Aircraft noise can be a very emotional subject for those who are affected by it in their day to day lives. Yes, like other aircraft enthusiasts, I love being next to an airport taking in the sights and thrilling at the gut shaking sounds of powerful jets. However, I have also lived with those same jets passing near my home. The disruptive effect on your day to day life cannot be over stated. Not being able to speak to someone else in the room or listen to the your favourite TV show gets very frustrating. In the 1980s I lived in Fulham, London. Twice every evening our windows literally rattled as first the Concorde from New York arrived followed some time later by the one from Washington DC. Thrilling at first, but it gets old rather quickly.
So what is being done about it? What is the solution?
Aircraft noise in most countries is taken very seriously. Its disruptive characteristics have a negative effect on those exposed to it at close hand. Loss of quality of life, loss of productivity by those who have disturbed sleep among other things.
Enter the Jeg Age.
In the early days of passenger air travel, piston driven propellor engines were the only form of propulsion. Whilst they were relatively noisy, they didn’t produce sounds in the high frequency range that jets do. When the jet age began with aircraft like the Boeing 707 and the Douglas DC8, a whole new ball game started. These early jets, compared with today, were fuel hungry and extremely noisy. Their engines were what you call pure jets, consisting solely of the jet engine turbine. The result was that the high pressure ignited fuel air mixture was forced out of the tail pipe into still air. The friction caused between the fast travelling air meeting the still air was significant and caused a large amount of the roaring sound that resulted.
Aircraft and particularly engine makers have for decades been working diligently to find ways to reduce the sound footprint of a jet engine. The most signficant break through was the bypass engine. The concept is to take the aforementioned pure jet, the jet turbine, and encase it in a second nacelle. The nacelle is the outer casing of the engine. Inside the front of this nacelle is a large fan. This fan sucks in air from the front of the engine and feeds some of it into the jet engine turbine, the rest of it flows around the jet turbine and is ejected back around the flow coming out of the exhaust tail pipe of the jet turbine. As well as adding to the thrust of the engine, the bypass airflow also serves to encapsulate the exhaust from the jet turbine. This serves to reduce the friction between the jet turbine exhaust and the still air, as well as dampening the sound.
New methods and materials used in the construction of engine naceles and the engines themselves have also been instrumental in reducing jet engine noise. Boeing for example have
adopted a new configuration for the trailing edge of their engine nacelles which can be seen on the Boeing 787, Boeing 747 8 and the new 737 Max aircraft models. The nacelle trailing edge is finished in a chevron configuration, like a sawtooth. This means there is a longer linear trailing edge which allows the air from the engine and the still surounding air to merge together over a larger area, spreading that shock over a larger amount of air particles. The smoother the transition through the air of an aircraft, the greater its fuel economy and the less noise it makes.
Aircraft design improvements around noise reduction are not just limited to creating quieter engines. When Airbus Industrie began their initial design of the giant A380, one of the design requirements was to make it as quiet as possible. The engines of course were designed to be state of the art and provide noise reduction to strict specifications. Airbus however, also looked at another factor. An aircraft has a much larger noise footprint when it flys close to the ground. That stands to reason, an aircraft flying low over your house makes much more noise than one flying twice as high. So what Airbus undertook to do was to design the aircraft so that it was capable of a steeper climb out. That is to say that the A380 is designed to be able to climb more steeply after take-off, thereby spending less time closer to the ground while departing a city.
It is not only what you are flying in that makes a difference. Airports located near built up areas are continually being presured to find ways to reduce their noise footprint. As our urban areas continue to sprawl, airports that may once have been located in the countryside now find themselves being surrounded by new housing and industry. It is tempting to think, well they knew the airport was there already so how can they complain? The truth of the matter is, many of our cities are getting overcrowded and whatever land is available must be used.
