Domestic travelers can hold the airline liable only if their injuries are caused by the airline’s negligence. But if the passenger is traveling internationally, then treaties called the Montreal and Warsaw Conventions apply. Under the Conventions, whether the airline was negligent is for the most part irrelevant. An airline is responsible only if the passenger’s injury was caused by an “accident.” So, for an international traveler, the key question is what, exactly, qualifies as an “accident.”

The U.S. Supreme Court has defined “accident” to mean “an unexpected or unusual event or happening that is external to the passenger.” Certainly, an aircraft running off the end of the runway would qualify as an accident. But there are plenty of injury-producing events which present more difficult questions.

Here’s what the courts have said:

• Accident: A passenger is injured when a fellow passenger opens an overhead bin and liquor bottles fall out.
• Not an Accident: A passenger slips and falls on plastic bag left in aisle (reasoning: after long flight, it would not be “unusual” to encounter trash in the aisle).
• Accident: A passenger burned by tea when tea spilled from tray table because the passenger seated directly in front of the injured passenger caused a “jolt” that upset the tray table.
• Not an Accident: A passenger falls while trying to walk up a broken escalator.
• Accident: A passenger seated near the smoking section asks to be moved, the flight attendant refuses, the passenger has an asthma attack and dies.
• Not an Accident: A passenger dies from an airline-induced blood clot.
• Not an Accident: One passenger falls on and breaks the arm of another passenger (reasoning: the passenger decision to try to climb over his fellow passenger not related to the aircraft’s operation.)

More at Chris Cotter’s excellent article: Recent Case Law Addressing Three Contentious Issues in the Montreal Convention.