Boeing says formation flight tests conducted with FedEx Express using two 777F freighters confirm the potential for reducing fuel consumption by up to 10% by flying in the wingtip vortex of the lead aircraft.
The test, details of which have been disclosed for the first time, was conducted during Boeing’s 2018 ecoDemonstrator program using an aircraft leased from the cargo carrier. The evaluation took place off the Washington-Oregon coast with the ecoDemonstrator flying in the wake of another FedEx 777F which was undergoing customer acceptance testing prior to delivery.
News of the Boeing test, which was the first by the company to investigate the potential efficiency benefits of wake surfing using two airliners, follows similar flight trials by the U.S. Air Force, DARPA and NASA.
In November Airbus also announced it plans to explore the fuel saving phenomena under the fello’fly technology demonstrator project which will involve formation flying with a pair of A350s in 2020. The following year Airbus also hopes to start operational trials on transatlantic flight in cooperation with airlines.
The Boeing-FedEx tests focused on validation of the wake prediction model used to assess the optimum cruise location for the in-trail aircraft, as well as the performance of the autopilot to maintain the correct computed position based on input from modified automatic dependent surveillance–broadcast (ADS-B) datalink and traffic alert and collision avoidance (TCAS) systems.
Using precise positioning information and the wake prediction model, the autopilot flies the aircraft in a zone where upwash inside the lead-aircraft’s vortex increases the angle of attack on the trail-aircraft’s wing. This rotates the lift vector and, because lift is an order of magnitude greater that drag, produces a significant reduction in drag for a small increase in lift.
Previous tests into wake surfing—such as NASA’s Autonomous Formation Flight program in the early 2000s, which used a pair of F/A-18s, and the Air Force Research Laboratory’s Surfing Aircraft Vortices for Energy project with paired C-17s in 2013—indicated the potential for fuel savings of 10-15%. In 2017 NASA also flew four wake surfing test flights with two Gulfstream G3 variants to validate performance improvements from a modified autopilot in the trailing aircraft and measured sustained fuel flow reductions up to 8%, with brief periods of 10%. 
“We took that technology and put it on a commercial aircraft,” said Doug Christensen, Boeing Technical Fellow and ecoDemonstrator program technical leader. “The system uses ADS-B (IN) and ADS-B (OUT), and it is really about the trailing aircraft knowing what the lead airplane is. Through ADS-B it knows where the aircraft is, its weight and speed. Knowing all that, the trailing aircraft can position itself in the wake where it is safe and ride the vortex to take advantage of the drag reduction. The system in real time predicts the strength and location of that vortex from the leading aircraft and position itself accordingly.”
“The system relies on really robust and good wake prediction models,” said Jeanne Yu, director of technology integration and ecoDemonstrator program. “Once you have that capability you are able to place yourself in the right position. During the flight they traversed the wake to test the model and validate whether they had predicted the sweet spot correctly and they discovered they had it spot on.” The work, which indicated savings between 5-10%, helps lay the foundation for future studies of wake surfing and potential methods for proving its practical and safe application in commercial operations, she added.
During the 2018 formation flight, which was performed at standard cruise speed and altitude, the two aircraft maintained 4,000 ft. separation—outside of the predicted region of wake effects but still close enough to gain significant benefit for wake surfing. The position of the wake was predicted by constant comparison of the location of the ecoDemonstrator and lead aircraft using ADS-B and own ship data.
“The autopilot was configured to track a position relative to the wake,” said Boeing, which partnered with TCAS supplier ACSS on an updated system which provided additional ADS-B (IN) data to the ecoDemonstrator. “This allowed us to improve the wake prediction accuracy. Position relative to the wake and the lead aircraft was also presented to the pilots on a custom flight test display,” the manufacturer added.