Wow- lots going on in this one!
Nancy, Bess, and George are on vacation on a dude ranch/flight school (?). Bess falls in love with a cowboy, forgeting about patiend Dave back home, and receives the first proposal of marriage out of the 3 main characters. Nancy is taking flying lessons, and when a plane and pilot go missing, she and her instructor help find him and solve the mystery, but the most interesting parts of this one are the plot-moving devices, you can see the end coming in this one. Bess eventually decides to turn down the cowboy and to wait for Dave, but it is clear that their lives will be changing soon.