Is working in unloading airplane baggage and placing passenger boarding stairs forbidden, knowing that alcohol is served on planes and some of it in the airport where the questioner works?

The work mentioned is permissible, and there is no objection to continuing it, because you are not assisting in a known evil, and the default assumption regarding travelers' belongings is that they do not contain anything forbidden. The airport's provision of forbidden items to customers has no bearing on you and does not affect the permissibility of your work.