Heathrow has two parallel runways, which run west-east. They are referred to as the 'northern' and 'southern' runways, and serve all of the five terminals (though terminal 2 is currently being re-built). Both runways can be use for both arrivals and departures, although the use of the northern runway for departures on easterly operations was previously restricted by the Cranford Agreement.
The Cranford Agreement prevents planes taking off over Cranford in normal circumstances. It came into force in the early 1950s. It was argued that, because Cranford was so close to the northern runway, take-offs would be unbearably noisy for its inhabitants.
The Agreement only applies when planes are taking off in an easterly direction, and means that, in normal circumstances, all easterly take-offs are from the southern runway, with no changeover during the day (see 'Runway Alternation' below).
The government have now confirmed the abolition of the Agreement, which opens up a new flight path over Cranford and on to Southall, north Hanwell and north Ealing. Work is required at Heathrow to implement this change. It is expected that planning permission will be requested for these works in 2013 for implementation in 2015.
Westerly preference and easterly operations
Aircraft normally land and take off into the wind. In the UK the prevailing wind is from the west, so for most of the time (70% approx), Heathrow's planes come into land over London and take off over Berkshire. Even when the wind is blowing from the east, provided it is less than 5 knots, planes continue to land and take-off as though the wind was westerly. This is the so-called 'westerly preference'. It was introduced because the noise of departing planes was greater than those landing, so having more departures to the west reduces the number of people affected by noise, because more people live to the east of the airport than the west. However, when the wind is blowing from the east and stronger than 5 knots, planes from Heathrow take off eastwards over Ealing and towards central London. This is termed 'easterly operations', and happens about 30% of the time. The actual split of easterly vs westerly operations varies from month to month, and year to year.
Ealing residents will suffer noise overhead from planes taking off from Heathrow for much more of the time than they do already if the airport gets its way and persuades the government to abolish the westerly preference for airport operations. When there is a strong easterly wind, normally more than 10 knots, planes must take of to the east, against the wind. This means that Ealing gets the noise up to 30% of the time, when there is such a wind. When the wind is from the west, the prevailing direction planes take off to the west, away from London. As there are far fever people living to the west of Heathrow than to the east, this means far fever people are disturbed. Because of this, the Government long ago decided that planes should take off to the west as much of the time as possible, and the airport therefore runs on a westerly operation even when there is a easterly wind, as long as the wind speed does not exceed 10 knots. This is known as the westerly preference. If the preference were abolished, Ealing would suffer the noise overhead approaching 50% of the time, instead of the current maximum of 30%. Some Ealing residents have already written to their MP's to say that such a change would be entirely unacceptable, and have asked to the inform the Secretary of State.
The table below shows the Heathrow westerly/easterly operation split from January 2011, from figures provided to the Heathrow Airport Consultative Committee (HACC):
The BAA Flight Performance Report 2011 gives the annual East / West split for recent years as follows:
For the latest information click here
Runway alternation and mixed mode operation
When the wind is blowing from the west at Heathrow, planes land on one runway and and take-off on the other. The runways are switched at 3pm, which allows residents under the final approach and take-off paths a break from the noise. This is called 'runway alternation'. If the alternation was abolished, allowing planes to land and take-off from the same runway, this would be known as mixed mode. Mixed mode has been proposed as a way to expand the number of flights at Heathrow, although the current government has ruled it out.
On easterly operations, the effect of the Cranford Agreement is that all planes take off from the southern runway, and there is no changeover during the day.
There is some more information on mixed-mode in this HACAN pamphlet.
Department of Transport and Commissions latest thoughts
The British government originally measured aircraft noise in decibels [dBA], but averaged noise pollution out over a 16 hour day. The result represented a continuous sound level which was always much lower than the actual noise from a single aircraft flying overhead.
From 2002 to 2008, the Department for Transport (DfT) ruled that no noise nuisance from aircraft occurred when the average noise measurement was less than 57 dBA Leq. This meant that areas like Ealing were judged to have no noise pollution from Heathrow.
Research by the World Health Organisation (WHO) found that people start to get affected at an average of 50 decibels and become severely affected at 55 decibels. WHO recommend that the effect of each individual flight should be taken into account in plans to reduce aircraft noise.
Working towards this, the European Union (EU) Environmental Noise Directive (2006) requires separate measurements for daytime and for the more sensitive evening and night-time periods. Evening and night-time measurements are weighted for lower background noise. The three measurements are then combined to produce a 24 hour average. This is expressed as a 'Lden' level. The EU have set the figure for the onset of community annoyance at 55 dBA Lden, so it is now a legal requirement for Heathrow Airport to consider noise issues affecting the area within the 55 dBA Lden contour.
Although the WHO research suggests this figure should be much lower, the EU figure of 55 Lden is an improvement on the old Department for Transport one. EANAG believes the mid-term aim should be to improve the environment of everyone suffering noise at 50 Lden, preferably by reducing the noise rather than through measures such as adding insulation.
Noise preferential routes (NPR)
Noise prefential routes are defined routes that are used by planes to take them from the runway to an altitude of 4,000ft. The NPR's have a 1.5 km swathe each side either side of the centre-line, within which the plane is considered to be on track.
The aim is to restrict noise disturbance to areas with fewer people. However, the problem at Heathrow is that west London is so highly populated that no effective NPR can be designed. There are two NPR's that cross Ealing, running from south-west to north-east, passing over Norwood Green, Hanwell, central Ealing and north Acton.
You can see the routes taken by planes arriving at, and departing from, Heathrow via the WebTrak system.
Continuous descent approach (CDA)
CDA involves a plane descending smoothly from the holding stack (around 6,000ft) to the airport, rather than dropping down in stages. This type of approach uses less fuel and also creates less noise.