Travelmath offers an excellent calculator for determining approximate flight times between cities. Users enter a departure and destination city and the system gives an estimate of flight time, allotting half an hour for takeoff and landing.
There are several factors that may affect the estimates for flight times, including airline delays and cancellations, aircraft speed and jet streams. The Bureau of Transportation Statistics shows that approximately 20 percent of flights in the U.S. are delayed and another two percent are cancelled. This has an obvious impact on actual flight times for travellers.
Travelmath also uses an average air speed of 500 mph. This can vary both by aircraft and weather conditions. Commercial jets can fly anywhere from between 480 to 525 knots (552 to 604 mph). MIT reports that commercial airlines usually fly much slower for economic reasons. Jet engines peak at optimum fuel efficiency at lower speeds and the wings on a slower aircraft need less sweep, resulting in less cost to build them, as well. An article in the Daily Mail also notes Class B airspace as a factor in airspeed. Class B airspace restricts aircraft to speeds under 250 knots (288 mph). It usually is any area below 10,000 feet. However, this ceiling is extended to higher elevations around busy airports.
Lastly, the jet stream can affect travel time. These high speed air currents meander from west to east across the planet at speeds between 57 and 247 mph. Commercial jets frequently enter a jet stream when they produce a tailwind to reduce fuel consumption and increase flight speed. Flying in the jet stream can reduce travel time by over one-third on some flights.