AirlineIcaoCode

3 character code as designated by the International Civil Aviation Organisation (ICAO) to uniquely designate an airline operator. Local (non-ICAO) codes can be added as required as long as they are unique for airline operators within the defined context.

Reference Document: ICAO document 8585"

• "AAA" - Three uppercase letters (A-Z). This is according to ICAO rules.
• "1AA", "11A", "A1A" - Combination of uppercase letters (A-Z) and numbers, but third character must be an uppercase letter. 

ICAO airline codes are normally only uppercase letters, 'A' - 'Z'. However IATA allows for three character airline codes with numbers as part of FlightId. This is typically used for testing purposes.