June 1, 2011 By: David Esler
At an NBAA International Opera?tors Conference a few years ago, a long-range business jet captain delivering a presentation on flying the North Atlantic Track System (NATS) related a hair-raising anecdote.
“We were on our way to Europe, high up in the flight levels,” he said. “It was a gorgeous morning, bright and clear, smooth as silk. We were monitoring the instruments, making our position reports to Shanwick Control, when overhead, a thousand feet above, came a Boeing 747-400 overtaking us.
“It was absolutely stunning,” the plot enthused. “We could see every rivet, every detail of that big, beautiful jetliner. We were absolutely transfixed – until suddenly, we were almost upside down!”
Both aircraft were flying precisely on the centerlines of their assigned tracks, and the wake turbulence of the much larger 747 had cascaded downward, causing an upset of the business jet. The flight crew quickly regained control of their aircraft, but the incident illustrated the unintended consequences of ratcheting down vertical separation from 2,000 to 1,000 ft., Reduced Vertical Separation Minima (RVSM) having been inaugurated first on the NATS to increase capacity of the oceanic airspace-management system. Normally, winds would have dissipated wingtip vortices, but in relatively still air, wake turbulence from widebody aircraft could descend to the track on the flight level below, causing havoc for smaller aircraft like business jets.
In response, NATS administrators (the track system is cooperatively managed by Canada, Iceland, the U.K., and Portugal) encouraged the voluntary use of the ICAO Strategic Lateral Offset Procedure – wonderfully acronymed SLOP – whereby aircraft fly either 1 or 2 nm to the right of an assigned route or track. Originally conceived to avoid collisions between opposite-flying aircraft when one has erroneously chosen (or been assigned) the wrong altitude or flight level, SLOP can also be used to mitigate wake turbulence encounters in scenarios where aircraft on different flight levels are operating in the same direction, as on an organized track system like the NATS.
Sometimes you can’t win for losing. Driving the unintended consequence of collisions from aircraft plying the wrong cruising altitudes or wake turbulence descending through the flight levels to harry aircraft below is the unprecedented accuracy of today’s Global Positioning System (GPS) and Required Navigation Performance (RNP)-certified navigation equipment and altimetry. Back in the day, a collision between aircraft at the same altitude might be avoided simply by the error tolerance of first-generation automated nav systems and air data computers, but with contemporary GPS and IRS guidance sensors operating through flight management systems (FMS) with improved software logic and position-synthesizing capabilities, aircraft religiously track the centerlines of their assigned courses at precisely the altitude dialed into their digital autopilots.
Counterintuitive Effects?
The issue of increasing precision in navigation is not new, John Hansman, Ph.D., professor of aeronautics and astronautics at the Massachusetts Institute of Technology (MIT), pointed out. In fact, performance has actually improved exponentially since the Pentagon turned off “selective availability” for GPS, allowing all users, civil as well as military, to access the satellite nav system’s most accurate positioning capability. (Selective availability was an intentional degradation of the GPS signal, ordered by the Pentagon, to reduce the precision of the GPS fix to approximately 100 meters.)
“Since then, GPS has been fairly accurate,” Hansman, who also directs the MIT International Center for Air Transportation, said. “It’s taken a while for the technology to get into all of the fleet, however. And there are effects in the increasing precision, some counterintuitive. As the altimetry precision has become greater, the probability of collision is increased if you erroneously level at an incorrect altitude. So that’s a potential effect.”
How might that impact the FAA’s Next Generation Air Transportation System, or NextGen, the long-awaited makeover of America’s aging air traffic management system? NextGen proposes nothing less than a complete paradigm change in the way the FAA distributes and separates aircraft en route and in terminal areas. Separation and control will shift from ground to cockpit, and the aircraft itself will become proactive in determining its position relative to other aircraft. Because heightened precision should allow closer intervals, the FAA says capacity in the system will be greatly increased to accommodate predicted hikes in air traffic density over the next two decades. To facilitate this, NextGen relies on the latest cutting-edge technology, like automatic dependent surveillance-broadcast (ADS-B) and navigation equipment capable of RNP values as tight as 0.1 nm. The good news is that all of this exists now.
