When aviation industry chiefs met in early April at the Aviation and Environment Summit to discuss their latest action plan to reduce the environmental impact of flying, they chose to focus on what has often been seen as the global air system's perennial whipping boy - namely, how effectively the world's air traffic is routed to its destination.

Just a year previously International Air Transport Association president Giovanni Bisignani and Airbus chief executive Tom Enders separately slammed progress on the Single European Sky concept for improving the continent's air traffic management, with Bisignani branding as "embarrassing" Europe's failure to implement a programme that could deliver millions of tonnes of carbon dioxide savings. "We have had nearly 20 years of talks and no results," he roared in customary style.

More pragmatic in tone, this year's declaration, which calls for a fast track deployment of performance-based navigation (PBN) throughout the global air transport system, will certainly demand unprecedented and unaccustomed levels of cooperation both in geopolitical and industry stakeholder terms if it is to ever become the basis of a globally meaningful network approach.

3D RNP approach

The International Civil Aviation Organisation in 2007 urged all 190 member states to have PBN implementation plans ready within two years. PBN essentially means a shift to more accurate and efficient aircraft trajectories by moving away from reliance on such ground-based navigation aids in favour of satellite-guided area navigation procedures. This in turn enables more direct routes and more efficient approaches and departures. It also allows substantial reductions in fuel burn and emissions to be achieved.

These sophisticated systems, which combine global positioning satellites with existing avionics on a modern aircraft's on-board flight management systems to guide aircraft along precise flight corridors with pinpoint accuracy, are well within the realms of technical feasibility. It is the implementation of such performance-based techniques across the whole stakeholder community that has little historical pedigree and will prove to be the critical factor in its success.

As Naly Rafalimanana, Airbus's product marketing manager for the A350 XWB, points out, the advanced twinjet will have the most advanced RNP 0.1 AR (RNP with authorisation required) capability, in addition to capabilities such as continuous descent approach or 4D trajectory management integrated in the aircraft's flight management system as soon as it enters service in 2013.

Nicolas Vitu-Barbier, Airbus's flight management system RNP manager, explains how RNP AR sets PBN at a new level, with RNP values as low as 0.1nm (0.18km) in approach, reduced obstacle protection area without buffer and curved flight path in the latest stages of the approach.

"This enables shorter tracks and flexible flightpaths that can reduce fuel burn leading to lower emissions, a type of navigation that requires dedicated aircraft definition, but also requires national authority operational approval and dedicated crew training," he says.

RNP AR capability is nothing new and is already certificated for Airbus's single-aisle aircraft, the A330 widebody, and soon the A340-500/600.

But will such PBN capabilities be ahead of the infrastructure it is flying within?

Rafalimanana says: "The aircraft is certainly dependent on the ground infrastructure as, for example, continuous descent approaches have to be certificated at each destination airport. However, what we are seeing is CDA uptake as a market-based shift to improve trajectory management. It's clearly a new trend and the A350 XWB will be able to fly all those patterns very easily."

Tim Tuttle, Boeing's ATM programme manager, concurs: "Aircraft can do the job very well on their own, equipped with a modern flight management system, global positioning satellites backed up by inertial reference units."

Boeing has done much work in perfecting tailored arrivals, with continuous descent approaches flown on datalinked 4D paths that are shaped for local constraints and timed for merging traffic, and all its current production aircraft have PBN environment capabilities.

But as he points out, major airspace redesigns to take into PBN operations to improve navigation will never be just one job.

"You have the airlines and their maintenance departments that have to be involved because PBN means changes in the way you operate aircraft. Then there is the regulator who has to grant operational approval for transport category aircraft. Air navigation service providers of course have to organise the way airspace is structured and then there are also the airports, which have to manage not only throughput but the whole community of environmental impacts."

PBN requires that holistic industry approach because air travel relies crucially on a network. A global system would require performance-based navigation to reach a critical momentum across that network so that pilot projects such as Aspire in the Pacific area and its Atlantic cousin Aire - which have both demonstrated that, in theory, at least it is possible to conduct fully optimised flights delivering efficiency improvements of around 4% - become the norm.

