Noise levels decrease the further you are from the sound source. This is one reason that aircraft in mid-flight are less noisy than those taking off or landing.
Noise screens, buildings and natural features (e.g. hills, grass) can help to block and absorb sound to decrease its spread over a wide area. Conversely, large hard surfaces (e.g. buildings, paved areas or even lakes) can reflect sound, helping it to travel further.
Wind speed or direction and even temperature can affect how sound is carried and therefore how loud a noise is or how far it travels.
Alongside formal permanent changes to aircraft routes, there are a range of factors that might change the noise impact of aircraft. These include weather conditions, flight destinations changing, different aircraft being used, and different procedures being adopted. Generally, these factors are out of the CAA's control. Often, they are also out of government, air traffic control, airline and airport control.
The CAA does not have a regulatory role in approving these types of change, but is currently consulting on best practice guidance for the aviation industry on communicating their impact to the public.
These factors might include:
Fore safety purposes, aircraft need to take off and land into the wind. Because of this, the way the wind is blowing affects the direction of aircraft travel. The UK experiences westerly and south-westerly winds around two thirds of the time, so aircraft mostly land travelling from the east, and take off towards the west. Wind direction at an airport may change on a daily basis, and therefore the direction of arriving and departing aircraft on any particular day will reflect that.
The Atlantic jet stream is a current of fast moving air that runs between America and Europe. Its position moves further north or south over time. Flights to North America need to avoid the jet stream as flying into the 200mph headwind would slow them down and lead to significantly more fuel being used. Because of this, when the jet stream changes its position, aircraft may be sent on different departure routes to avoid it, changing the places they fly over and their noise impact.
Weather conditions have several impacts on aircraft performance. In warmer air, aircraft need to fly faster, and in how temperatures engines cannot generate as much power and consequently tend to climb slower. The spread of noise is also affected by temperature and humidity. The wind is not a significant factor on how aircraft noise spreads, but it can require aircraft to adjust power to stick to their flight path, which may cause noise levels to vary on windy days.
Aircraft navigate on the basis of magnetic compass heading - which is based on the location of magnetic north. However, the position of magnetic north changes over time because of changes to the earth's magnetic core. This means that an aircraft following a given compass heading will travel over the ground along a slightly different track over time, causing a gradual change to noise impact.
Most airports have set departure routes that aircraft use. Aircraft are usually sent along departure routes in line with their ultimate destination. This stops aircraft having to travel further than they need, and importantly for safety, minimises aircraft cutting across each other in the air. In reducing the distance flown by going on the most direct route aircraft reduce the amount of fuel burned and carbon emitted. It also means that if there are changes in destinations served by an airport, aircraft may begin to use different departure routes.
Airlines may begin to fly to new destinations from an airport, changing departure routes and noise impact, for a number of reasons:
Generally, newer aircraft are quieter than older ones - and in almost every area older aircraft being replaced will cut overall noise impact. However, in certain limited circumstances the way new aircraft are operated might increase noise for some. If older aircraft start operating from an airport, noise may increase. This may happen if a new airline starts flying from an airport using older planes; or if an existing airline increases their flights using older planes instead of their usual aircraft.
Another way changes to aircraft type may impact on noise is if larger aircraft begin to operate from an airport. While allowing more passengers to travel, larger aircraft tend to be noisier than smaller ones. Again, there are a variety of reasons larger aircraft may start using an airport - such as airlines flying more people to the same destination or new airlines offering new destinations.
Noise impact can be affected by how aircraft are flown, either generally as part of an airline's preferred practice or tactically depending on conditions on the day.
The way an aircraft operates has a big impact on the noise it makes. When its wheels are down, it is significantly noisier, so if an airline or a pilot decides to lower its wheels earlier, that noise will impact more people. The CAA enforces rules that say aircraft must be ready for landing with their wheels down five miles from the end of the runway, but there is no outer limit - so some airlines have procedures that tell pilots to be ready for landing much earlier than this. If a new airline starts operating at an airport, it may have a different policy that could change the noise levels caused by landing aircraft.
Unlike landing, where standard rules apply, every aircraft has a series of options as to how they take off called Departure Noise Abatement Procedures. There is usually no single one that is best environmentally, as each procedure affects noise, local air quality and carbon dioxide emissions differently. To reduce complexity and protect safety, airlines are only allowed to adopt two different procedures for each type of aircraft they use. As noise impact is highly dependent on where people live around an airport, a procedure that cuts noise at one airport may not do so at another - but airlines are not allowed to have different procedures for every single airport they use. An airline changing its adopted Departure Noise Abatement Procedures is likely to change noise on the ground.
Noise impact can also be affected by decisions made by Air Traffic Control about how aircraft should be flown within agreed routes or airspace, depending on the conditions of the day, such as the number of aircraft in the area.
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