The buzz phrase describing the aircraft-centric system is “performance-based navigation,” or PBN, and, according to Steve Fulton, of GE Aviation PBN Services (formerly Naverus) in Kent, Wash., represents “the idea that we are in a transition from ground-based navigation systems to airborne nav systems.”
Fulton, now a GE “technical fellow” who founded Naverus, explains, “We’re going from rules-based to performance-based operations. In the new world, we will have different capabilities that are a function of the sophistication of the airplane. And it will affect the entire spectrum of aviation, commercial to general.”
Noting that aviation still relies on procedures dating from the Eisenhower era, Fulton, whose company designs and implements RNP procedures, touted the advantages of NextGen. “Real benefits are flowing to the operators,” as a result of RNP.
But with the keystone of NextGen being precision – precision of navigation and altimetry and precision of timing (see “4-D Nav is Coming,” September 2009, page 28) – there is concern in some quarters that unintended consequences could emerge once the system is in full operation. For example, with aircraft following each other exactly in trail on RNP approaches or even en route, will wake turbulence be a problem as it was that day on the NATS? And what about aircraft noise? With hundreds of aircraft tracking the centerlines of RNP approach and departure courses exactly over the same houses day after day (as opposed to the natural dispersion that exists on non-RNP procedures today), will noise complaints, pardon the pun, go through the roof? Are these and, perhaps, other unintended consequences on the metaphorical radar screens of NextGen planners and engineers?
“Any time we change from one continuum to the next, we have to manage the process of change very carefully,” Fulton observed. “From the human perspective, change is not something we like to do. While the new continuum may offer advantages, the transition, the discontinuity we go through, is a place where we want to pay close attention.”
A Major Challenge
One of those planners is MIT’s Hansman, who advises the FAA on NextGen issues. “In order to achieve the potential from NextGen, we will want to reduce the separation standards, but we have to take into consideration wake turbulence, and if the risk is too high, then we’ll not be able to accomplish that,” he said. “So this is a major challenge. We do not have good models of the encounter dynamics that cause upset. There are so many variables, it is hard to predict whether any one wake encounter will be hazardous or not. So it has been very difficult to set wake turbulence criteria.”
As an example, Hansman points to the significant difficulties the industry has faced in determining a safe separation distance behind the Airbus A380 mega transport. (In the United States, it has been set at 8 nm for large trailing aircraft and 10 nm for small.) “We don’t have precise engineering criter
ia for wake turbulence avoidance,” he said. “As a consequence, it becomes difficult to move away from the present criteria, which provide safe avoidance. The acceptance in the broader community, however, is that the investment in NextGen will allow us to reduce separation standards as one of the ways to increase the capacity of the system.”
Historically, with the exception of final approach spacing, separation standards have been driven by uncertainty in the position of the aircraft or the ability to measure the separation interval. “While not well documented,” Hansman observed, “it appears that the radar separation standards that apply today were set by the accuracy of the radar systems at the time the standards were introduced. And those numbers are big enough that, by being 5 mi. separated laterally, you were more or less protected from wake vortex. As we now start to consider reducing those separation standards, wake vortex considerations will become an important factor in determining what a safe separation standard should be.”
This is also a fundamental issue with RNP when reducing separation standards in order to move routes closer together in terminal areas, Hansman believes. “The closest you can get an airplane to an obstacle is two times the RNP value,” he said. “As the RNP values go down, it will be possible to get standard routes much closer together, especially in terminal areas, so in those cases, wake turbulence considerations will become more important in terms of [defining reduced lateral separation intervals]. It’s more complicated in the terminal area because aircraft are maneuvering. Normally, you will also be getting turbulence from other sources in the lower altitudes [like weather and terrain effects], so it may be difficult to determine where the turbulence is coming from. At the higher altitudes, where it’s smoother, turbulence from convection is not generally a factor.”
