A Scandinavian Airline System (SAS) Boeing 737 descends noiselessly to the threshold of Arlanda Airport at Stockholm, Sweden, having just completed a descent with engines at idle or nearly so. This is not considered remarkable; rather it is one of several hundred "green" arrivals that SAS has now made at Arlanda.
In more than 650 continuous descent arrival (CDA) approaches made by last year, the airline saved an estimated 130 tonnes of fuel, 204 tonnes of carbon dioxide emissions and 715 kg of nitrogen oxide emissions. Scaling up to the 36,000 approaches SAS expects at Arlanda annually, the airline could save $5.8 million worth of fuel, 23,000 tonnes of CO2 and 73 tonnes of NOx per year, while also exposing the Stockholm area to less noise.
Small wonder that SAS, Sweden’s air-traffic control provider LFV and the country’s airports operator are intending to extend green approaches to other airports throughout Sweden and beyond. Next in line is Landvetter Airport at Gothenberg, where each green approach is expected to save around 100 kg of fuel, 314 kg in CO2 and 1.1 kg of NOx.
Capt. Peter Larsson, SAS project manager for CDA approach research, undertaken as part of the European Union’s North European ADS-B Network Update Project (NUP), said the airline plans to extend use of CDAs into more intensive traffic environments such that eventually it will be operating at all traffic densities and in "mixed mode" along with aircraft making conventional approaches. The NUP project at this writing was in an extended second phase.
U.K. Beginnings
The European airports where CDAs are most intensively used are the three airports that serve London. Some 95 percent of Heathrow arrivals are CDAs, with 90 percent at Gatwick and more than 80 percent at Stansted. CDAs were pioneered in the U.K. by NATS, formerly National Air Traffic Services, with British Airways.
As described by NATS spokesman Patrick Horwood, in a conventional approach, a flight descends to around 3,000 feet, about 15 miles from the airport and flies level until it picks up the signal from the airport’s landing system. With a continuous descent approach, the flight stays higher for longer and then descends to the point at which it will intercept the landing signal. This reduces noise at ground level and, depending on the type of aircraft, can save up to half a tonne of fuel per flight.
NATS is recognized as a world leader in the development and operation of CDAs, and its expertise has been sought by operators as far afield as the United States, Middle East and Scandinavia. It has a "CDA outreach" initiative within the Sustainable Aviation program to help other airfields develop their procedures. In order to drive CDA compliance still higher, NATS, along with airport operator BAA, the CAA and the Department for Transport, has recently published a guide for airlines and airports. The "Arrivals Code of Practice" has been widely distributed around the industry.
CDAs, clearly a powerful way of cutting down on fuel burn and emissions, are made possible by 4-D trajectory management — managing aircraft trajectories in time as well as the three spatial dimensions — which Europe, like the United States, sees as a key part of the way ahead for air-traffic management (ATM). CDAs can work only if each aircraft can be relied on to arrive at a particular 3-D (latitude, longitude and height) position from where it can descend on minimal power to the threshold, avoiding the conventional and wasteful process of stepping down through different flight levels in a non-precision approach. Arrival at that point must be at a designated time.
Achieving this requires a flight management system (FMS) on the aircraft that can manage the aircraft’s trajectory, through en-route speed adjustment, in response to a requested time of arrival (RTA) uplinked from air-traffic control. Regular estimated time of arrival (ETA) updates from the aircraft close the loop.
Modern flight management systems are RTA-capable. For instance, Smiths Aerospace, now part of GE Aviation, has had the capability on board Boeing 737s since 1986. ATC computers on the ground carry out the task of dovetailing multiple 4-D trajectories so CDA aircraft can be merged without conflicts. Wasteful holding in stacks over terminal areas is avoided.
SAS also is involved, through its regional subsidiary Wideroe, in another approach initiative which, although mainly safety related, has environmental benefits, too. Last Oct. 29, a Wideroe Dash 8-100 turboprop touched down at remote Bronnoysund airport in Norway under Global Navigation Satellite System (GNSS) guidance, the first time a passenger aircraft had done so using a certificated GNSS precision approach system.
Twin Universal Avionics GLS-1250 receiver/processors linked with the FMS were responsible for keeping the aircraft on a designated trajectory that avoids high terrain and other obstacles in the airport vicinity. The pilot flies as though making an ILS approach, using similar guidance cues from the flight deck instrumentation. A ground-based augmentation system (GBAS) uplinks differential corrections to the on-board GPS, bringing the required precision to the system. A VHF data link from Park Air Systems uplinks the data from a processor unit at the airport.
The system, supporting Special Operations Category 1 (SCAT-1) operations, is capable of guiding aircraft down to Category 1 decision height on a defined trajectory that starts about 20 miles out from the threshold and subsequently follows a 3.9-degree glide slope to touchdown. It can be used day or night and in restricted visibility.
