Heathrow Airport currently operates its runways in what is known as ‘Segregated Alternated’ mode:

• Segregated mode – means arriving aircraft are allocated to one runway and departing aircraft to the other. 
• Runway alternation – means the use of the two runways is switched over at fixed periods so that noise from arriving aircraft is shared between communities below the approach paths.

One way of increasing capacity at Heathrow would be to introduce mixed mode onto the existing runways. This would mean that both runways would be used simultaneously for a mix of arrivals and departures. 

Arriving aircraft would be shared between the two runways, rather than being concentrated at any one time on a single runway, as now. So there would be longer intervals between arrivals but there would be no segregation or runway alternation. 

Mixed mode could potentially deliver an extra 15% runway capacity at Heathrow by around 2015, allowing up to 540,000 flights per year, compared with today's limit of 480,000.