The Department for Transport’s report, “Improving the Air Passenger Experience”, provides appropriate estimates of aircraft stacking above Heathrow. This report is available at:
Figure 27 on page 25 shows that in 2006-07, 56 per cent. of arriving aircraft were held in a stack and for those held, there was an average 8.5 minute hold. This equates to roughly 50,000 tonnes of carbon dioxide emitted by aircraft stacking at Heathrow in 2006-07.
As the capacity constraint is reached at Heathrow, these delays and emissions are forecast to increase in future without additional capacity.
Although the effect on stacking of a third runway has not yet been quantified, it is anticipated that a third runway will reduce the need for aircraft to be held in stacks. It will also enable the airport to operate with greater flexibility for slot re-timing with potentially fewer delays for passengers.