Park Air Systems, a subsidiary of Northrop Grumman, with headquarters in the U.K. and Norway, provided the ground station system under a contract awarded by Norwegian air navigation service provider Avinor. Under the contract, Park Air Systems over the next three years will provide SCAT-I satellite-based landing systems at up to 25 airports where terrain constraints rule out conventional ILS.
While safety is the main driver behind the system — tragically, Wideroe lost a Dash 7 in 1988 in a controlled flight into terrain (CFIT) accident that killed all 36 passengers and crew — fuel and emissions savings should result from the ability to maintain scheduled operations without go-arounds and diversions.
Progressive though such initiatives are, European authorities are keenly aware that attending to the approach phase of flight is productive only if airport capacity is sufficient to avoid having to delay aircraft in holding stacks before they can land. This consideration explains pressure for expansion at Europe’s busiest airports — witness schemes for additional runways at Heathrow and Stansted in the U.K. alone. However, in the present climate, telling a skeptical European public that airport expansion can make a contribution to emissions reduction is likely to be met with incredulity.
RNAV, RNP
CDA and GNSS-guided precision approaches, likely extending to full landing systems eventually, are not the only emissions-savings game in town. More efficient utilization of the horizontal plane of operation may be equally important. Freeing aircraft from the confines of "aerial motorways" defined by ground navaids and enabling them to navigate point to point via virtual waypoints clearly has fuel saving potential.
While not quite the "free flight" that was once mooted, area navigation (RNAV) does represent a degree of fuel-saving liberation. If, at the same time, aircraft could be equipped to navigate very precisely, they could be allowed to fly closer, both horizontally and vertically, to other aircraft, so that greater use can be made of favorable altitude bands and jet streams, such as across the Atlantic.
Again, a lower fuel and emissions payoff can be expected. To achieve this, aircraft must meet a standard of Required Navigation Performance (RNP) defined according to factors such as the phase of flight and flight area.
RNP is in daily use, the best known European example being in Austria. Two years ago, the Austrian civil aviation authority approved RNP 0.3 procedures for use at Innsbruck Airport, where approaches and departures take place through narrow valleys between 9,000-foot mountains. Participating aircraft must have dual FMS and positioning systems.
Austrian Airlines had to upgrade the navigational avionics on its Boeing 737s to fully meet the RNP standard. But resulting benefits of eliminating step-down approaches, fewer missed approaches and reduced decision height translate into fuel and emissions savings. Upgrades to RNP 0.2 or 0.15, now in prospect, will further improve certainty of arrival, especially during the sudden snowstorms and "white-outs" to which the area is prone. Avoiding diversions means less fuel is used and less CO2 emitted.
While RNP is a natural for "difficult" airports like Innsbruck, the technology should provide savings across the board as it comes into more general use. Eurocontrol believes that a full-up RNP environment, with 4-D trajectories followed from gate to gate with high precision, may not be realized for another two decades, but that an initial one-mile PRNAV standard (RNAV-1 in U.S. parlance) could be achieved within a year or so.
This may ultimately require mandatory equipage, adding to those mandates required for the surveillance and communications aspects of ATM, such as Mode S reporting, Automatic Dependent Surveillance-Broadcast (ADS-B), Reduced Vertical Separation Minimums, and 8.33 kHz VHF spacing. Airlines will lament the costs involved in entering the new CNS/ATM era, but will have to bite the bullet if they are to have the required system capacity while at the same time delivering on their environmental commitments.
Although RNP is a performance standard that can be met by any sufficiently capable system, satellite-based navigation is widely seen as the cornerstone of RNP capability and, in this respect, Europe is confident that its Galileo satellite system, though slow to emerge, will eventually be a valuable complement to GPS, the two systems providing greater accuracy and integrity than would be possible with either system alone. The Russian GLONASS system could be integrated too.
Ground-based augmentation, such as that provided locally and by the European Geostationary Navigation Overlay System (EGNOS), Europe’s equivalent to the Wide Area Augmentation System in the United States, will help secure the necessary precision, enabling progressively tighter RNP levels to be met.
Space-based systems, suitably backed up with terrestrial "fallbacks" such as DME/DME, inertial navigation and LORAN will, it is felt, eventually supply the accuracy and service assurance needed to support full precision approaches and automatic landings. This would save fuel and emissions by limiting the incidence of go-arounds and diversions to alternative airports.
There are other approaches to fuel economy and reduced emissions. Four-dimensional trajectory management and precision trajectory following will permit more flexible use of airspace. This is epitomized in the "business trajectory," a path through space that is more ideal from the user’s business point of view and that may include point-to-point routings at best speed and height, fuel-efficient cruise climbs and CDAs.
