Aviation English is the de facto international language of civil aviation. With the expansion of air travel in the 20th century, there were safety concerns about the ability of pilots and air traffic controllers to communicate. In 1951, the International Civil Aviation Organization (ICAO) recommended in "ICAO Annex 10 ICAO (Vol I, 5.2.1.1.2) to the International Chicago Convention" that English be universally used for "international aeronautical radiotelephony communications." Despite being a recommendation only, ICAO aviation English was widely accepted.
However, because many pilots and controllers lack proficiency in English, accidents have continued to result from miscommunication, including the Tenerife accident in 1977 that killed 583 or the Charkhi Dadri mid-air collision in 1996 that killed 349. The ICAO has acknowledged that "communications, or the lack thereof, has been shown by many accident investigations to play a significant role". In 2003, the organization "released amendments to annexes of its Chicago Convention requiring aviation professionals involved in international operations to demonstrate a certain level of English language proficiency."
Aviation English is a type of English for Specific Purposes, with several specific idiosyncratic structures: for example, any epanorthosis must be conveyed using "correction". The need of standardization has led to the creation of the Test of English for Aviation, not endorsed by the ICAO.