From FlightSafety Foundation:

A number of accidents, some of them fatal, and incidents have been attributed at least in part to communication issues related to the language proficiency of air traffic controllers and pilots.

The International Civil Aviation Organization (ICAO) mandated that pilots and air traffic controllers demonstrate language proficiency sufficient to deal with the linguistic challenges presented by quickly changing and dynamic abnormal situations and emergencies that require extended use of language outside that of standard radiotelephony (RT).

The language proficiency requirements are applicable to non-native English speakers but, according to a statement in ICAO Doc 9835, “Native speakers of English, too, have a fundamentally important role to play in the international efforts to increase communication safety.”

Still, it seems that the onus for safeguarding successful communication is on the non‑native English speaker. In many cases, non-native speakers are tested and taught how to approximate to native speaker norms when, in reality, many of them will have less opportunity to interact with native speakers.

English, the language of aviation, is a first language or widely used national language in approximately 60 ICAO member states, ICAO said several years ago in Doc 9835. But the document also says that “there are more speakers worldwide of English as a second or foreign language than as a first language, and most of the contexts in which English is used occur among speakers of English as a second or foreign language. Non-native users of English outnumbered native users at the start of the 21st century by approximately 3 to 1.”

So, it stands to reason that the majority of aeronautical radiotelephonic interactions are between speakers for whom English is not the first language; in other words, it is a lingua franca — a language used for communication among groups of people who speak different languages. I won’t go into too much, but these interactions are qualitatively different from the interactions that take place between native speakers.

When non-native speakers engage with other non-native speakers in English, either in an aeronautical or a non-aeronautical context, they come to the speech event with their own language ability, their own cultural expectations, their own first language interference and a host of other unique dimensions. These interactions are “de-territorialized speech events”¹ not tied to any one specific culture and so are very “hybrid in nature.”²

Native speakers tend to take so much for granted: connected speech, complex localized language structures, lexis (vocabulary) and much more. This puts the native speaker at a disadvantage as these features of native English speech are particularly problematic to non-native speakers at lower levels of proficiency.

Native speakers are in the minority³ and so, it has been argued, it is as incumbent on the native speaker as on the non-native speaker to meet part way by bridging the gap in safeguarding successful communication.⁴ It would appear, from the evidence and the literature, that there is a need for native-speaking pilots and air traffic controllers to undergo training to learn how to accommodate their non-native English-speaking interlocutors in order to safeguard communication and mitigate against possible incidents.

From ICAO:

​Montréal – 4 July 2013 – The International Civil Aviation Organization (ICAO) has announced the launch of a new and improved Aviation English Language Test Service (AELTS) website (www.icao.int/aelts).

First launched in 2011, the website for this voluntary service has been made significantly more intuitive and user-friendly, responding to ongoing feedback from the aviation English language testing community.

“Aviation English language tests are designed to measure the speaking and listening ability of pilots and controllers, a key factor in the day-to-day safety of air transport operations,” noted the UN body’s Secretary General, Raymond Benjamin.

“As aviation continues to grow, with almost 100,000 flights a day today and 200,000 daily expected by 2030, it’s imperative that ICAO continues to evolve and refine its safety support tools,” continued Benjamin. “This helps to ensure that passengers around the world can continue to look to air travel as their safest means of rapid global connectivity.”

ICAO’s AELTS directly supports the UN standard-setting body’s Doc 9835, the Manual on the Implementation of ICAO Language Proficiency Requirements. By measuring test performance against its Language Proficiency Requirements (LPR) criteria, ICAO is able to provide important information on test quality so that States, pilots and controllers can make the most informed selection possible when choosing a test provider. 

An international AELTS Steering Committee, comprised of highly qualified experts from States, associations and non-profit organizations, advises ICAO on best practices and provides guidance on how to develop, implement, manage and improve the test assessment service.

