We know that noise from planes can be disruptive to communities around Heathrow. During the day, when planes are landing and taking off to the west (westerly operations), we alternate the use of our two runways to provide local communities with respite.
Communities around Heathrow place great importance on the alternation system and we make every effort to stick to it. The alternation pattern means that for part of the day we use one runway for landings and the other for take-offs then, halfway through our day at 3pm, we switch over.
This gives some communities approximately 8 hours of respite a day.
At the end of each week we switch completely. What we did in the evening during the previous week, we now do in the morning and vice versa. This is so that communities get respite from planes in the morning one week and in the evening the next.
To help you plan ahead, we publish an annual schedule of runway use. It tells you which runway we're using any day or night of the year. The schedule covers landings only because that's where runway alternation makes the biggest difference.
On easterly operations, we do not alternate the runways at 3pm because of the Cranford Agreement which prevented us from using the northern runway for departures during the day. Although the Cranford Agreement has now ended, we need to undertake works to the airport’s infrastructure before runway alternation on easterly operations will be possible.
With an expanded Heathrow we intend to introduce runway alternation on both easterly and westerly modes of operation, giving respite to communities to the east and west of the airport.
In practice runway alternation is more complicated than the simple explanation above. At any given time, our choices of runway and flight direction also have to take account of the six factors listed below.
Daytime runway alternation follows a daily cycle and a fortnightly cycle. Alternation starts at 6am and continues till the last aircraft departs at the end of the day.
Runway alternation gives everyone an equal share of respite from aircraft noise. The diagram above shows how it works when the wind blows from the west. Runway alternation during the day is not currently possible when the wind blows from the east.
We make every effort to adhere to the published runway alternation schedule. However, sometimes there may be a build-up of flights being held in the holding stacks. When this happens, the Government has set rules permitting NATS to land aircraft out of alternation, i.e. on the departures runway. In these circumstances, both runways will be used for arrivals for a temporary period. This is called Tactically Enhanced Arrivals Mode (TEAM) and is allowed after 7am on westerly operations when severe inbound congestion occurs, or is anticipated to occur, involving delays to arriving flights of 20 minutes or more. Under these circumstances we can land up to six aircraft an hour on the runway designated for departures. These rules have been in place since the alternation system was introduced in the 1970s.
Although not described as TEAM, we are also able to use both runways for arrivals between 6–7am without being limited to a set number because this hour is the busiest time of day for arrivals into Heathrow.
There are other occasions when unforeseen circumstances mean that arriving aircraft need to land on the departures runway. For example, this can occur if an aircraft landing on the designated arrivals runway develops a problem which prevents it vacating the runway in time for the next aircraft to land. Subsequent aircraft will then need to use the other runway until the aircraft on the designated arrivals runway is able to vacate safely.
There are other times when it is not possible to stick to the published runway alternation schedule. We refer to this as ‘de-alternation’ or ‘out of alternation’.
Routine maintenance of runways, taxiways or associated equipment is often scheduled to coincide with the pattern of runway alternation. However, there will be occasions when it is not possible to stick to the published runway alternation schedule. For example, this may be to facilitate maintenance or works of a longer duration (e.g. the 2014 runway re-surfacing) or work of an unforeseen, urgent nature.
Weather can also affect the use of alternation. For example, strong south-westerly winds can blow across the various buildings in Heathrow’s maintenance area and affect the approach for aircraft landing on the northern runway on westerly operations. When this occurs, we switch to landing on the southern runway for safety reasons.
Air traffic controllers may also decide to switch the use of runways for other safety reasons, for example if visibility on one runway is temporarily impaired by patchy fog or a nearby off-airport fire.
Since there are very few aircraft taking off or landing at night there’s more scope for runway alternation whether we are on easterly or westerly operations. We can switch landings between the northern and southern runways and, if the weather allows it, we can bring in aircraft from the east or the west.
Those four options allow us to operate night-time runway alternation on a four-weekly cycle.
Since the wind direction and strength can have an impact on this pattern, we always specify a primary and a secondary (alternative) runway in our schedule.