BILL READ reviews the latest work being done by regulators, aircraft manufacturers, airports, and air traffic managers to tackle the problem of aircraft noise. [caption id="attachment_8915" align="alignnone" width="373"] An Airbus A380 landing at the 2012 Farnborough Air Show. Improvements in technology have enabled the A380 to be one of the quietest aircraft currently flying. In 2012, the A380 won the John Connell Silent Approach Award from the Noise Abatement Society for significant products which reduce noise for the benefit of the community while enhancing the environment.[/caption] The annual RAeS Greener by Design conference on 15 October looked at the problem of noise from aircraft in the air and on the ground. Speakers included representatives from airports, aircraft and engine manufacturers, air traffic management, environmental research groups and government.

While aviation brings many economic benefits, it also has its negative environmental aspects, including pollution, greenhouse gas emissions and the noise from aircraft endured by residents who live under flight paths or close to an airport. The health effects of excess noise are sometimes hard to quantify but one of the most cited effects is lack of sleep during the night. While noise (and indeed other forms of pollution) are also created by other transport modes, such as trains and road traffic, the effect of aircraft noise is often perceived to have the worse effect. Much negative publicity against aviation has come from local residents concerned that airport expansion proposals will result in more noise. "Noise is an emotional issue," observed Dr Andy Jefferson, Programme Director from Sustainable Aviation.

Perceptions of noise

Noise reduction at UK airports. (Airbus)

While it is possible to accurately measure levels of airport noise, the effect of the noise on the local community is more difficult to quantify. Ian Flindell, a consultant and lecturer at the University of Southampton Institute of Sound and Vibration Research, looked at a series of surveys which attempted to quantify the effect of aircraft noise on local residents. One of the first of these was a social survey of people living within ten miles of Heathrow conducted in 1961 on behalf of the Wilson committee. This was followed by the Aircraft Noise Index Study (ANIS) in 1982 which looked at people living near Heathrow, Gatwick, Luton, Manchester and Aberdeen and the 2005 Attitudes to Noise from Aviation Sources in England (ANASE) survey which interviewed 2,733 random residents living close to the 20 busiest UK airports. There have been a number of other surveys on airports in mainland Europe, including Paris in 1998, Amsterdam in 2002 and Frankfurt in 2005. These surveys identified that noise was perceived to be at its worst, the louder or more frequent it was. "Individual interviews provide interesting insights which you can’t get from standardised questionnaires," said Ian Flindell. "Actual individual and community responses can be very complex showing that noise perception depends on many other variables and trade-offs and is likely to differ between different communities and airports. Many residents report that, although they find aircraft noise annoying, they appreciate the economic and social benefits brought by the nearby airport."

However, the perception of aircraft noise on those who have to listen to it is often not consistent, "The perception of noise varies between people," explained Andy Jefferson. "For instance, a loud aircraft event on a windy morning generally results in fewer people being annoyed than the same aircraft event on a still, foggy morning."

Regulating noise

Recognising that aviation noise is a problem which needs to be addressed, the International Civil Aviation Organization (ICAO) has recommended for many years that national regulatory authorities adopt technical standards to limit noise from civil aircraft. These standards have been adopted by 180 countries which require new aircraft and engines to be certified for compliance with ICAO noise standards before entering service. Over the years, ICAO has issued new noise standards (referred to as ‘Chapter’ numbers) which progressively reduce the permitted levels of noise. The latest Chapter 4 standard which applies to all aircraft built after 2006 is 10db quieter than the previous Chapter 3 standard. In addition to reducing noise from aircraft, ICAO has adopted a ‘balanced approach’ which also looks at other aspects, such as land-use planning, noise abatement procedures and operating restrictions.

In addition, the European Commission’s Advisory Council for Aviation Research in Europe (ACARE), in its FlightPath 2050 report, has set the ambitious target of reducing perceived aircraft noise by 65% from 2010 levels to 15db for aircraft departures and arrivals by 2050.

Practical steps

In recent years, the aviation industry, together with government and regulators, have done much work to try and tackle the problem of aircraft noise. However, it must be recognised that noise from aircraft is influenced by a number of variables, only some of which it can influence (see right). Established in 2005, Sustainable Aviation (SA) is an alliance of UK airlines, airports, aerospace manufacturers and air navigation service providers. In April, SA published an Aviation Noise Roadmap which looks into practical ways in which noise could be reduced. The Roadmap focuses on five areas in which the issue of aircraft noise can be addressed: aircraft and engine technology, operational improvements, land use planning, operational restrictions and communication and community engagement. "We see the Roadmap as a tool kit’ which industry can use to move forward," said Andy Jefferson.

Holistic approach

                Aircraft design criteria. (Airbus)

Regarding engine and aircraft technology, both airframe and engine manufacturers are working to design ever quieter aircraft. "Low noise is achieved by addressing the aircraft as a system," explains Andrew Kempton from aero engine manufacturer Rolls-Royce. "The noise made by the airframe (especially the landing gear and the wing flaps) is just as important as the noise from the engine at the approach to landing condition. The aircraft performance is also important as the thrust required and the altitude gained greatly influences the noise on departure. Finally, the flight operational procedures (such as cut-back on departure and continuous descent on arrival) greatly influence the noise on the ground."

However, it must also be remembered that reducing noise is not the only goal. "Aircraft design balances a lot of criteria," points out Daniel Carnelly, Airbus Technical Marketing Director, Aircraft Performance & Environment. "However, only some of these are the environmental aspects, by which I don’t just mean noise but also CO2 and NOx emissions. Other criteria that need to be considered are that the aircraft is safe, that it achieves its range, minimises costs for the airlines, and so on."

