The last U.S. airmail stamp was issued in 2012 ... although the last true U.S. airmail was probably issued in 1993 as the remaining issues were the Scenic American Landscape series intended for international mail. Do you not have access to a Scott U.S. Specialized Catalogue? Any recent one would have issue dates, presumably for Scott C77 thru C150? Added: If you don't need exact dates, Mystic has a catalogue that you can access online at no charge. It shows range dates for various U.S. stamps.
The Tagging Database at Stampsmarter also has this info. If you follow the link below it will take you to the main page. Click on "View Database" then select "View By Series". In the selector window that comes up select "Airmail". It will bring up only the Airmail issues. If you want specific issue dates that is available by clicking on Details for each stamp, where the issue date as well as other info is displayed.