RNAV/RNP: Eurocontrol’s Vision
Europe, with its multiple states, diverse airspace and projected 6 percent annual growth in air traffic, poses a difficult air traffic management (ATM) challenge. Eurocontrol, the senior body charged with meeting the challenge, certainly in terms of en-route airspace, believes that RNP/RNAV (required navigation performance/area navigation) is the way ahead. Avionics Magazine questioned Alex Hendriks, Eurocontrol’s head of airspace, flow management and navigation, about Europe’s implementation plans. As an air traffic controller with 20 years’ experience, as well as a qualified commercial pilot, Hendriks is well qualified to answer.
Avionics: There are diverse definitions of RNP and what it means. What is your understanding?
Hendriks: You are right, and the uncertainty this has caused has led us to avoid using the term recently. Fortunately, there is much work under way, sponsored by ICAO [International Civil Aviation Organization], which is coming to consensus. We are comfortable with the general understanding arising from these discussions that everything is RNAV and that RNP is "RNAV plus extras." Those extras are primarily containment, integrity and continuity.
Avionics: What do you mean by containment?
Hendriks: This refers to certain limits around the intended trajectory within which an aircraft would be contained by virtue of the technology used. In relation to RNAV performance, 95 percent containment is quoted: the aircraft would be contained within the stated limits for 95 percent of the time. For RNP there is a requirement that the systems should give an alert if the trajectory strays outside of 2xRNP limits. To ensure that a timely warning is given, the alerting system must be of very high integrity, with a not more than one in 100,000 chance of failure.
Avionics: Europe was first to mandate RNAV equipment. What benefits have followed?
Hendriks: RNAV has enabled us to substantially redesign European airspace, giving us new and optimum routes and reducing previous reliance on fixed routes defined by VOR and NDB ground aids. That, together with reduced vertical separation minima [RVSM], has given us an overall 10 to 15 percent increase in en-route capacity across Europe, with up to 30 percent in some areas. This will suffice for the present, but given predicted traffic growth averaging 6 percent per annum, we constantly have to look ahead to what new measures may be needed.
Avionics: Is air travel in Europe at risk from future capacity constraints?
Hendriks: En-route capacity is probably adequate now for the next few years. Constraints will first be felt around airports. And while we can support states and airport authorities in increasing airspace and runway capacity, there are many factors outside Eurocontrol’s influence. It’s hard to predict what may happen, as much will depend on community and environmental pressures.
However, something that affects us all is the tendency we see for aircraft numbers to grow faster than passenger numbers. This is due to growth of the low cost operators and a trend away from hubs towards more direct routes between regional airports, flown by smaller aircraft. These pressures may be exacerbated by the appearance of very light jets [VLJs], especially if these wish to make use of major airports, as well as smaller regional ones. We’re OK with these developments–we’re keen to give the traveling public what it wants–but there will be repercussions for ATM.
Avionics: Are there alternatives?
Hendriks: We believe that RNAV is not in question and that, for the future, there is no real alternative to GNSS [Global Navigation Satellite System], suitably backed up, probably by DME. The main questions now are around phasing: how quickly and in what stages do we implement RNP, when and to what degree we can dispense with existing ground aids, and when, if at all, equipment mandates are appropriate, etc. The time scale is driven more by considerations of cost-benefit rather than technology. However, environmental and political factors can impact the decisions.
Avionics: U.S., European and Asia-Pacific approaches to future air navigation differ. Are present harmonization efforts sufficient?
Hendriks: There will always be some differences in emphasis because, for instance, the U.S. has a far larger GA [general aviation] community than Europe, and Asia-Pacific has a less developed radar environment than either. Broadly, however, and thanks to harmonization efforts over the last few years, there has been substantial convergence, certainly between Europe and the USA. There’s now a common direction and momentum, such that the differences between us [Europe and the United States] are now minor. And if we get things right, Asia-Pacific and other regions will wish to follow suit.