If all else fails, a solution may be to operate aircraft more precisely at existing separation standards and accept slight increases in wake encounters due to the enhanced ability of RNP-compliant avionics and control of separation being transferred to pilots as opposed to controllers adding buffer – meaning that today aircraft are rarely sequenced at the actual minimum separation standards. “One of the places where we expect to get benefit is on an increased number of parallel approaches to key airports, like San Francisco and Newark,” Hansman said. “So we have had to be very careful in considering wake vortex in defining close parallel approach procedures.”
The reduced separation promoted by NextGen advances actually applies more to a lateral than an in-trail application, said Bob Lamond, NBAA director, air traffic services and infrastructure and a former air traffic controller. “My gut tells me that the separation standards that we have will not change a lot,” he continued, “and will still accommodate wake turbulence considerations. You can only put one airplane on the runway at a time.”
Another application for NextGen RNP procedures will be high traffic density areas with closely clustered airports, e.g., Southern California and the New York/Newark metro area. “We think that there is significant potential to increase the efficiency and capacity of the very busy terminal areas by de-conflicting airports that are in close proximity,” Hansman claimed.
This will require the use of the signature RNP curved approach or shorter turns to final while allowing two airports in close proximity to operate at maximum capacity. “In New York, when Kennedy is in a big push, LaGuardia is required to run procedures that are of lower than maximum capacity. So by using RNP procedures, you can theoretically allow each airport to operate at its max potential at the same time. However, this requires a change in arrival and departure procedure trajectories. In many cases today, that could require an environmental impact study, which could slow down implementation of new procedures.”
Wake Turbulence and RNP
In the terminal environment where the real value in capacity and efficiency resides, the in-trail separation requirement still has to be respected, GE’s Fulton maintained, “so it doesn’t matter whether you have a small airplane following a heavy one, those rules aren’t being modified right now. What’s new is that the traffic will be organized more predictably.”
And this is the “greatest opportunity” for RNP, Fulton believes, since “in the places where we have ADS-B tracking, we can now really measure the performance of the airplane precisely. With RNP and in the presence of ADS-B surveillance, we have data that would support the understanding that these airplanes are tracking the centerline within their wingspans. If you’re flying an ILS, the most critical place is over the runway threshold, about a 50-ft. box where wake turbulence would be most critical, and as a result we have developed appropriate separation standards for in-trail operations.”
Then Fulton asserted unequivocally, “There is nothing that RNP is doing in terms of the level of precision that has any relevance to the wake turbulence concern in my mind.” It is the in-trail distance that is the limiting factor, the primary element of concern, “not how closely you’re tracking the path of the airplane in front of you. Also, wake turbulence is an energy event, and so the way we reduce the concern is to ensure we are a proper distance or time behind the in-trail airplane. Energy dissipates over time, not distance.”
Steve Bergner, an instrument procedures expert and retired business pilot who worked with the NBAA to assist flight departments in qualifying for RNP/AR (Authorization Required) approaches, said he foresees no significant wake turbulence problems in the terminal areas.
“We’ve been doing ILSes with big and little airplanes in trail for a long time,” he added. “I really don’t see that as a unique issue for NextGen.” Wake turbulence will continue to be a challenge, regardless of the implementation of NextGen and RNP procedures, Bergner believes, with runway occupancy time and wake turbulence intervals remaining “finite limits.”
The “magic” to RNP/AR is twofold, the NBAA’s Lamond pointed out. “First, you get a very narrow ground track because of the flight deck containment and warning to the crew – you can get down to a 0.1 tolerance.” Second, is the “radial-to-fix turn,” or RF leg, a pure curved course, as opposed to the RNAV FMS-generated turn composed of straight segments connecting waypoints. “Imagine a 180-deg. turn,” Lamond proposed, “where all you need is a fix where you want to start and a closing fix, and the airplane will fly a true curved path.”
