Talk about an elevating experience, literarily! This idea will work if you are planning a trip together around the time of your proposal. The idea is unique, exciting, and fun. You are sure to receive great cheer and excitement from the fellow travelers and staff!
You can make arrangements with the pilot and flight attendants beforehand so that they can help you on this exciting plan. You can have the pilot make an announcement that will vary depending on how you want to go about this. You can have the pilot make an announcement: "There is a man on this plane that is in love. He's so lucky that the woman he's in love with is sitting right next to him. Raise your hand if you are that man." That is when you raise your hand, look at her, and you loudly express to her how you feel. "I am that man. These past xx years have been magical. I want to spend the rest of my years in magic with you. Will you be my wife?"
You can make this proposal at the end of your flight, the beginning, or middle, its all up to you. You will however need to make sure that the timing is okay with the pilots and staff. You can plan your celebration at your arrival destination. The entire vacation can be a celebration as a newly engaged couple!