Easterly operations
On easterly operations, we do not alternate the runways at 15:00 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.
How does runway alternation work?
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 patterns of runway alternation are different to night-time patterns: at night there are only a handful of landings and very few take-offs (see full explanation below)
- Aircraft land and take off into the wind, so the wind direction has a big effect on flight patterns
- Government policy currently favours aircraft taking off towards the west. Even when the wind blows from the east, aircraft could still fly in from east and take off towards the west. This is because we have a westerly preference
- A historical restriction means that aircraft cannot take off towards the east from the northern runway; this is because of the Cranford Agreement
- The hour between 06:00 - 07:00 is the busiest time of the day for arrivals so we are able to use both runways for landings
- Occasionally we have to suspend runway alternation for airfield maintenance, in response to bad weather or when air-traffic control want to avoid a build-up of arriving aircraft
Daytime runway alternation
Daytime runway alternation follows a daily cycle and a fortnightly cycle. Alternation starts at 06:00 and continues till the last aircraft departs at the end of the day.
- In the morning we use one runway for take-offs and the other for landings. At 15:00 – roughly halfway through Heathrow's working day – we swap over
- We continue this morning and evening pattern of runway use for the rest of the week
- The following week we switch completely. What we did in the evening during the previous week, we now do in the morning. And vice versa
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.
Using both runways for arrivals
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 07:00 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 06:00 - 07:00 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.
De-alternation
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 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.
Night-time runway alternation
Since there are very few aircraft take-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.
- Week 1: Aircraft fly in from the west to land on the northern runway
- Week 2: Aircraft fly in from the east to land on the northern runway
- Week 3: Aircraft fly in from the west to land on the southern runway
- Week 4: Aircraft fly in from the east to land on the southern runway
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.