From Easyaviationenglish.com:

English has long been the common language of aviation.  Pilots and air traffic controllers of varying nationalities have been required to communicate using english.  Previously it was up to each country to create their own standard of aviation english.  However, these standards often vary and as a result miscommunication in the english language has contributed to many aviation accidents.  The International Civil Aviation Organization (ICAO) created an international standard for language proficiency requirements including a rating scale to measure the level of english proficiency.  Of this scale, ICAO level of 4 or higher was officially recognized as being english proficient in aviation.

ICAO set an initial deadline for 2008 for pilots and air traffic controllers to achieve the minimum english proficiency of ICAO level 4.  Many countries were not able to meet the deadline so an extension was given until 2011.

The purpose of an international standard of english is to enhance global aviation safety

These english standards are generally accepted by ICAO member countries around the world.  However, each country may set their own english standards beyond what was set by ICAO.

Anyone can take the ICAO english test but pilots and air traffic controllers involved in international flight operations must achieve at east level 4 of english proficiency.  Even pilots who fly between two non english speaking countries must first pass the ICAO english test.

The ICAO english test measures the ability to speak and understand english in an aviation environment (reading english is not required).  This includes how well one can efficiently communicate routine and non routine situations both face to face and over the radio.  In particular the test measures the following:

• Comprehension – to be able to understand english through various accents and dialects.
• Communication – to communicate information clearly and effectively particularly during emergency situations.
• Radio communications – communication over the radio can be very different than speaking in person.  Proper radio communication skills require the use of standard phraseology.  It also requires the ability to understand distorted english created by radio interference.

During the test the examiner evaluates the applicant based on the following areas:

• Pronunciation – to speak english with a dialect or accent that is easy for the listener to understand.
• Structure – grammatical structures and sentence patterns.
• Vocabulary – the speak accurately and efficiently using the correct words.
• Fluency – the continuous flow and rhythm of speech.
• Comprehension – to understand and make sense of what is heard.
• Interactions – how well one responds in a conversation.  This may include checking, clarifying or confirming information if necessary.

Each category is graded on a scale between 1-6 (1 is the lowest, high is the proficient).  The lowest score determines the final ICAO english rating.  For example, an applicant may be scored 4 for every category except comprehension where the score was 3.  As a result, the applicant will receive a final rating of 3.The international standard to be english proficient is level 4 or higher.

Those who have ICAO english level 4 must retake the exam every three years while those with ICAO english level have up to 5 years to be reassessed.  Achieving ICAO english level 6 is considered an expert level and therefore does not require a reassessment.

From AOPA Pilot:

We have received many questions about the English-proficient endorsement for pilot certificates. Pilots want us to clarify who’s affected, how to get the certificate, and clear up the confusion about the compliance date. The initial deadline was March 5, 2008, but the FAA was flooded with applications and has extended the compliance date a year—until March 5, 2009.

Pilots who fly from the United States to any destination outside of the United States, will be required to have a new certificate with “English Proficient” on it when acting as a required crewmember after March 5, 2009. This is a result of the International Civil Aviation Organization’s (ICAO) language proficiency standards for operating internationally.

The requirement applies to all holders of private, commercial, and airline transport pilot certificates with powered ratings, as well as flight engineers and flight navigators. If you hold an instructor certificate, it will not have an English-proficient endorsement and you do not need to order a replacement for it.

Pilots with a U.S.-issued certificate will not need to pass a language test, just obtain a replacement certificate by requesting one from the FAA. The plastic replacement certificate costs $2 and takes about two weeks for online processing, and four to six weeks for paper processing through the mail.

Here’s something to consider if you’ve been meaning to order a new certificate with a number different from your Social Security number, but haven’t gotten around to doing it. Since all new pilot certificates will automatically be issued with the endorsement, you could accomplish both things with one request—and you aren’t charged the $2 fee for a new certificate, only for a replacement.

Pilots can download the paper application for SSN removal from the FAA’s Web site for a replacement certificate ($2).

If you already have an account, just log in. If you are not yet registered, you’ll have to create an account and enter your personal information.

Place a checkmark in front of the $2 box and select “English Proficiency” as the reason. Follow the steps to receive your new certificate in about two weeks.

AC 60-28B FAA English Language Standard For FAA Certificate

ICAO English Language Proficiency Requirements