Many airports have adopted various noise abatement procedures to help reduce the noise impact of their operations. For example, they can adopt air traffic control procedures that vary the aproach paths to the airport. That way fewer aircraft will fly over more suburbs rather than a few suburbs bearing the full brunt. Aircraft can be guided over water or forested areas as much as possible. During off peak times secondary runways can be used to allow those living under the main runway(s) approach path to have a break.
The way aircraft are controlled in the landing phase can also make a difference. In the landing phase most aircraft generate a significant amount of noise due to the configuration of flaps and additional engine thrust required to compensate for the extra drag caused by the extension of flaps. Traditionally most approach patterns for landing at an airport have consisted of stepping the aircraft down to lower altitudes as it gets closer to the airfield. For example, it gets cleared down to 10,000 feet where it flies for a while, then down to 5,000 feet where once again it flies for a while.
During this time it is overflying populated areas at these relatively low altitudes generating noise. A new approach, literally, is the constant glideslope. This means the aircraft is not asked to start descent until it is clear all the way to the runway. It means the aircraft will descend at a constant rate all the way to the ground and not spend any time flying over the ground at lower altitudes waiting to get further clearance to descend. Like the A380s take-off above, the aircraft will spend the minimum amount of time close to the ground where it is the noisiest.
Curfew is an option adopted by many airports. This restricts the operations of jet aircraft to certain hours of the day. For example, there may be no jet operations permited between 10pm and 6am. This ensures that there is quiet time when most people are trying to sleep. Curfew can cause problems for airlines. Flight delays for aircraft travelling to the curfew airport can be further exacerbated if that delay means they may arrive after curfew comes into effect. If they were only delayed by an hour to start with, they may find that the curfew will add a further 8 hours to the delay as they need to now arrive after 6am.
Another scenario affecting airline competitiveness is where we have two airlines, one based in city A where there is a curfew and one in city B where there is no curfew. Both airlines want to maximise the amount of flights they can do between cities A and B to profit from carrying more passengers. The airline operating from city B with no curfew has the advantage as they can start operating earlier and finish later.
By leaving at 4am for example and arriving just after the 6am curfew the airline from the non-curfew city is already halfway through their first return trip before the airline from the curfew city has even started. Similarly the non-curfew city airline can depart on their last leg just before 10pm curfew whilst the curfew city airline needs to conclude their last flight by 10pm.
Another innovation to make airports quieter is the provision of electical services for aircraft at the terminal gates. You may have noticed when you are at the airport that even though a jet might be stationary at the gate, you can still hear a jet engine whine. This is caused by what is known as the APU or Auxilary Power Unit. The APU is a small jet engine that usually sits in the tail cone of a jet aircraft. It doesn’t provide any thrust as its sole purpose, as the name implies, is to provide power to the aircraft whilst its main engines are not running. This power is what is used to run lighting, air conditioning and other electrical functions whilst the aircraft is parked. The APU may be much smaller than the main engines, however, its noise output is still significant. If you live next to an airport the jet noise is constant. To alleviate this type of noise, many airports are providing land based power which an aircraft can plug into instead of firing up their noisy APUs before shutting down main engines. A significant amount of noise is avoided as well as unnecessary polution.
It is accepted that airports are not the best of neighbours. Some airports however, make an effort to try and make life better for those who live close. I use an example from Sydney, Australia , which is the largest city in Australia and a very important commercial hub. Sydney’s Kingsford Smith International airport is located around 7 kilometres from the city centre which is handy for travellers but also ensures many parts of the city are exposed to aircraft noise. Sydney city undertook to compensate the worst affected suburbs by providing the homes with sound proof double glazed windows. This of course helped those residents immensely, but at what cost? Well, subscribing to the concept of user pays the users of the noisy aircraft paid. A levy of A$3.60 was applied to each ticket that involved an arrival or departure in Sydney. Once the expense of the double glazing was covered the levy was removed.
It is doubtful we will ever completely resolve the issue of aircraft noise, but finding ways to reduce it and manage it better goes a long way to improving the lives of those who are subjected to it. Finding ways to observe noise abatement helps us all.