In contemporary air travel, one thing you might notice about transatlantic flights is that they are usually flown overnight going toward Europe, but flown during the day going toward the Americas.

There's a simple reason for this: a Europe-bound flight consumes more relative time than an America-bound flight, thanks to the wonders of time zones. Going from the U.S. East Coast to Britain takes about twelve hours in terms of local time, whereas a trip in the opposite direction takes about three hours. (The actual flight duration is shorter going east, and can be noticably shorter if the aircraft can pick up tailwinds from the jet stream: you'll be able to tell because you'll feel turbulence through the whole flight.)

So if a plane took off from London at 6 AM, it would arrive in New York around 9 AM; and if it left New York at noon, it would reach London at midnight. This might sound fine and dandy, but consider the traveller: they lose an entire day going to London, and then have the longest morning of their lives coming back. Instead, the plane leaves London in the afternoon, allowing the traveller an entire morning to hang out or get business done, and then returns overnight, allowing the traveller to sleep it off. Much better.

Another factor in scheduling transatlantic flights is that the aircraft themselves have to be serviced and fueled at each end. Most airlines want to be able to perform maintenance and rotate aircraft in and out of service at their base airport, so they schedule their planes to spend a minimum amount of time (usually an hour or two) overseas and a longer amount of time at home. British Airways' schedule, for instance, looks like this:

Fig. 1: British Airways flights from and to London

                From London       To London
              Leaves  Arrives  Leaves  Arrives
New York       4 PM    7 PM     8 PM    8 AM
San Francisco 11 AM    2 PM     4 PM   11 AM
Toronto       12 PM    3 PM     6 PM    6 AM
Vancouver      4 PM    6 PM     9 PM    2 PM
Note that none of the aircraft spend more than three hours overseas: once they return to London, they're thrown back into the available aircraft pool for that day's departures to North America. Now let's look at American Airlines, London's biggest foreign operator, and how they schedule their flights:

Fig. 2: American Airlines flights to and from London

                 To London       From London
              Leaves  Arrives  Leaves  Arrives
Chicago        8 PM   10 AM     2 PM    5 PM
Dallas         5 PM    8 AM    10 AM    2 PM
Miami          6 PM    6 AM    10 AM    3 PM
Raleigh        7 PM    7 AM    12 PM    4 PM
The dead times at either end are the same, since American can service or replace their aircraft at either end with the same level of difficulty. In fact, it would be easier for American to fix a 777 in London than in Raleigh, which only gets one widebody flight a day compared to Heathrow's thirteen.

To complete our study, let's look at two other U.S. majors that only have minor operations in London:

Fig. 3: Delta and Northwest flights to and from London

                 To London       From London
              Leaves  Arrives  Leaves  Arrives
Atlanta        6 PM    7 AM    10 AM    2 PM    (Delta)
Cincinnati     7 PM    8 AM    11 AM    3 PM    (Delta)
Detroit       10 PM   10 AM     2 PM    5 PM    (Northwest)
Minneapolis    7 PM    9 AM    12 PM    3 PM    (Northwest)
Each airline schedules more time in America than they do in London: obviously, they want to get their plane the hell out of hostile territory.