While many flight numbers may appear random, there is definitely a method behind the madness. Individual airlines create their own numbers based on internal methodology, but they must coordinate their efforts with other airlines in order to avoid confusion in the flight control towers. United Airlines and American Airlines, for example, cannot both have two incoming planes with similar flight numbers arriving at the same time. Airlines are governed by the International Air Transport Association (IATA), and it often falls on the IATA to approve these designations before they can be implemented by an airline.
Many flight numbers are assigned according to the general direction of the flight itself. Planes headed east or north are usually assigned even numbers, while planes headed west or south are assigned odd ones. Return flights are often assigned those that are one number higher than the departure flight, so passengers can easily remember which return flight to take. For example, the fictional WiseGEEK Airlines may have a flight from New York to Las Vegas departing at noon. Since it is traveling west, it may be designated as flight 711. The WiseGEEK Airlines flight from Las Vegas back to New York would most likely be called flight 712. Both flight numbers would have to be approved by the IATA, in case there happened to be an existing United Airlines flight 712 also arriving in New York at about that time.
Airlines also assign flight numbers according to a set of agreed-upon parameters. One major airline may agree to limit its possible choices to 1-499. Another airline, possibly a smaller carrier working with the major airline, could have 500-749. Another major airline may have numbers from 750-1000, and so on. Through a practice called code sharing, smaller regional airlines working together with major airlines coordinate their numbers to avoid confusion and to designate the flight as a joint effort. Some airlines also designate flight numbers according to the destination of the plane or the type of passengers it will carry. Domestic flights, for example, may have two digit numbers while international flights may have three digits. This practice varies from airline to airline, but they are usually consistent. Chartered flights may have special numbers that instantly allow airline employees to recognize them.
Assigning flight numbers isn't all business, however. Airlines are allowed, within reason, to designate planes with significant or whimsical numbers. A flight to San Francisco, for instance, may receive one such as 49 or 1849 as an homage to the famous Gold Rush of 1849. An airline providing service to Philadelphia might select 1776 or 76, reflecting that city's historical role in the formation of the United States. Some flights to casino towns like Las Vegas or Reno, Nevada could have numbers with 7s and 11s for luck. As long as the airline does not violate IATA regulations concerning flight numbers, it can select numbers for reasons of its own. These numbers can also be changed if they become controversial, as in the case of American Airlines Flight 11, which crashed into the World Trade Center on 11 September 2001. When commercial flying privileges were restored, the airline changed Flight 11 to Flight 25.