The introduction of such a common set of navigation standards fortunately underpins airspace modernisation programmes such as the Single European Sky and its US equivalent NextGen, which place precise air navigation concepts at the centre of airspace safety and capacity considerations.

Both these have identified the need for a 10% environmental improvement over the next 20 years, and it is hoped that, in time, the industrial momentum caused by these two heavyweight programmes will tip the balance in favour of a universal system, which dispenses with the wasteful airborne zig-zagging between beacons on the ground.

The Intergovernmental Panel on Climate Change's (IPCC) influential report on Aviation and the Global Atmosphere said in 1999 that improvements in air traffic management and other operational procedures could reduce aviation fuel burn by up to 18%. The influence of ATM over CO2 emissions was estimated at 12%. "The rate of introduction of improved ATM will depend on the implementation of the essential institutional arrangements at an international level," it noted.

As Civil Air Navigation Services Organisation (CANSO) director general Alexander ter Kuile notes, any global airspace efficiency initiative is potentially cast against a background of reckless self-interest on the part of nation states: "Aviation remains a highly strategic state-influenced and controlled sector. We are not a consumer product in the same way as a McDonalds hamburger business, which is free to decide its strategic direction on market grounds alone."

Co-operation issues aside, PBN essentially has the potential to whittle down further inefficiencies in much the same way that the advent of reduced vertical separation minima (RVSM) had in increasing the number of aircraft that could safely fly in a particular volume of airspace.

Even so, CANSO believes that such low-hanging fruit has already been picked, and the next steps will be considerably more challenging, with PBN one of the principal tools in wresting further system efficiencies.

Phil Stollery, chairman of the organisation's environment working group, frames those IPCC figures, however, in light of a recent CANSO study that has given the industry much greater clarity in terms of ATM's potential contribution to the efficiency debate.

Stollery explains that efficiency in this sense is the difference between the exact point-to-point distance of a flight at the most fuel efficient altitude and speed, and the actual flight mileage flown.

"One of the things we wanted to do was to put the record straight. The IPCC report estimated that ATM had an influence over 12% of system inefficiencies and our report reflected back on that. We reckon that between 1999-2005, improvements allied to a better overall assessment, as well as the introduction of initiatives such as RVSM, generated a 4% improvement in system efficiency and that on average the global ATM system is operating at around 92% efficient today," he says.

That is 4% down with 8% still to go.

"Of the remaining 8%, half is locked up in interdependencies. The other half, 4%, we have set at the goal to recover, which amounts to an ambitious target considering forecast growth."

CANSO members who are responsible for supporting 80% of the world's air traffic aim to close that gap and reach around 96% efficiency by 2050, although CANSO points out that 100% efficiency is not possible due to the effect of those numerous interdependencies, which are governed by factors such as safety, noise, capacity, weather and even airline behaviour and military operations, which are unrecoverable.

Even here, however, Stollery believes that a concerted effort incorporating a performance-based approach could reduce the effect of the interdependencies by around 2%.


The targets are impressive because, as Stollery points out, aircraft fuel efficiency declines as congestion increases so the CANSO targets will be a stretch. "On a 'business as usual' case we would expect to see emissions increase as more and more aircraft are forced to take longer routes or are placed in holds in order to cope with increased traffic numbers," he says.

It is a variable picture too, with Europe's estimated current efficiency between 89% and 93%, leaving much more room for improvement than Australia, which is estimated at 98-99% efficient, so the very gradual improvement in efficiency to 2050 may not be a smooth curve.

"Depending on demand levels, efficiency will vary over the short to medium term, especially in areas where significant regional and civil/military co-operation is lacking," adds Stollery.

One thing is clear, it is going to be a long hard slog to secure a highly efficient global airspace, even with the engines of the Single European Sky and NextGen behind it.

Hugues Subra Salafa, head of Airbus air traffic management, is leading the European airframer's Single European Sky contribution and has been involved in the definition phase together with the 14 other candidate members within the SESAR Joint Undertaking public-private partnership, which is headed by the European Commission and Eurocontrol.

