If Your Flight Was International: The Montreal Convention
The Montreal Convention of 1999, formally the Convention for the Unification of Certain Rules for International Carriage by Air, is a treaty that governs passenger injury claims on most international air travel. The United States is a party to it, and for covered international flights it has the force of federal law. It replaces many state-law questions with its own rules.
Article 17 makes the carrier liable for a passenger's bodily injury when the injury results from an "accident" on board the aircraft or during boarding or disembarking. Up to the applicable Special Drawing Rights limit, the carrier's liability does not depend on proof of negligence. Special Drawing Rights are a unit of value set by the International Monetary Fund, and the limit is adjusted over time, so no fixed dollar figure is given here.
Article 21 changes the picture above that limit. For damages beyond it, the carrier is not liable if it proves the injury was not due to its own negligence or other wrongful act, or that it was due solely to the negligence or wrongful act of a third party.
The word "accident" carries a lot of weight. Courts have read it to mean an unexpected or unusual event external to the passenger, and they have examined whether particular turbulence encounters qualify. The answer depends on the circumstances, so not every bump in flight meets the definition.
Article 35 sets a two-year period for bringing a claim, measured from events including the date of arrival at the destination and the date on which the aircraft ought to have arrived.
Domestic U.S. flights are not governed by the Montreal Convention. For those, state tort and product-liability law sets the rules for the claim, while federal aviation regulations govern how the flight was operated. Your ticket and itinerary show where the trip began and ended, the first fact in deciding whether the Convention applies, and are worth keeping for an attorney's review.