Under the flight path
Living or working under a flight path and hearing planes taking off
When planes take off, they have to follow specific routes in the sky which were set by the Government. These routes, which are 3km wide, were specially designed to make sure planes avoid flying over areas where lots of people live, until they reach 4000ft. The routes are called Noise Preferential Routes or NPRs for short
Air Traffic Control makes sure that pilots stay on these flight paths until they reach 4,000ft. Once they have reached that height, Air Traffic Control can direct them off the flight path towards their destination.
The technical name for these special flight paths for take-off is ‘noise preferential routes’. They were set by the Department for Transport several decades ago.
Taking off and the importance of wind direction
Another basic aspect of aviation safety is that planes need to land and take off into the wind.
They can take off in the same direction as the wind, but this is only allowed if it’s a very gentle wind – up to 5 knots, which is little more than a breeze. (In aviation, you measure speed in knots – the same as for travel on water. A knot is 1.15 miles per hour.)
Most of the time at Heathrow, the wind comes from the west. This is known as a ‘westerly’ wind.
So, because planes need to fly into the wind, most of the time they’ll fly towards the west. Therefore, the Government has set the take-off flight paths for when the wind is blowing from the west as the map shows.
Of course, we can’t control the direction of the wind. Because of this, the Government has also set take-off flight paths for the opposite wind direction (towards the east) from the other end of the runways. The map above also shows the take-off flight paths for this wind direction.
Heathrow and the ‘westerly preference’
But at Heathrow, the wind is not the only factor that governs how planes take off. There are also rules and agreements to take account of people as well as nature.
So that planes don’t take off over central London every time the wind comes from the east (which in some years can be as much as half the time), Government policy is that planes should always take off to the west, towards Windsor, unless the wind is too strong.
This policy, with a few changes, has been in place at Heathrow since 1962. A relatively new element of this is that late at night there is a ‘westerly preference’ (a preference for taking off towards the west) one week, and an easterly preference the next. This is so the burden of the noise of planes at night is shared more fairly between communities to the east and to the west of Heathrow.
So, at Heathrow all flights take off into the west, unless there is an easterly wind of 5 knots or more.
What does this mean locally?
It means that on average, approximately 70% of take-offs head towards the west and only approximately 30% of take-offs head into the east, in the direction of London. This also means, of course, that approximately 70% of landings are over London and approximately 30% are over Windsor. It is important to note that the exact percentages vary from month to month and year to year.
The split in wind direction of take-offs – is almost completely dependent on the wind direction and speed so varies from year to year and month to month. In fact, the length of time that the runways operate in one direction can vary from a few hours to a few months: it all depends on the weather.
What this means for noise
If you live under one of these take-off flight paths, then you will probably hear some noise when that particular flight path is being used.
If you live beyond the flight path – past the point where the plane reaches 4,000ft – then you might sometimes hear some noise when the plane leaves the flight path to head towards its destination.
If you live alongside (but outside) a flight path, you might hear noise if a plane flies outside the flight path. This can happen if Air Traffic Control tells a pilot to leave a flight path for an operational reason (such as to avoid bad weather). We take good ‘track keeping’ (staying on the flight path) very seriously at Heathrow and around 95% of aircraft here are ‘on-track’.
This website will tell you what we are doing about this type of noise and how you can find out more or complain if you need to.
Other local procedures
There are also a few operating procedures that planes must follow just at Heathrow.
Alternation
You might have heard people use the phrase ‘runway alternation’ when they talk about Heathrow and noise from planes. This refers to our voluntary system of alternating the landings between Heathrow’s two runways (the north and the south runways) when the wind is from the west. Runway alternation only applies to arriving aircraft (see section on Arrivals Noise) however the other runway is normally used for departures
The Cranford Agreement
In most cases, aircraft do not use the northern runway to depart when the airport is on easterly operations due to the ‘Cranford Agreement’. This is an oral agreement between the Government and the residents of Cranford which has existed since the mid-1950s. Cranford is a town located very close to the end of the eastern end of the northern runway.
The agreement was designed to protect the residents of Cranford from the high noise levels experienced on the ground from departing aircraft which are at low altitudes over Cranford due to its proximity to the runway. The Cranford Agreement says that there should be no take-offs over Cranford unless necessary. This is usually only on occasions when the southern runway is closed, e.g for maintenance work or for safety reasons which happens very rarely.
This means that planes can’t use the northern runway when they take off towards London. Because of this, the vast majority of aircraft will use the southern runway to depart. There is therefore no system of alternation during the day on easterly operations. However in 1999 the Government introduced a system of runway alternation at night on easterly operations. This is because during the early morning, there are no departures and therefore either runway can be used for arriving aircraft.
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