Useful article from the CAA
http://www.caa.co.uk/docs/7/DAP_EIS04.pdf
Quote:
In air traffic control
terms, distances of 30 miles and less are comparatively
small and take only a few minutes flying time. Typically,
aircraft inbound to the UKâ₠¬ÃƒÂ¢Ã¢â‚¬Å¾Ã‚¢s major airports are routinely
required to intercept the Instrument Landing System (ILS)
facility, an aircraft-interpreted aid comprising centre-line
and glide-slope elements radiating from the touchdown
point and aligned with the runway concerned, from a
height of approximately 2000 - 2500 feet. During the night
the joining level is often set higher. The fact that aircraft
need to be established on the ILS centre line at
approximately 10 miles from touchdown will explain why
many areas of the southeast are over-flown at relatively
low level. If the aircraft were significantly higher they
would be unable to achieve a safe intercept of the ILS
glide-slope. The need for aircraft to over-fly built up areas
at all has also been questioned. Unfortunately, the
west/east alignment of Heathrow Airportââ‚à ‚¬Ã¢â€žÂ¢s main runways
and the location of the airport in relation to London mean
that aircraft on final approach to the westerly runways will
inevitably fly over congested areas of population en-route
to the ILS and when established on the facility.
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