One sign of convergence is that agreement has become likely that efforts should focus on RNAV1 capability initially. Although equipment manufacturers can better this by a large margin, the 1-mile standard moves us forward without barring operators of legacy equipment and can probably win general support.
Avionics: Why is there a mismatch between airborne and ground capability?
Hendriks: Airborne capability is way ahead of ground capability for three reasons:
- Aircraft manufacturers have spearheaded new technology in their latest-generation jets;
- Operators welcome technologies that promise operating cost reductions and are prepared to invest in them; and
- The airborne side benefits from economies of scale–after all, there are thousands of A320s and B737s–whereas, on the ground, there are fewer installations and every state wants a different, bespoke, ATC [air traffic control] system. That’s why we [at Eurocontrol] are pushing for common operational concepts and equipment functionality.
Avionics: How can we achieve efficiency without penalizing operators of older aircraft? What role should mandates play?
Hendriks: Clearly the fact that ATC may be dealing with up to five generations of equipment on aircraft is an issue. It may not be possible to treat all equally while maximizing overall capacity. We want to facilitate airspace use by aircraft not equipped for RNAV/RNP for as long as possible. But there comes a point at which a small percentage of unequipped aircraft can prevent progress for the majority who are equipped. Our approach is to encourage equipage and avoid having to mandate until absolutely necessary. Perhaps if 60 percent are fitted, you don’t mandate, but once 80 percent are compliant and the solutions have been shown to meet cost-benefit criteria, it might be necessary. Eventually, one might have to say, "Sorry, you must equip to a specified navigational standard or you can’t be accommodated in certain airspace."
Avionics: What about state aircraft, including military?
Hendriks: State aircraft are treated differently from civil aviation. They have not been required to conform with the RNAV or RVSM requirements. With previous implementations, our studies showed we could accommodate some 2 to 3 percent of noncompliant aircraft without detriment to system operation overall. However, if the proportion becomes significantly higher, the promised benefits fail to materialize. While the military is exempt, they do not want restrictions on their operations and have been proactive in equipping their transport aircraft to meet the ECAC [European Civil Aviation Conference] requirements. (It is frequently not possible to equip combat aircraft.) Within Eurocontrol we are developing ways to integrate civil and military aircraft operations within shared airspace.
Avionics: And unmanned air vehicles [UAVs]?
Hendriks: There’s no way around it: ground controllers will have to be able to treat sizable unmanned aircraft just as if they were manned. This means that UAVs must meet ICAO criteria for sense and avoid, see and be seen communications, etc. We believe this is achievable. In fact, I hazard a guess that within 20 years unmanned aircraft may be flying, say, the North Atlantic.
Avionics: What are the repercussions for general aviation [GA]?
Hendriks: GA is not a uniform category. Corporate operators, for example, keen to have unrestricted airspace access, can be in the vanguard when it comes to investing in enabling technology. Recreational aviators, on the other hand, may be content to stay out of the system, or be treated on an exceptional basis, and therefore will not need sophisticated navigational fits.
Avionics: Are you happy with ICAO’s harmonization efforts and approach to common operational requirements?
Hendriks: Broadly, yes. We are comfortable with their definition of RNP and think it should clarify matters. We also welcome guidance over common operational requirements, though we wonder whether this will be definitive enough. We note the difficulties organizations like ICAO have in trying to find solutions that are acceptable to everyone and the fact that they are subject to political pressures.
Avionics: Are we over-reliant on GNSS?
Hendriks: Yes, we are very concerned, given the weakness of the signals and their vulnerability to jamming, and to a lesser extent, spoofing. Current augmentations still rely on the space segment and are therefore just as vulnerable. Although the advent of dual frequency and other features with GPS and Galileo will make GNSS more robust, we do not envisage sole source navigation based on GNSS becoming viable in the foreseeable future for all phases of flight. There will have to be ground-based backups. For RNAV en-route and in terminal airspace we prefer DME/DME because the infrastructure is already there and thousands of transport aircraft use it. The issue of drift with inertial systems means we would not want to rely on INS [inertial navigation systems] fully, while LORAN would mean having to revitalize the infrastructure for that system and produce the necessary multimode receivers. For GA, we’ll have to keep VOR, at least for now. For landing, ILS is there, is reliable, affordable and will stay for the foreseeable future, possibly complemented by MLS [microwave landing systems] in certain situations.
