When Airbus in November announced it would demonstrate energy-saving automated formation flight in 2020 using two A350s, and Boeing subsequently revealed it had flight-tested the concept in 2018 with two 777s, it was a signal that a long-known technique could soon be ready for prime time.

Air-wake surfing for efficiency has a long pedigree of consistent research results that show an aircraft can save fuel by flying within the updraft created by the wingtip vortices shed by another—reclaiming energy left in the atmosphere by the lead aircraft’s lift-induced drag.

• Significant promise but substantial challenges for wake surfing
• Aircraft systems, planning tools and operating procedures are key

Since researchers at Germany’s Technical University of Braunschweig proved the concept in flight in 1984, achieving a 15% power reduction by manually flying two Dornier Do 28s wingtip behind wingtip, multiple demonstrations with different aircraft types have shown fuel savings from wake surfing ranging from 8-18%.

So why are operators, commercial or military, not already using the technique to improve efficiency? And why is it being looked at now? One reason is the availability of the avionics required to enable automated wake surfing, particularly automatic dependent surveillance-broadcast (ADS-B).

“Why now? ADS-B is now mandatory in the U.S., so we have the avionics to make this possible,” says Al Sipe, chief engineer for aviation efficiency at Boeing. “Why are we not doing it? The fuel savings are not small. We have the capability. But the regulations say don’t do it. So the next step is getting the regulations changed.”

With the benefits firmly established, Boeing’s test in 2018 with its 777F ecoDemonstrator flying behind another FedEx 777F and Airbus’ Fello’fly demonstration this year are developing systems and procedures airlines can use to safely and robustly take advantage of wake surfing.

“We have done enough tests to be confident we can do this with commercial aircraft,” says Sipe. “We think we will do it first with cargo aircraft, and we are working with Airbus on the regulatory aspects,” he told the American Institute of Aeronautics and Astronautics SciTech conference in January.

Attention is shifting to the next steps: developing the safety case and taking it to the regulators to get the rules changed; agreeing on the procedures between aircraft, operators and air traffic control (ATC) that will enable wake surfing; and developing the business case for implementation.

There are two things wake surfing is not. The first is a drag reduction technique. Instead, to maintain steady, level flight within the updraft from the wake vortex, the aircraft must pitch nose-down so it is descending relative to the upward moving air. The lift vector, normally vertical, is tilted slightly forward. This counters some of the drag, requiring less thrust to maintain horizontal flight. This reduces fuel consumption.

The second is traditional formation flying. Military aircraft fly in close formation, a few wingspans apart. Commercial aircraft would fly in extended formation, up to 1 nm apart, on what the industry prefers to call “cooperative trajectories.” This greater distance—10 or more wingspans for commercial aircraft—reduces the fuel-saving benefit but eases the workload on the pilots.

While the physics of wake surfing are on a firm footing, there are many technical and operational questions still to be answered. In a formation, only the trail aircraft sees a fuel saving. “Who gets preference? Is that an airline problem or an ATC problem?” asks Sipe.

Nelson Brown, a test engineer who worked on NASA Armstrong Flight Research Center’s Automated Cooperative Trajectories (ACT) flight-test project in 2017, cites the example of existing airline agreements to exchange pilot jump-seat rides and suggests a “benefit today, pay back tomorrow” marketplace could emerge.

There is also the question of how big the fuel savings will be in actual operations. Tests of a prototype automated wake-surfing system conducted by DARPA, the U.S. Air Force Research Laboratory and Boeing in 2012-13 using a pair of C-17 airlifters achieved a 10% fuel saving over more than 90 min. in extended formation.

But 10% is a best case and will not be achieved all of the time. “If we can argue that the trail aircraft in a formation can see an 8% reduction in fuel burn consistently; the pair will save approximately 4% during this phase of flight,” Tristan Flanzer, Boeing flight controls engineer, told the conference.

“The aircraft are not in formation all the time, so accounting for other phases of flight, such as departure and landing, the savings may be reduced to 3%,” he says. There is then the matter of what fraction of an airline’s fleet will be in a formation at any time. “If half of all flights are paired off, the system level  saving is 1.5%. So the real benefit is an order of magnitude lower,” says Flanzer.

This is still a significant saving, but wake surfing for efficiency is not just a technical challenge. “There needs to be a clear value proposition for the airlines, a well-defined conops [concept of operations] to understand the technical and operational needs, a provably safe system architecture and a plan to certify the system,” he says.

Boeing is proposing a conops with four distinct phases: premission, departure and single flight, cooperative trajectory operations, and split and post-flight. It assumes: aircraft are equipped with upgraded autopilot and autothrottle, ADS-B “In” and flight deck alerting updates; pilots have been trained in manual recovery from wake-induced upsets; and operations have been approved. Unless they are ex-military, commercial pilots will not have experienced flying in extended formation, so training is important.

The premission phase begins months in advance, with strategic planning to adjust flight schedules and maximize the opportunities for cooperative flying. Once aircraft pairs and schedule times have been identified, detailed flight plans are produced for each aircraft. Prior to departure of the first aircraft, the crews of both will conduct a preflight briefing of lead/trail responsibilities and rendezvous information. Both crews will monitor the continued viability of cooperative operations up to departure.

The aircraft may be leaving from different airports, at different times, so their departures must be closely coordinated. “Any delay is most efficiently absorbed while the second aircraft is still on the ground,” says Flanzer.

