Towing the line

Deep Dive


Ask a battery supporter about hydrogen and they will say the energy density is too low. Ask a fan of hydrogen about batteries and they will tell you something similar.

So how do you get meaningful zero emissions performance without waiting for technology to evolve?

The team at Magpie Aviation believes it has the answer – towing. Aerotowing is not a new concept having been performed for a century or more – usually to launch gliders or transport military aircraft. But California-based Magpie Aviation thinks the concept can be applied to zero emission aviation and thus achieve meaningful distances out of otherwise incapable aircraft.

So how does it work? By automatically connecting an electric passenger or cargo aircraft – known here as the ‘main aircraft’ – to an electric ‘tow aircraft’ in the sky; with a safe distance of approximately 1,500ft (500m) between them. The tow aircraft, which only carries batteries, can tow the main aircraft at high speeds for around 300 miles per charge (with today’s batteries); no electrical power is transferred through the line, propulsion is purely mechanical. This means the main aircraft only needs power to takeoff, land, and then during transition (with reserves) – the rest of the time it glides whilst being towed. For longer routes, the tow aircraft can then be safely swapped out in the sky and it lands at a clear secondary airport to recharge for its next tow.

Taking an example flight from San Francisco (SF) to Los Angeles (LA), a charger can be placed at Hollister Airport just south of SF and one at Lancaster Airport immediately north of LA. With those two bases any potential flights from the greater area of each city can be catered to.

Upon hearing about this concept for the first time a number of questions immediately arose: How safe is this? Surely wingspans have to increase? There are not enough pilots, but now aircraft requirements have at least doubled? Three aircraft, how much does that cost? But Magpie, backed by a programme of record with the FAA’s Center for Emerging Concepts and Innovation, has an answer for each. The firm has recently completed its first in-flight aerotowing tests (pictured below) crewed by veteran US Air Force and Airbus programme test pilots Tim Gardner and Jim Payne. Magpie’s automated technology enabled its two prototype aircraft to connect in the sky repeatedly, and with “centimetre-level” precision. All the pilot has to do is hold a closing speed and let the active hook do the work.

“What we are building is an enabling technology,” Oliver Haas, head of Partnerships and Strategy tells Revolution.Aero. “What we are really building is the ability to connect two aircraft safely and reliably in the sky. We can make this work really well on a retrofitting basis too. For the tow aircraft, you can electrify an existing airframe, rip out the passenger infrastructure and fill it with batteries. For the passenger or cargo aircraft, you need to make some minor structural modifications to the front for attaching the tow device, as well as the capability to operate on low-power. This adaptability allows Magpie to move more quickly towards commercial operation.”

The benefits of the system really start to fly when the tow aircraft is customised. It is hard to miss the extra-wide wingspan in the renderings produced by Magpie of a tow aircraft. That wingspan allows the two aircraft to glide down to its secondary base once it has completed towing, allowing it to reach maximum tow range without the need for extra power to get back home. But Magpie is not building those aircraft today, it is going to retrofit at Part 23-scale first, says Haas.


Coordinating two aircraft in the sky requires a robust safety case and Magpie is confident that the 500m towing distance allows for optimum safety levels. “It is lower than the separation distance today between aircraft and this is something we are addressing currently with the FAA. The tow aircraft also flies slightly below the main aircraft, such that if any incident were to occur the main aircraft has full autonomy to drop the connection and alter its course or altitude. There are also templates for how the regulator thinks about this already, such as military refuelling in civilian airspace.”

 More aircraft also means more pilots and you would have to have been stuck under a very large rock to not know pilots are in high demand these days. Automation of the tow aircraft could be key here, but Haas notes that the company is not betting on this technology for its path to market. “Pilot shortage is top of mind for many,” says Haas. “There are ways we can really benefit some of the challenges happening right now. We are exploring the option of making the tow aircraft a single-pilot operation, but we think it could also make a great training platform for junior pilots to build hours. It is also a way to attract a more senior pilot who may have retired from long-haul into that aircraft. Because there are no passengers onboard you have a slightly different context on who can sit up front.”

Economically, you have more aircraft flying. With the help of an independent consultant, Magpie built a model to compare costs against other forms of sustainable aviation, both operational and proposed. Looking at an ATR aircraft, typically fuel and maintenance will make up around 30-40% of the operating cost. “If you were to theoretically electrify an ATR, it would fly a few tens of miles plus reserves. Not a very useful aircraft, but the fuel and maintenance costs drop significantly due to the electrification. So now you need to add the towing cost. At scale, with 2030 batteries, we believe we can roughly reach parity of existing operating costs. The reason we are excited about parity is because if you compare that to SAF or hydrogen we are significantly lower than what those are predicted to cost.

 “We are not trying to sell snake oil. As a company we want to be as open and honest as possible about our approach. This is a scaled view assuming you have aircraft cycling through a full 12-hour segment of a day with reasonable charging times – not 20 minutes for example. Because you get to electrify a whole aircraft, you can basically use the significant savings of that platform to roughly pay off the cost of your towing on an operating cost basis,” says Haas.

Magpie’s founders have pedigree in the future flight world. Before Magpie, Damon Vander Lind established the Heaviside eVTOL aircraft division of Kitty Hawk. Also, prior to that he ran Makani Power, a firm that developed tethered airborne wind turbines before being acquired by Google. “While the idea might sound crazy, there is no reason why it isn’t possible” says Vander Lind.

As a number of startups ‘toe the line’ to build the future of aviation with yet-to-be delivered breakthrough technologies like new batteries or green hydrogen, Magpie’s bet on towing stems from a more pragmatic view. The majority of the world’s flights are over 1,000 miles but there currently aren’t any zero-emission solutions that cover this distance. The Magpie approach looks to solve this using existing technology. “We like to bet on one miracle at a time,” Vander Lind tells us.

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