Why is it numbers are not always pronounced in a phonetic way? For example a flight number of 2390 is pronounced "twenty-three ninety" instead of "two three niner zero"
The FAA's ATC orders (section 2-4-20) say that the 'group form' should be used by controllers when talking to any aircraft with an FAA authorized call sign. So you would expect to hear it with airline traffic but not private flights:
- Air carrier and other civil aircraft having FAA authorized call signs. State the call sign followed by the flight number in group form.
NOTE- “Group form” is the pronunciation of a series of numbers as the whole number, or pairs of numbers they represent rather than pronouncing each separate digit. The use of group form may, however, be negated by four-digit identifiers or the placement of zeros in the identifier.
EXAMPLE− “American Fifty−Two.” “Delta One Hundred.” “Eastern Metro One Ten.” “General Motors Thirty Fifteen.” “United One Zero One.” “Delta Zero One Zero.” “TWA Ten Zero Four.”