Aircraft Noise

The emotive topic of aircraft noise.

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 overstated.  Not being able to speak to someone else in the room or listen to 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 sometime 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 quarters. Loss of quality of life, and loss of productivity by those who have disturbed sleep among other things.

Enter the Jet 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 tailpipe into still air. The friction caused between the fast travelling air meeting the still air was significant and caused a large amount of 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 significant breakthrough 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 tailpipe 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 nacelles and the engines themselves have also been instrumental in reducing jet engine noise.

Boeing for example has 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 surrounding 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.

This Vietnam Airlines Boeing 787 shows off the chevroned rear of the engine nacelle that ensures a smooth laminar airflow over the engine casing.

Aircraft design improvements around noise reduction are not just limited to creating quieter engines. When Airbus Industrie began its 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 flies 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.

Flying Quieter

It is not only what you are flying in that makes a difference. Airports located near built-up areas are continually being pressured 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 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.

As our cities get bigger and spread around airport areas, more people are finding themselves living with aircraft noise. Of course, airports provide cities with the lifeblood of their economies. Having an airport near the centres of business encourages companies to base themselves in those cities.

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 approach 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.

The Continuous Descent Approach ensures that the landing aircraft stays as high above the ground as possible during the whole landing approach phase. Thereby it minimises the noise footprint over populated areas it passes over by being higher above them.

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.

A 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 permitted between 10 pm and 6 am. This ensures that there is a 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 the 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 6 am.

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 number 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.

Here we can see that the airline that operates out of the airport with a 10 pm to 6 am curfew is compromised by having to start later and finish earlier than its competitor based at the non-curfew-affected airport. In this comparison, the airline from the non-curfew airport can do 3 return trips against its competitor's 2.

By leaving at 4 am for example and arriving just after the 6 am 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 the 10 pm curfew whilst the curfew city airline needs to conclude their last flight by 10 pm.

Another innovation to make airports quieter is the provision of electrical 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 pollution.

Friendly Neighbour

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 soundproof double-glazed windows. This, of course, helped those residents immensely, but at what cost?  Well, subscribing to the concept of the user paying 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.