The Demo Flight (also known as the introductory flight, or discovery flight) is the first flight that you take at a flight school. In my opinion, the purpose of the demo flight is twofold: 1) to experience what it is like to fly, and 2) to learn if you like the flight school and instructor. If you are toying with the idea of learning to fly, I highly recommend that you look up a flight school near you and take one of these introductory flights just to get an idea of what it is like. One of the great things about the demo flight, is that it often comes with a discount. It's really in the flight school's best interest to give you a good deal on the demo flight because the more people they get to take a demo flight, the more long term customers they get. I have heard that at some flight schools, you can even get a demo flight for free, particularly on International Learn to Fly Day, which is an event that the Aircraft Owners and Pilot's Association (AOPA) came up to encour