To save energy and tackle pollution, the aviation industry is taking some surprising aerodynamic cues from birds. Aircraft manufacturing giant Airbus is developing a system, called fello’fly, in which two commercial planes mimic migrating birds by traveling in tandem with one close behind the other. The company says this arrangement could enable the follower aircraft to reduce its fuel requirements by as much as 10 percent per trip.
Fello’fly was inspired by the way some birds, notably geese, often fly in formation to save energy. Like a bird, a moving airplane leaves a wake of disturbed air behind as its wings generate swirling vortices. These vortices move in a circular motion and expand outward until they dissipate, creating an updraft around the wake. If a second aircraft follows the leader at a specific distance and slightly away from the center of the wake, this updraft provides extra lift.
Such a boost allows the follower to cut down on engine thrust, fuel use and emissions. But to reap these benefits, airliners have to position themselves carefully: if the follower gets too close to the center of the leader’s path, the vortices will actually push it downward instead of up. Planes also have to greatly reduce the distance they usually maintain. “Fello’fly operations will reduce longitudinal ‘tail to tail’ separation from around 55 kilometers in oceanic airspace to three kilometers,” says project leader Nick Macdonald, a senior engineer at Airbus.
The multinational company conducted three tests of the system in July. Two Airbus A350 aircraft flew in France’s western Atlantic airspace, remaining three kilometers apart for four hours. “These tests demonstrated that it is possible to stabilize the aircraft within the wake updraft on autopilot and that the fuel savings are significant,” Macdonald says.
Fello’fly is intended to work with compatible craft from different airlines that happen to be in the same airspace at the same time. For the next stage of testing, which is designed to develop safety protocols and is set to take place over the Atlantic Ocean as early as next year, Airbus recently signed agreements with the airlines French Bee and Scandinavian Airlines (SAS). Other planned participants include international air navigation service providers such as France’s Directorate of Air Navigation Services (DSNA), the U.K.’s NATS Holdings (formerly National Air Traffic Services) and the Belgium-based EUROCONTROL.
Aircraft flying so close would need to coordinate meticulously to prevent accidents and excessive turbulence, and this coordination would need to start on the ground. Airline pilot David Mrak, who is not affiliated with the fello’fly project, says challenges would likely include setting rules of engagement between the two planes, deciding on the role of air traffic controllers, maintaining communication between all parties involved, creating plans to deal with emergency situations or bad weather, and divvying up the economic benefits. “For example, with two different airlines envisaged in a fello’fly flight, the issue becomes ‘Who gets to save the gas?’” Mrak says.
EUROCONTROL is partnering with Airbus on this project. The organization will focus on codifying the requirements and procedures for the predeparture phase, including enhancing communication between the aircraft and air traffic controllers. Giovanni Lenti, who heads EUROCONTROL operations that deal with airspace data maintenance, flight plans and flow management, says strict rules of operation, in combination with specialized software, will aid in tackling such challenges—and in choosing which planes to pair up. “The software that will help in finding the suitable partners and pairing the aircraft [by taking into consideration their respective flight plans] is being developed by Airbus,” he says.
A pair of airplanes using fello’fly would take off separately, and their pilots would then rely on an onboard system to safely bring the two craft into formation. Currently, air traffic controllers are largely responsible for maintaining horizontal separation between two aircraft at the same altitude, Lenti says. But “to reduce the distance to around 1.5 nautical miles (around three kilometers), which is necessary for fello’fly,” he adds, “it will have to be done by using new onboard technology, embedded in aircraft avionics, by the pilots.”
Although the minimal separation might sound potentially dangerous, Mrak suggests that operating planes so near each other may not be out of the question. “As aviators, we undergo extensive training,” he says. “Flying aircraft in formation is a matter of proper training and protocol.”
As a further fail-safe, Lenti says, the air traffic controller would have to keep a dedicated flight route empty about 305 meters below both aircraft. That way, if either plane encounters a problem that requires it to change its flight level, it can immediately disengage from the pairing and descend to a safe altitude.
Airbus also has to contend with the fact that flying in such proximity could create too much turbulence for passenger comfort. But Macdonald claims that previous fello’fly tests have already addressed this issue. He says these tests found that if the follower plane stays at a suitable point in the leader’s wake updraft, it will only experience turbulence similar to what an aircraft typically encounters during normal operations.
Airbus hopes to roll out the concept in commercial airline operations around 2025, although, Leni notes, it would initially only be used on oceanic routes in order to avoid heavier air traffic. “At the beginning, it will not be possible to implement it on continental, aircraft-congested routes,” he says. “However, on the ocean, it can work very well—with significant fuel savings for the follower aircraft.”
Mrak estimates that if the system works, “it could potentially save the airlines thousands of pounds [several metric tons] of fuel per transatlantic trip.” And according to Airbus, every metric ton of fuel saved means roughly three metric tons less carbon dioxide emitted.
A version of this article with the title “Tight Flight” was adapted for inclusion in the February 2021 issue of Scientific American.
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