Rolls-Royce Trent XWB engine (Rolls-Royce)

As far as engines are concerned, there are also compromises that need to be made between different environmental objectives. Engines that are optimised to reduce noise are often heavier, resulting in additional fuel burn and emissions. The sound from a jet aircraft was originally dominated by the noise of the jet behind engine mixing with ambient air but technology has now greatly reduced this noise, so that the focus has moved on to controlling internal engine noise (see right)." Over the years, Rolls-Royce has succeeded in reducing the noise levels of each new engine it produces. The company has continually invested in new technology to bring about further reductions in aircraft noise, including high-powered computing to run detailed large calculations, university technology centres (UTCs) working on industrial challenges, including a noise UTC in Southampton University, and a jet noise test facility at QinetiQ. "Aircraft design requires many attributes to be balanced," explains Kempton. "The open rotor is more fuel efficient than the turbofan but the turbofan is quieter. Meeting the ACARE targets may also require novel aircraft configurations, such as the blended wing body."

Operational aspects

Aircraft noise levels can be adjusted through such operational measures as changing the angles of approach and climb and when an aircraft’s undercarriage is lowered. (Sustainable Aviation)

On the operational side, noise levels on the ground can be changed by the ways in which aircraft land and take-off, such as the angle of approach and departure and the deployment of flaps and landing gear. "From some of the measurements we identified we found the difference in noise between wheels down and wheels up was the equivalent of a technology change of an aeroplane," comments Darren Rhodes of the UK Civil Aviation Authority. Another method is to use steeper approaches. "Evidence suggests that approximately 5-10% reduction in noise footprint area can be achieved for every 0·25° increase in glide path angle," says Darren Rhodes. "At Heathrow, 3·25° would give more noise improvement than all the fleet changes over the past ten years."

The UK air traffic control provider NATS has been looking at continuous descent approaches (CDA) into airports which can both save airlines fuel and reduce noise levels. NATS has developed a tool which uses radar data to track flight profiles and identify any inefficiencies. Using this flight profile monitor (FPM), NATS conducted a one-year trial with ten airlines operating into Edinburgh Airport between December 2011 and December 2012 which revealed significant variations in airline CDA performance. During the trial CDAs improved by 20%, mostly due to new radar, but also because of increased focus on CDAs. A second CDA trial is currently in progress with easyJet at Bristol Airport. “CDA improvements are there for the taking. They can reduce noise and save fuel and do not require any investment in capital or technological improvements,” adds Carrie Harris, Head of ATM Environment, NATS. Other contributions that air navigation can provide to manage noise levels include improved aircraft navigation capabilities, such as area navigation or ‘RNAV’ which allow aircraft to fly more precise routes, potentially avoiding noise sensitive areas, and other innovative ATM practices which can offer route variation and noise respite.

Operator restrictions

Another method used to reduce aircraft noise is to place restrictions on operators as to what they can do at airports, such as:

Air traffic movement limit

Noise- preferential routes

Continuous descent

Noisy planes ban

Preferential runways

Higher charges for night operations

Closure of airport overnight

Restrictions on ground noise (reverse thrust, APU, engine run-up)

These restrictions can be imposed by regulatory authorities, the European Union, national or regional governments and by the airports themselves. A number of continental airports also have noise restrictions, one example being Frankfurt Airport, which has an system of ‘active noise abatement’ administered by the airport, airlines, ATC and local government.

Good news and bad news

Sustainable Aviation’s prediction of how new quieter aircraft designs will offset the predicted total increase in aircraft fleets. (Sustainable Aviation)

Adding all these different approaches together, the good news is that aircraft noise from individual aircraft has greatly decreased and is expected to continue to fall as new quieter aircraft replace older designs. However, the bad news is that much of this gain may be offset by a predicted doubling in the total number of aircraft by 2050"Despite a steady reduction over time in average aircraft noise, annoyance levels have not gone down because the number of aircraft movements have increased," said Jonathon Counsell, Chair, Sustainable Aviation.

In December 2013 the Airports Commission chaired by Sir Howard Davis is due to present its interim report to the Government on the short and long-term requirements for future airport capacity in the UK. A final report is due in the summer of 2015. One of the discussion papers produced by the Airports Commission is on the subject of airport noise. "This paper sought input on the different metrics that can be used to measure and quantify noise; the different ways of measuring and quantifying the effects of noise; and ways of mitigating noise," explained Phil Graham, Head of Secretariat at the Airports Commission. These metrics, which included the frequency of noise events, also compared similar noise levels from road and rail transport and concluded that noise from aircraft still annoyed more people.

Noise factors

There are still other problems yet to be overcome. "Although ICAO has imposed noise restriction rules, they are not in force everywhere," explains Geoff Maynard, Managing Director of Altra Capital Limited and a member of the Greener by Design Committee. "ICAO only has the power to regulate international flights and there are large numbers of flights in the US and China which are not covered."

Another problem is that the number of people living within noise contour bands of major airports is increasing. There is little that can be done to change existing residential areas but, when planning for new airports or airport extensions, consideration could be given as to what sort of buildings should be located in noisy areas. The nature of the noise problem can often change over time, sometimes as a result of attempts to reduce its impact. Initiatives to concentrate the impact of noise on fewer people will benefit those who escape it but will make it worse for the communities that still experience all of the noise.

While welcoming many of the initiatives being taken, Tim Johnson from the UK-based environmental group Aviation Environment Federation. warned that some old problems still remain. "Unlike other areas of aviation environmental policy, the absence of quantifiable targets and a timescale provides no cap or limit on aircraft noise, creating uncertainty," he said. "Communities feel strongly that the evidence exists to inform long-term targets but a fear of the consequences of compliance is preventing decision-making."

Bill Read
13 December 2013