Air traffic control (ATC) voice communication is built upon a readback-hearback loop: Controllers send messages to pilots who listen and then recite back their contents. Successful communication requires participants to conduct and understand ATC radiotelephony in the same language. Since inadequate language proficiency was involved in some aviation accidents (e.g., 1996 Charkhi Dadri; 1995 Cali; 1977 Tenerife), the International Civil Aviation Organization (ICAO) is requiring its contracting states to ensure that ATC personnel and flight crews are proficient communicators of the English language when operating in airspace where the English language is required.