noun used as an adjective

Nouns used as adjectives remain invariable unless they are made possessive. Thus, in the example

• Flight Numbers AB123 and CD456 are now ready for boarding.

the noun flight does not take an s because it is acting not as a noun but as an adjective modifying the noun Numbers.

However, nouns used as adjectives may be in the possessive form, in which case they are followed by ’s:

• The flight’s passengers were fortunately not injured in the incident.