I've seen the following sentence in the book which teaches English as a corrected sentence. My question is why the word passenger -in the following sentence- is referred as a singular while almost never airliners isn't aimed for a singular passenger... therefore should it not be "passengers airliners"? In addition why there is not an indefinite article before the word 'long'?
"To fly big passenger airliners calls for long training and experience"
Because when a noun is used as a modifier of another noun, it is nearly always used in the singular, even though you might expect a plural. Eg
book club = club for discussing books
hand drier = drier for the hand
key safe = safe for keys.
There are exceptions, but that is the basic rule.
Passenger airliners is an example of a noun used as an adjective. It is not a noun in that phrase. Passenger airliners versus cargo airliners.
Training is an uncountable noun: Training is important if you want to become a teacher. No a.
Passenger can be a noun or an adjective. In this example, passenger is an adjective, modifying airliner. It doesn't take the plural.
The airliner is designed to carry passengers.
The airliner is intended for passenger service.
The first sentence above uses passenger as a noun, the second uses passenger as an adjective.
