An international airport is an airport with customs and border control facilities enabling passengers to travel between countries around the world. International airports are usually larger than domestic airports and they must feature longer runways and have facilities to accommodate the heavier aircraft such as the Boeing 747 commonly used for international and intercontinental travel. International airports often also host domestic flights, which often help feed both passengers and cargo into international ones (and vice versa). Buildings, operations and management have become increasingly sophisticated since the mid-20th century, when international airports began to provide infrastructure for international civilian flights. Detailed technical standards have been developed to ensure safety and common coding systems implemented to provide global consistency. The physical structures that serve millions of individual passengers and flights are among the most complex and interconnected in the world. By the second decade of the 21st century, there were over 1,200 international airports and almost two billion international passengers along with 50 million metric tonnes of cargo passing through them annually. History In August 1919, Hounslow Heath Aerodrome, in London, England, was the first airport to operate scheduled international commercial services. It was closed and supplanted by Croydon Airport in March 1920. In the United States, Douglas Municipal Airport in Arizona became the first international airport of the Americas in 1928. The precursors to international airports were airfields or aerodromes. In the early days of international flights, there was limited infrastructure, "although if engine problems arose there were plenty of places where aircraft could land". Since four-engined land planes were unavailable for over-water operations to international destinations, flying boats became part of the solution. At the far end of the longest international route (which became the Kangaroo Route), on-water landing areas were found in places such as Surabaya and in the open sea off Kupang. In Sydney, Rose Bay, New South Wales, was chosen as the flying boat landing area. International airports sometimes serve military as well as commercial purposes and their viability is also affected by technological developments. Canton Island Airport, for example, in the Phoenix Islands (Kiribati), after serving as a military airport during World War II, was used as a refuelling stop by commercial aircraft such as Qantas which stationed ground crew there in the late 1950s. The advent in the early 1960s of jet aircraft such as the Boeing 707 with the range to fly non-stop between