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After a Boeing 777 crash-landed at Heathrow, the "public safety zones" around a proposed third runway take on greater significance.
While Thursday's crash is the first on either landing or takeoff at Heathrow since 1972, the issue of building another runway at the airport raises fears among some that increasing the number of aircraft landing and taking off will have an effect on safety.
Back in November, the government outlined its support for airport operator BAA's aim to build the third runway and sixth terminal building by 2020.
This would enable Heathrow to increase the number of flights it could handle each year to 605,000.
A study by National Air Traffic Services (Nats) suggests three runways operating in "mixed-mode" would see movements rise to 130 aircraft taking off or landing each hour.
Hillingdon Council, the London borough which covers the site of Heathrow's proposed third runway, says the development should be rejected because of the risk it poses to safety.
In the council's view "the high density of population in the vicinity of Heathrow, combined with the increase in the number of aircraft movements that a third runway would allow is predicted to cause a significant increase in safety risk".
Yet, based on 1994's movement figures of 423,385, the DfT calculated an average rate of 0.168 crashes or over-runs per million movements.
However, any new development at Heathrow would see land beyond the enlarged airport boundary included in new public safety zones (PSZs).
'Individual risk'
These are based on the risk posed to the public when an aircraft is landing or taking off, so extend beyond the runway threshold in contours - like those shown on maps to provide an indication of land heights.
The risk of an individual being affected by an air crash diminishes the further they are from the airport - the outer limit of public safety zones are designed to reflect a one in 100,000 chance of an individual being at risk, if they lived or worked in that area for a year.
The Department for Transport (DfT) states that the creation of a PSZ is "based on objective assessments of the individual risk to people on the ground in the vicinity of each airport".
When using a computer to model the risk to an individual on the ground, the DfT factors in three criteria:
Using these factors, the DfT maps the risk contours usually from a one in 10,000 risk to an individual, out to a one in 100,000 risk.
The significance of public safety zones is that they can also lead to a restriction on the amount of buildings that can be developed within them.
The government's policy states that there should be "no increase in the number of people living, working or congregating in PSZs". Local planners are urged to reduce the numbers over time.
'Maximum tolerable' danger
Airport authorities are also expected to buy and demolish properties that fall within the one in 10,000 contour, because that is the "maximum tolerable" risk.
This, of course, is part of the fate that awaits the village of Sipson, near Heathrow, should a third runway get the go-ahead.
Not only would properties be demolished to build the runway and associated taxiways, it would also be expected that some homes would be within the highest risk contour.
According to the pressure group Aviation Environment Federation - which is made up of environmental groups, local authorities and individuals - "hundreds if not thousands of properties may well be affected," once PSZs for any new runway are drawn up.
But a DfT study from 1997 suggested "there appears to be a small number of houses just within this [one in 10,000] contour at Heathrow".
As that relates to the existing two-runway airport, there is still a question over whether the development of a third runway will mean many more properties will have to be demolished because of the risk of a crash.
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