Dozens of RNP approaches are in use now, Lamond, said. “In the future we hope to have more aircraft that can do them, as it will require upgraded avionics and aircrew training. Several airlines have upgraded to do it, including Alaska, Southwest and Horizon. It’s not rocket science any longer. Gulfstream offers it in its product line, as does Cessna. We have about 15 NBAA member flight departments that are equipped to do RNP/AR.”
Like Fulton, Lamond is skeptical about the possibility of RPN nav precision setting aircraft up for wake turbulence encounters. “We could have wake turbulence in the United States today, but I have not heard of any upset incidents due to GPS RNAV. Could you have opposite direction turbulence? Yes, I suppose, but you should be 2,000 ft. apart vertically for same-direction of travel [in U.S. domestic airspace]. On the oceanic tracks they’re all going the same direction [with 1,000-ft. vertical separation], whereas in domestic airspace, you would normally have 2,000-ft. separation based on cardinal heading or direction of travel [i.e., 1,000 ft. apart vertically but moving in opposite directions].” So according to Lamond, “NextGen isn’t bringing [wake turbulence] on,” as it is accommodated today even in the presence of GPS accuracy.”
Flow Corridors and Variable Glideslopes
Rick Heinrich, Rockwell Collins’ director for strategic initiatives and, like Hansman, a NextGen advisor to the FAA, also asserts that wake turbulence would be a consideration no matter what the navigation paradigm. The issue in his mind is airspace design, or assembling “flow corridors” based on the characteristics of the airspace. “The goal is to integrate wake vortex as one of the many ‘constraints’ in the overall flow,” he said. Higher performance separation standards implicit in NextGen will require that consideration of wake impacts be a component of flow management, “and part of the assessment will be the impact of the wake vortex on the operational flow, its degree of significance, and what phase of flight is being considered. Wake vortex is more of a concern during closely spaced landing operations than it is en route. That is why new concepts are being considered like variable glideslopes.”
At Honeywell, which participates in NextGen planning as a member of an FAA industry advisory committee, Chris Benich, vice president, government relations, said that rather than unintended consequences deriving from NextGen, he saw only “intended opportunities” to apply technological fixes. “First, as you are lining up aircraft for arrivals, everyone is on a vertical profile,” he explained. “The turbulence is dissipating below you, and the winds can shift the vortices, so as long as you keep the smaller aircraft higher, you’ve got them above the wakes. So by procedure, you can affect some of that issue.”
New technology like GBAS (Ground Based Augmentation System), produced by Honeywell under the trade name SmartPath, offers multiple approaches to each runway and thus enhanced flexibility in procedures design. “So you can bring lighter aircraft in on a 3.2-deg. glideslope while keeping the heavies below them at 3.0 or 2.8 deg. for a touchdown zone at the runway threshold. Meanwhile, the lighter aircraft will touch down 1,000 ft. down the runway, descending above and in front of the wake turbulence. Procedurally, that’s one way.
“Similarly,” he continued, “you can do the same thing with RNP, i.e., multiple vertical descents, or ‘pseudo glidepaths,’ but it’s slightly less precise because you’re using barometric vertical guidance versus precision GPS vertical.”
While these tools are procedural, a second solution could be based on using ADS-B In to bring information on other aircraft to the cockpit, providing awareness to the flight crew of where wake turbulence might exist, since the pilots would then know the exact position of the preceding aircraft plus other data including winds. Thus, a situational awareness model of the whole approach situation could be constructed in the aircraft’s FMS. “This is farther down the road,” Benich said, “the mid- to longer-term part of the NextGen program requiring ADS-B In.”
