Originally Posted by
rutankrd
There are approximately 18 slots available after 23.01 and 06.00 every night .
Almost all were at one time used by freighters but these days are repurposed to long haul arrivals.
Depending on season the earliest arrival are 04.40 from Boston or 04.50 from Hong Kong .
This is correct for scheduling but in practice any delayed late departures or very late arrivals are able to be handled, and the balance is maintained by bringing in one or more of those due before 0600 in just after 0600. Because the limit is a global one across a season rather than a fixed amount per day this can be done when convenient. So you do get the odd 4 hours late 0200 arrival. As regularly has to be reinforced, Heathrow is a proper H24 airport, just with a night movements quota.
It is extraordinary to look at a timetable from the 1960s, when in summer there were very considerable movements in the small hours by 707s, Comets, etc, off to Mediterranean points, plus all the freighters mentioned above.