A holistic suite of analytical tools being developed by the US Federal Aviation Administration will be central to better understanding the interrelationships between noise and engine exhaust emissions. Output from the test-verified programs, which the FAA plans to have in place by 2009, will be key to developing or evaluating technical, operational and market-based approaches to minimising greenhouse gas emissions and their effect on local and regional air quality.

As well as being an essential ingredient for making policy decisions about the agency's Next Generation Air Transportation System (NextGen), the tools more immediately will be used by the international community to forecast long-term trends in environmental impact goals and by the USA to assess the need for more stringent oxides of nitrogen standards. These will be evaluated by the International Civil Aviation Organis­ation's Committee on Aviation Environmental Protection (CAEP) in time for the next ICAO assembly in 2010, part of the organisation's next general assembly in 2010.

An emission that has both local and potential climate effects, NOx is a concern for populations near airports and has been the target of increasingly tighter ICAO emissions standards, sometimes at direct odds with optimising reductions in jet aircraft engine noise or fuel consumption. Lower-noise aircraft are typically powered by engines with combustors that tend to operate at higher pressures and temperatures, creating more NOx. Attempts to reduce NOx directly can also have a negative impact on fuel efficiency.

Included in the FAA's projects to disentangle the complex interactions among noise, emissions and fuel burn are the Aviation Environ­mental Design Tool (AEDT), an integrated package of noise and emissions analysis tools the Environmental Design Space tool, a model to help aviation authorities understand the trade-offs in technology development for new aircraft designs and the Aviation Portfolio Management Tool, a model to help aviation authorities asses technological, operational and policy measures to gain the most significant environmental benefit in terms of health and welfare at the least cost.

"The first use [of the tools] is to better understand the correct standards we should be setting, as well as the policies," says Carl Burleson, director of the FAA's Aviation Policy, Planning and Environment Office. "We're not saying we have all the answers, but it is better public policy to build models able to look at interdependencies," he says.

Farthest along is the AETD, which the FAA assembled using four existing, but augmented, standalone programs that compute airport noise, airport emissions, global noise assessments and global emissions inventories. With AEDT, the FAA says analysts will be able to study the complex relationships between competing environmental factors using a single program with common airframe and engine and aircraft trajectory. This will provide a more accurate assessment compared with today's method of using multiple independent programs that differ slightly.

Less mature is the FAA's Environmental Design Space tool, aimed at giving aircraft designers more information on environmental trade-offs at the conceptual design stage. "You can optimise an aircraft for noise or fuel burn or for less NOx. If you try to optimise for any one of those, you will have trade-offs for the others," says Burleson. "We want to understand, 'What are those trades?' The reality is that the choices are impacting the system."

The Airport Portfolio Manage­ment Tools (APMT), also a work in progress, will help analysts evaluate the impacts of environmental procedures around airports. The agency is evaluating the software as an aid to understanding the complexities of reduced thrust take-off and climb procedures, where reductions in overall noise exposure, particulate matter and nitrous oxide are counterbalanced by increases in fuel burn, carbon dioxide and sulphur oxides. CO2 and NOx are considered greenhouse gases, while SOx is primarily a local pollutant linked to poor air quality and acid rain. Burleson says the FAA will also use APMT to do a "shadow assessment" of the cost-benefit relationship of any NOx changes proposed by CAEP once the tool is complete.

Along with building the software, the FAA is also involved in environmentally friendly flight procedures, the results of which are helping to verify and validate the environmental analysis tools, increasing the believability of the computer models in the absence of flight tests.

In addition to information gained from continuous descent arrival (CDA) procedures already completed in Louisville, Kentucky by UPS and in Atlanta by Delta Air Lines, the fidelity of the computer models will also benefit from data acquired in two proposed tests in 2008 - a surface operations trial at New York Kennedy in late spring or early summer, and "flexible track" en-route tests over the Atlantic Ocean by mid-summer.

The Kennedy tests are part of a collaborative effort called the Atlantic Interoperability Initiative to Reduce Emissions (AIRE) between the FAA and Europe's Single European Sky ATM Research (SESAR) arm. The idea is to implement "quick wins" through better operations and best practices such as CDAs, where an aircraft throttles back to idle for most of an approach, and single-engine ground operations, says Patrick Ky, executive director of the SESAR project.

At Kennedy, the test will focus on using the ASDE-X surface radar and surveillance system to optimise ground movements and departures for minimal fuel burn and reduced emission in the airport vicinity, says Burleson. Airlines participating in AIRE include Air France, Delta, FedEx Express, KLM and Virgin Atlantic.

Flexible track tests will include flights between Europe and Miami, where aircraft will be given more freedom to modify routes and altitudes to minimise both travel time and fuel burn. ICAO reports that fuel burn on average can increase as much as 110kg per 185km (242lb per 100nm) flown if altitudes are as little as 6,000ft (1,815m) below the optimum cruise level. US airlines for flexible track include American and UPS.

Data from completed trials and operational efforts such as reduced vertical separation minimum (RVSM) are also available increasingly available to model fidelity. A study by the Volpe National Transportation Systems Centre of 220,000 flights performed pre- and post-RVSM reveal that there was on average a 1.8% improvement in fuel burn and a 3.1% reduction in NOx for the same origin and destination cities and aircraft types, post-RVSM. For CDAs, Delta reported saving as much as 499kg of fuel per approach during its test approaches in Atlanta, where it used the idling approaches for as many as 13 flights a night using Boeing 737-800s, 757s and 767s.

Once completed and test-verified, the FAA will ideally be able to use the tools to optimise environmental and operational benefits using computer models to uncover optimal configurations or procedures without waiting for the results of time intensive and costly flight trials.

Burleson points out that CDA tests were initially designed to reduce noise, but ultimately the FAA found in the actual tests that emissions were cut as well. "One of the most interesting issues for NextGen is that often, as you experiment and test, you will get some positive 'ah-hahs!'," he says. Integrated models will be key in accelerating the frequency of those ah-hah moments.

Creating quieter engines is often at odds with efforts to cut NOx

Source: Flight International