In the new system, everyone will be included in the information loop in real time, Collins’ Heinrich emphasized, reducing or eliminating the problem of conflicting information from different sources leading to less-than-optimal decision making. (This ability avoids the “latency” in the present system where the transit time from the aircraft to the control center on the ground and back to the aircraft is reduced or eliminated. Actually, the real issue in latency is not transit time – which occurs at the speed of radio waves – but the decision-making time involved in the process. The aircraft-centric model keeps the decision process in the cockpit within reasonable parameters and in this case, “reasonable” means that the pilot has not had to ask the controller for a deviation from the planned operation.)
“I may have wake turbulence or a weather event or a closed runway,” Heinrich speculated, “things that have to be entered into the information exchange that will impact the decision process. So my wake vortex becomes another input that allows the pilot to be proactive in selecting their desired resolution to the constraint – for example, offsetting to one side of course, within a predetermined containment volume, to avoid the wake turbulence event. From the controller’s perspective, he can still see the flow of aircraft but doesn’t have to be involved in the decision to offset.” The use of lateral offsets is just another tool in the NextGen toolbox, Heinrich implied. “Assume crosswinds from the west on a northern approach would dictate that the following aircraft be given a western lateral offset to put the wake on the opposite side.”
Wake vortex solutions are bundled into the evolving NextGen weather suite, Heinrich reported. Departures and optimized profile climbs should not represent a problem, as there will be plenty of time for the wake to dissipate before an in-trail aircraft encounters the vortices of the leading aircraft. Meanwhile, terminal operations are being designed for RNP 0.3, placing aircraft closer together. “This will require improved sequencing to put lighter aircraft in the lead or to increase or decrease the slot sizes, respectively, for heavy and lighter aircraft in order to maintain arrival rates. Again we can apply variable glideslope principles to keep arrival rates optimized.”
Wake events are less a problem in the en route phases, “but again avoidance will rely on interval management and merging and spacing design for the transition from en route to terminal phases,” Heinrich explained.
Making Noise With ‘Slot Car Tracks’
Addressing the noise issue – airplanes tracking so accurately over the same routes and nav fixes that they would generate a constant noise impact above the same spot on the ground – Hansman at MIT admitted that, “There is an interesting concern about RNP in the terminal area in terms of noise footprints.” When non-RNP procedures are flown, a “natural dispersion” of tracks develops laterally, Hansman explained, “so when you go to an RNP procedure, you wind up with a potential noise concentration [due to the accuracy of the onboard nav equipment], a concern to some communities. If you are right under the RNP track, you now get hundreds of airplanes flying over you, another potential downside of the high precision we run in the system. However, it can be a positive factor, too, in that we can now move the trajectories to avoid noise-sensitive areas.”
Benich at Honeywell elaborated: “With RNP technology, you don’t have to put everyone on the same road, as you can move the [approach or departure courses] to different places. Ideally, you’re lining it up in a place that is known not to produce noise impacts. And you have the strategic ability to align the track in different places on different days, too, as arrivals can be adjusted and the level-offs minimized. Throttle-off, idle descents can become the norm, as the known VNAV path will allow an idle-thrust approach, lowering noise in the terminal area.”
RNP eliminates the randomness of turn anticipation, pilot Bergner pointed out, “as the RF leg under RNP is like a model slot car track. On the arrival, the aircraft aren’t generating a lot of noise on the ground, as the engines are idling – it isn’t until the final approach phase of flight when the aircraft is dirtied up and the power is up that you get the noise. You can also overlay the tracks on less-sensitive areas like freeways. A good number of the RNAV arrivals we have now pretty much emulate what used to happen in radar vectoring.”
Fulton and GE Aviation PBN Services don’t deny that there are “concerns” being raised around RNP noise issues. “RNP is a tool, and if not used wisely, it could present a problem,” he admitted. “The opportunity we have with trajectory-based, time-sequenced operations is to do things differently in terms of managing airspace. The challenge is to take full advantage of the technology.
