Just to add that, as covered above (but contrary to certain beliefs) Heathrow is a 24-hour operation airport. But there are restrictions on the number of "night" (defined as 2330-0600) movements.
Time was when these used to be taken by night freight flights in the small hours and some other deep night passenger schedules. But over time operators have tended to concentrate their usage on early morning long-haul passenger arrivals, finding this is their best usage of them.
On average there are about 14 movements per night, which typically (if there is a typical night) is probably made up of one delayed departure and, later in the night, 13 long-haul arrivals, spread over about 0430-0600. Inbounds from Hong Kong tend to be the first in. Actually it's unusual to actually have a touchdown at 0559 as that is "wasting" a slot which could be avoided by arriving a minute later.
By about 0545, if there have been stronger than expected tailwinds across the Atlantic (or lighter than expected across Asia) the stacks around Heathrow can be quite full with those waiting for the end of the restricted period. I am often getting up around this time and our house is about 10 nm from the Lambourne hold, so a look out of the window is a good weather guide in the dark for if it's CAVOK or not, seeing the parade of lights going round.
Not always realised is that immediately after the restrictions finish, for one hour, Heathrow's normal runway alternation between arrivals and departures is not used, and both runways are fully used for arrivals, with parallel approaches. This is because there is not much traffic departing before about 0645, but there are a lot of inbounds.
There is a certain skill in scheduling all this, and BA for example, who have the largest allocation of night slots, will know what the averages are for them having delayed departures which need to use a night slot. They will adjust their inbound arrivals for what they find is the most efficient usage of what remains for them.
If there is an exceptionally severe delay at Heathrow in the evening the Department for Transport can authorise that the limits are suspended, although they are very reluctant to do so.
Incidentally, the "24-hour" aspect applies to aircraft operations. Now, trying to get bags loaded/unloaded, etc, or a cup of coffee in the terminal, at 0200 is another matter altogether.
Last edited by WHBM; 3rd Dec 2008 at 11:31.