Avionics: How quickly can the present infrastructure be dispensed with?
Hendriks: NDBs could virtually go today, except that we still have nonprecision approaches based on them. We think we can resolve this issue. We’ll probably have to hang on to VOR for another 10 to 15 years because DME/DME–retained as a backup to GNSS–is not suitable for GA. We don’t see RNP replacing ILS, as we think there will always be a need for an independent landing system. ILS has an affordable infrastructure and is unlikely to be superseded by satellite landing systems or enhanced vision systems for a long time. We think radar is here to stay, despite dependent surveillance. ADS [automatic dependent surveillance] will probably make inroads, but there is always the risk that transponders may fail or not be turned on, so an independent means of surveillance will still be needed.
Avionics: How quickly can RNAV/RNP be implemented in Europe vs. the U.S.?
Hendriks: On neither side of the Atlantic is there much confidence that a full RNP environment can be achieved before about 2015. There are certain functionalities that have yet to be delivered, such as fixed-radius turns en-route, which we see as being needed before our airspace colleagues can develop more new routes.
By 2015 the RNAV/RNP functionality needed to achieve the required level of two-dimensional navigation will have been achieved. A start will have been made in the integration of vertical navigation capability into an ATM environment in a manner that can achieve efficiency and capacity gains. The main future challenge will be bringing in the fourth dimension, time. In terms of the latter, we need more clarity about what time concepts are most appropriate, for example, "lozenge in space" or basing trajectories on times of arrival [TOAs]. Realistically, it will be a further 10 years before we see fully functional 4D navigation, supported by advanced flight management systems in the air and controller support tools on the ground. Nevertheless, by 2025, 4D RNAV/RNP should be greatly easing the workload of controllers and enhancing their efficiency.
It should be possible to start transforming terminal area operations with RNAV/RNP well before then, however. From about 2010 we’ll see more and more approach, missed approach and other RNAV TMA [terminal area] procedures published, with increasing numbers of aircraft using them. Fully transforming the TMA picture will require 3D and 4D RNP, and this will follow as it becomes available for en-route.
Avionics: Will responsibility for trajectories and separations ever pass to pilots?
Hendriks: In reality, we see limited scope for responsibility to pass to the pilots except perhaps those who are crossing oceans with 25-mile separations. For denser traffic environments an airborne separation assurance tool would be desirable, but much more than this seems problematic. Even if all the necessary surveillance and intention information could be fed to the flight deck and even if the pilots could interpret that information rapidly and reliably enough, it would be extremely difficult to maintain a tight control loop from the flight deck. Airborne control might work for part of the time, but what happens when you take failure modes into account, especially when separations have been further reduced? How, in the absence of ground controllers, can you organize related ground operations, such as push-back and sequencing onto runways? Even if systems were to become smart enough, meeting integrity criteria would probably require triplex, or maybe quadruplex systems with prohibitive expense.
We think an element of ground control will remain necessary. This should still allow, however, for a more dynamic ATC environment in which pilots can negotiate more favorable routings and other changes, as flight circumstances alter. Air traffic control might, indeed, become less prescriptive and more a two-way, air-ground collaboration.
Avionics: Finally, what is your vision for the future of RNAV/RNP?
Hendriks: It’s different from what it was a few years ago. While we never saw it as a panacea, we thought it could perhaps replace more present infrastructure and practices than now seems likely. Nevertheless, we do have great faith in it and understand that, realistically, it’s the only show in town. Our vision is that by 2025 a very differently structured airspace, built around a high level of 4D RNP, will accommodate about two to three times as many aircraft as we see today, with safety levels at least equal to and hopefully greater than now.