The JU and the 15 members will soon officially sign a membership agreement to take charge with Eurocontrol of the working programme throughout the eight years of the JU programme until 2016.

 What is performance-based navigation?

Performance-based navigation, sometimes abbreviated to PBN, provides a basis for the design and implementation of automated flightpaths as well as for airspace design and obstacle clearance. The two main components of a PBN framework are area navigation (RNAV) and required navigation performance (RNP). RNAV specifies the routes, while RNP specifies the performance criteria.

What is RNAV?

Area navigation, or RNAV, enables aircraft to fly on any desired flightpath within the coverage of ground- or spaced-based navigation aids, within the limits of the capability of the self-contained systems, or a combination of both capabilities. As such, RNAV aircraft have better access and flexibility for point-to-point operations.

What is RNP?

Required navigation performance is RNAV with the addition of an on-board performance monitoring and alerting capability. A defining characteristic of RNP operations is the ability of the aircraft navigation system to monitor the navigation performance it achieves and inform the crew if the requirement is not met during an operation. This monitoring and alerting capability enhances situation awareness and can enable reduced obstacle clearance or closer route spacing without intervention by air traffic control. Certain RNP operations require advanced features of the on-board navigation function and approved training and crew procedures. These must receive approvals.


RNAV and RNP specifications facilitate more efficient design of airspace and procedures that collectively result in improved safety, access, capacity, predictability, operational efficiency and environment. Specifically, improved access and flexibility for point-to-point operations help enhance reliability and reduce delays by defining more precise terminal area procedures. They can reduce emissions and fuel consumption. RNAV procedures can provide benefit in all phases of flight, including departure, en route, arrival, approach, and transitioning airspace. For example, standard terminal arrivals can:

  • Increase predictability of operations.
  • Reduce controller/aircraft communications.
  • Cut fuel burn with more continuous vertical descents.
  • Shorten the distance flown in terminal radar approach control airspace.
  • Decrease interaction between dependent flows in multiplex airspace.

He says the principal goal is to achieve a system-wide implementation even though it will be achieved step by step in a mixed equipage environment aiming for a first level capability. "Mixed equipage will be a significant consideration even after 2020," he says. "Short-term goals have therefore been set based on current technological capabilities and PBN is a central part of this."

Management of the aircraft will, however, rely heavily on a collaborative decision making approach, in which ground based ATM and airlines - flying anywhere in the world - co-operate in authorising the flightcrew to apply the optimum trajectory. "That means optimum information sharing and operating within an optimum airspace," he says.

For Joe McCarthy, manager of the US Federal Aviation Administration's RNAV and RNP group charged with implementing these foundation pieces within the US air traffic system, the PBN airspace design effort today centres on ensuring operational compatibility within a highly complex environment.

To date, the FAA has authorised more than 265 RNAV procedures at 90 airports in 30 states. And yet, even though it established the design type process five years ago and the whole area of PBN has attracted high levels of industry engagement, as he says: "the challenge is to fit in these exotic beasts within competing traffic."

Integrating these procedures within a highly organised national airspace system represents a mammoth task, one best accomplished by aiming for maximum effect early. "The best way to do one of these redesigns is to identify a hub system. It's a huge project, but the end results are great. The challenge then is to make all the pieces of the jigsaw fit together including the complicated high altitude routes between major hubs," he says.

McCarthy's FAA team is scrutinising US hub airports for their specific characteristics and hopes to have a pretty cohesive plan drawn up soon through working with the Performance Based Aviation Rulemaking group made up US industry stakeholder interests.


"Hubs are high-density areas and we are looking at those ones where our efforts would allow us best to tick off all the boxes," he says, adding that on the list of characteristics would be the sort of airline operations, the impact of surrounding airports and airspace etc.

Boeing's Tuttle sums up: "Each and every factor within the aviation supply chain has to agree and yet each one of those folks has a different goal, whether it is safety or throughput. System changes that have to be broadly applied take time to percolate through but there is a high likelihood that the rest of the world will most likely follow the example of the USA and Europe."


Source: Flight International