military authorities. The documents notes various failure modes for both low-speed and high-speed aircraft. Potential malfunctions with low-speed vehicles include diversion from planned trajectory, uncontrolled landing, loss of control over speed or altitude, structural failure, and loss of communication. Possible problems with fast-movers include catastrophic failure and fragmentation, impaired performance or control, loss of telemetry, and uncontrolled descent or re-entry.

Under the Single European Sky Air Traffic Management Research (SESAR) program, an ECHO-2 project is currently underway to build on the original ECHO research. Expected to continue into 2026, ECHO-2 focuses on real-time monitoring of space launches and ground and air-ground operational integration procedures.

As higher-airspace operations become more common, countries that fly in this domain will have to work together. The ECHO concept of operations says, “An international level regulatory coordination will be needed for certain types of vehicles planning global operations.” ECHO also calls for international cooperation on researching new procedures for flights at the fringes of space.

Image: mivet29/Shutterstock

Thomas W. Young is a retired airline captain and a former instructor flight engineer with the West Virginia (U.S.) Air National Guard. Young has logged nearly 12,000 hours of pilot and flight engineer time.