You can physically choose not to show up, but being a "no-show" without canceling in advance has significant consequences. Most airlines follow a strict policy where, if you miss the first leg of your trip, all subsequent connecting and return flights on that same itinerary are automatically canceled without a refund. Furthermore, you typically forfeit the entire value of your ticket. Some airlines also charge a specific "no-show fee" if you attempt to rebook later. To avoid this, it is always better to cancel or change your flight at least ten minutes before departure. If you are running late due to an emergency, many airlines have an unofficial "flat tire rule" and may put you on the next available flight for free or a small fee, but only if you contact them immediately.