Current winds aloft strongly influence achievable formation benefits, and airlines will have to be flexible on departure times to get a wind-optimal routing for a formation that avoids incurring a large detour that costs fuel, Tobias Mark, a researcher at German aerospace center DLR, told the conference.

Once both aircraft are airborne, their onboard flight-management systems or ground-based interval management is used to adjust speed and coordinate arrival at the rendezvous point under ATC-assured separation with 2,000 ft. vertical spacing between the aircraft.

The cooperative trajectory operations phase has four subphases. The first is merge, the transition from ATC-assured horizontal separation of 5-50 nm to station-keeping managed by the crew of the trail aircraft. Using data transmitted by the lead aircraft, the trail aircraft stabilizes approximately 1 nm behind it and 500 ft. to the left or right of the predicted position of the lead aircraft’s wake vortices.

The two aircraft then join, the crews informing ATC they have assumed responsibility for separation and the lead aircraft taking leadership of the flight. ATC then deals with the pair as a flight of two operating within a 2,000-ft. block of altitude.

Now in stabilized formation, the trail pilot engages cooperative-trajectory mode to begin wake capture and tracking. Lead-aircraft data and own-ship information is used to predict wake position. With autopilot and autothrottle engaged, the wake capture function moves the trail aircraft into the ideal position—about 200 ft. from the vortex—based on ride quality and fuel saving.

The C-17 flights and NASA’s ACT tests involving a pair of Gulfstream GIIIs reported only “light chop” when surfing the wake, but it is a trade between ride quality and fuel saving. The crew can “turn the knob,” Brown says, surfing the wake more closely for fuel saving when passengers are awake, but moving further away at night to improve ride quality while they sleep. There will also be some threshold of atmospheric turbulence beyond which wake surfing is not possible, he says.

Once joined, the trail aircraft flies automatically, a situational-awareness display showing the crew its position relative to both the lead aircraft and its predicted wake. If the lead aircraft needs to change speed, altitude or heading, the intent is transmitted to the trail aircraft, which may have to move away from the wake during the maneuver. To avoid crossing the wake, a prevention and recovery function can command the trail aircraft away from the wake to avoid upsets and loads.

On reaching a preplanned point on the route, the pair will split, the lead aircraft notifying ATC, which reestablishes standard separation between them. “[Cooperative trajectory] operations will only be conducted during the cruise phases of flight, so ATC-supportable separation must be achieved at the latest by 150-200 nm from destination,” says Flanzer.

The takeaways from this conops, he says, include the need for tools for both strategic and detail mission planning, the importance of ADS-B and of leveraging existing interval management tools during the transition, and the requirement for new autopilot submodes.

The concept is based on predicting, rather than measuring, the wake position. When the trail aircraft is following behind by 20 wingspans, or about 4,000 ft, the vortices shed by the lead can drift significantly. The rate at which the wake sinks depends on the lead’s type, weight and speed, while the rate at which it drifts depends on winds aloft. Predicting position requires data from the lead aircraft.

“Absent a sensor to detect the wake, we have to use an aircraft-to-aircraft data link,” says Flanzer. “ADS-B is appealing because of its prevalence, but does it provide enough information and resolution to predict the wake? And how do we provide enough integrity to maintain safe separation?” The consensus is that ADS-B alone is not adequate.

For the level of reliable wake prediction necessary for routine commercial cooperative-trajectory operations, ADS-B data alone is likely insufficient for the actual wake-surfing phase, he says, adding: “For other aspects of the join, it does provide enough accuracy.” There are also ways to improve or augment ADS-B.

Accuracy of the wake prediction could be increased by adding wind data to the information broadcast by the lead aircraft, and a proposal to add these messages to the next standard of ADS-B, Version 3, is on the table, says Sipe. Mode S Enhanced Surveillance (EHS) already has this information, but the aircraft’s transponder must be interrogated for the data, and normally only ground radars do this.

For Boeing’s 777F ecoDemonstrator flight test, Aviation Communication & Surveillance Systems modified its traffic collision alert system (TCAS) to perform airborne interrogation of the lead aircraft for EHS data. The system sent ADS-B and EHS data to a flight-test laptop that hosted the wake prediction algorithm on the trail aircraft. The laptop then sent a command to the autopilot’s localizer control law, which was used to keep the aircraft on station relative to the wake. “ADS-B plus TCAS may work,” says Flanzer.

For maximum aerodynamic benefit, wake position must be known to within a few percent of wingspan, around 10 ft. for large commercial aircraft. This is beyond the 30-m (100-ft.) accuracy normally assumed for unaugmented GPS. If both aircraft are viewing the same satellites, they can navigate relative to each other with much greater accuracy, but this would require new ADS-B messages, and “we can’t count on the aircraft in a formation always seeing the same satellites,” he says. “We are going to need some way to sense position inside the wake,” says Sipe.

Despite the challenges that still lie ahead, wake surfing for efficiency continues to show promise, for both commercial and military operations. Maj. Will Guthrie, a U.S. Air Force Reserve tanker pilot who is also a FedEx pilot, says the cargo airline has several aircraft flying in the same direction, to Los Angeles and San Francisco, London and Paris, just minutes apart every day. “There is no reason not to link up so the trailing aircraft can take the benefit,” he told the conference.