G-Nick, I think you are mixing up issues here.
To start from the beginning, there are many factors regarding runway selection.
The basic one is the current (and forecast) wind. Obviously the ideal is to operate into wind. If the wind is light and variable, one will usually take into account the forecast wind (runway end changes usually cause delays at busy airports, so by setting up early or during a quiet period of calm wind, delays can be minimised).
That's the main one. If the wind is below 5kts, then one can choose either end to operate off. In that situation, there are many other factors that come into play.....
-Serviceability of navigation aids (for example, only one end of the Instrument Landing System is serviceable).
-Serviceability of runway/approach lighting.
-Taxiway closures.
-And, where applicable, any airport authority edict that says one end should be preferred over the other, usually due to noise considerations.
In Heathrow's case, The westerly preferential system was devised when aircraft generated a great deal more noise than they do today on climb out, hence it was preferred to have the aircraft on approach over London, and climbing out over the more sparsely populated Windsor, Slough, Bracknell area.
Preferential procedures tell us which runway to use if there is no wind, all other factors being equal.
The prevailing wind is merely from which direction the wind is more likely to blow. In most of the south UK that is WSW, hence most airfields have a runway aligned from W to SW (Heathrow 27, Stansted 23, Gatwick 26 etc).
Last edited by Gonzo; 14th Mar 2007 at 12:46.