“In the United States today we have a four-corner post architecture for airspace management [i.e., a quadrant system],” Fulton continued. “Aircraft are brought from a corner post to an arrival track to the downwind leg, [thence] to assigned headings to
the ILS approach course. And in that vectoring activity, because controllers are using their on-the-fly judgment, there is a natural dispersion of routing. We have level segments, too, because the vertical nav is not continuous, and you’re generating a lot of noise with the flaps down and power up.”
Imagine now, Fulton suggested, that aircraft are not collected over the corner posts but arriving from different directions, all converging, “and that we allow them to stay on their different compass headings and align with the runway 2 mi. out. There might be an airplane that flies over my house – but not every airplane.”
Heinrich at Rockwell Collins noted Europeans are addressing the noise issue at airports such as Schiphol in the Netherlands. There they place noise-monitoring equipment at specific locations, he explained, “and when a composite noise threshold has been met, the runway is closed, and operations shift to other runways. While that is not always the solution, it is part of managing the noise impact.”
Other Unintended Consequences
With its heavy reliance on telecommunications and Internet-like technology, information security will be one of the priorities of NextGen. “There are a lot of details to be worked out as we deploy this,” Honeywell’s Benich said. “One thing that has to be watched is security, that is, having the [aircraft] tail numbers on the flight tracking websites, as we will be broadcasting more information. This is not an unintended consequence but an issue that needs to be addressed. With more information flowing around, we have to be more careful of how we manage and protect it.”
Then there’s the ever-present possibility of equipment failure. At Rockwell Collins, the avionics manufacturer approaches this from what Heinrich termed a total-performance concept. “So if GPS were to be compromised, I can continue my operation because I will still have a DME-DME and IRU capability,” he began.
“RNP is really an airspace parameter; for example, an RNP 3 procedure, which tells me my containment volume – in this case 3 nm either side within 95% containment – so that means how the aircraft has to fit within the airspace. Now the onboard performance monitoring reverts to ANP, or actual navigation performance, which is where I monitor my sensors and how the data are being processed, that is, my adherence to the containment. As long as my ANP stays below the RNP, I am authorized to operate within that airspace designation. “So, if I monitor my ANP and lose my GPS sensor,” Heinrich continued, “my other sensors will still meet the performance requirement, just not as precisely. If I lost the DME sensor and went strictly to the inertial sensor, I could still monitor the drift – my actual nav performance. Until it gets above a certain threshold, I can still maintain the operation.”
If the aircraft’s navigation degrades to the point where its avionics can no longer maintain the required tolerance, then the crew has the option to perform a “management-by-exception” procedure where ATC intervenes and provides guidance using input from the aircraft’s ADS-B equipment.
Collaboration or Conflict?
Considering unintended consequences resulting from NextGen, Hansman cited the introduction of TCAS, which resulted in controllers adding spacing to equipped aircraft, as prior to TCAS, no monitoring of actual separation took place. “If one of the aircraft was at, say, 2.9 mi. separation according to the TCAS readout, the pilot would alert the controller,” the professor said.
“Today, in ATC operations,” he continued, “there is dual authority between the pilot and controller, so as a procedural matter, the pilots will tend to defer to the controllers in areas of separation, but the controllers will defer to the pilots in terms of weather, because the pilots can see or are operating within it. When we move to NextGen, where there will be equivalent sources of information, there are questions of whether this will result in more collaboration or conflicts between pilots and controllers. In a study we did in which both shared the same level of information, it did result in more collaboration, even though we designed the study to create conflicts.”
Emphasizing that the FAA and the industry need to carefully manage the process of change to NextGen, Fulton observed, “Certainly we don’t want to let the fear of unintended consequences bully us into doing nothing. This is a reminder of the necessity to properly plan. In all complex systems, you build and test and build and test. I don’t know of anyone who planned NextGen as a ‘big-bang event.’ We will plan for incremental deployment.” BCA
Editor’s Note: We invited FAA to provide input for this report, but the agency, acting through its Public Affairs office, declined to participate.
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