Air-traffic controllers will use ADS-B-based surveillance and tools able to predict conflicts up to 20 minutes ahead so that they can initiate trajectory adjustments in good time. Air crews will be able to negotiate their business trajectories, which they then "own" for the duration of the flight. The collaborative decision making that enables this can also involve airports and airline operating departments, as well as the air crews and ANSPs. A network-centric communications system called SWIM, for System Wide Information Management, will support the process.
Although most attention is paid to movement in the air, movements on the ground by aircraft and ground vehicles alike, can be just as wasteful. Figures published for Manchester Airport in the U.K. suggest that 60 percent of all emissions generated there — 430,000 tonnes of CO2 in 2005 — are due to ground movements and that 23 percent are created by taxiing aircraft.
Surface Movement Guidance and Control Systems (SMGCS) offer savings by ensuring smooth traffic flows at airports, though their main role is safety enhancement.
The future alternative of extending 4-D trajectory management to cover entire aircraft journeys, gate to gate, using augmented satellite-based technologies and multilateration, could save significantly by minimizing ground taxiing and queuing, with engines running, for departure. Ground vehicles can be brought within system oversight and control by equipping them with simple broadcasting beacons. Single-engine ground operations by aircraft are another fuel-saving measure.
Unfortunately, air navigation service provision in Europe lags behind airborne equipage. Both Airbus and Boeing have made RNP 0.3 (accurate to 0.3 of a mile) capability standard on their latest-generation aircraft, and are targeting RNP 0.1. Operators acquiring these aircraft only wish they could fully exploit this capability, especially in terminal areas.
European ANSPs are hampered by the fragmented nature of the continent’s air-traffic control infrastructure. There is a pressing need to unify large blocks of airspace across national boundaries and to reduce the number of ATC centers, ultimately to two or three. These are aims of the Single European Sky ATM Research (SESAR) program, but achieving this grand vision is a political minefield.
Last year, Giovanni Bisignani, director general and CEO of the International Air Transport Organization (IATA) scorned the Single Sky initiative, describing it as a 15-year circus of talks that had failed to deliver the 12-million tonne reduction in CO2 that it could have done. While this criticism may be unduly harsh, Europe certainly has a long way to go, as present difficult negotiations over functional airspace blocks (FAB) indicate.
Eurocontrol and the SESAR program need committed political backing if they are to succeed in meeting the stated aim of delivering an updated ATM system by 2020. Earlier this year, Eurocontrol accepted the SESAR ATM deployment sequence, proposing three successive implementation packages covering the period from 2008 to 2020 and beyond.
Europe knows that prompt action on the plan is vital if it is to prevent fuel usage and emissions from rising at anything like the expected rate of expansion in passenger miles flown. This, in turn, is vital if pressure from an environmentally concerned public to restrict aviation growth is to be resisted. Within the plan, ground-based ATM provision must move forward in step with that in the air if all potential improvements are to be delivered.
While Europe is under the strongest environmental pressure, the underlying trend is worldwide. Last year, Bisignani appealed (again) for governments and ANSPs to eliminate the 12 percent inefficiency claimed to be inherent in global air-traffic management. He accused governments worldwide of dragging their feet and declared that halving ATM inefficiency by 2012 could save 35 million tonnes of CO2.
Europe intends to deliver an ATM solution that will both expand capacity and enable aircraft to burn less and emit less. By aiming for a 4-D trajectory-based use of airspace along with a consensual form of trajectory negotiation between air and ground, Europe plans to deliver an ATM infrastructure that will both expand capacity and enable aircraft to burn less and emit less. High commonality with the NextGen system in the United States is intended, as manifested in the Atlantic Interoperability Initiative to Reduce Emissions (AIRE) between FAA and Europe’s SESAR program.
Future avionics and ground-based equipment will merge functionally to deliver reduced reliance on radar in favor of space-based surveillance and navigation systems, greater use of data links for air-ground communications, and all-encompassing wide-band network communications for shared situational awareness.
Ironically, though safety and system capacity remain primary drivers, environmental pressure may be the final lever that gets the ball rolling. The promise of 12 percent emissions reduction, maybe more, achieved by overhauling ATM infrastructure and practice, might just be enough to unlock the necessary reserves of finance and commitment.
CDAs Endorsed
The European Regions Airline Association (ERA) in January endorsed the use of continuous descent approaches (CDAs) "at as many airports as possible" because of the flight-safety and environmental benefits.
The association is comprised of 200 companies involved in intra-European air transport, including 60 airlines, 40 airports and 100 associate and affiliate members such as aircraft and engine manufacturers and avionics suppliers.
"Regional airports are less restricted than some of the busier major hubs such as London Heathrow or Frankfurt, and are therefore best placed to apply continuous descent approaches," said Mike Ambrose, ERA director general. "No special equipment is necessarily required, but it does entail enhanced cooperation between airlines, airports and air-traffic service providers."