When in 1982 did ICAO start using three-letter airline codes?

Question

All airlines have an alphabetic identification code issued by ICAO. From 1947, when ICAO was formed, until 1982, ICAO codes were two letters long;¹ in the latter year, due to the increasing number of airlines in the world, ICAO lengthened its airline codes to three letters (although the preexisting two-letter codes remained valid until 1987).

Does anyone know when in 1982 ICAO introduced three-letter airline codes? (I'm writing about an accident that occurred in 1982, and I can't tell if I should list the two-letter or three-letter ICAO designator for the flight.)

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¹: When IATA got into the business of issuing airline codes in the early 1950s, its members adopted their preexisting ICAO codes as their new IATA codes (this is why IATA codes are two characters long²).

²: Although the IATA system makes formal provision for three-character airline codes, it has never actually issued those, because airlines don't want to have to replace ancient equipment that can't handle airline codes longer than two characters.

Answer 1

The "planning" for it started after November 1981. "[T]otal conversion" from old to new (three-letter) took place in November 1987. In-between, "registration/change" was open.

In November the Council agreed that the planning should begin immediately for conversion from the current ICAO two-letter to a future three-letter designator system for aircraft operating agencies, aeronautical authorities and services. It approved 1 November 1987 as the date for total conversion to be completed. It also agreed to the standardization, encompassing the three-letter designator system, of an eight-character addressee indicator using a single character for internal distribution and approved the rules for the registration of or change in three-letter designators.

Source: ICAO Annual Report of the Council - 1981. p. 75. (PDF; icao.int)

End of answer.

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Inconsistency on Wikipedia, domestic three-letter codes, and IATA:

After checking the Wikipedia article you linked, I've found the example table matching what I found/wrote above (I'm reproducing the applicable row only):

Period	ICAO	IATA	Remarks
1982 to 1987	AA (AAL)	AA	ICAO issued 3-letter-codes but kept the 2-letter-designators as official system

(bold emphasis mine)

The article however is a bit unclear in its use of the word "introduced", so now I understand where the confusion comes from. Wikipedia is a great place to start learning about a topic, but always look in the citations used. If they're absent, unavailable (not online), unclear, or unreliable, question the statements made (example here).

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According to a 1984 FAA report, domestic operators (but not all) were given three-letter codes, and international operators two-letter codes. So expect domestic variations prior to the standardization.

I'll also bet IATA was not always alphanumeric, since the document quoted above (and others) refers to ICAO's original system (which according to the question, was mirrored by IATA) as two-letter, not two-character.