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"

Anyway, where did you see there referring to the second question here about the article before the word passenger?
– Virtuous Legend Oct 29 '16 at 22:46