Airlines could be forced to pay compensation to passengers for a delayed flight, even if it touches down on time.
The EU's top court, the European Court of Justice, decided planes should only be considered to have "arrived" once the door has been opened.
The ruling comes after a passenger sued the budget airline Germanwings for compensation, after his flight landed almost three hours behind schedule.
The European Court of Justice said that although the plane landed within three hours of its scheduled time, the doors weren't opened until after the three-hour window - and so the passengers on board are entitled to compensation for the delay.
Current rules for the compensation of passengers dictate that if a flight is more than three hours late arriving at the final destination, passengers are entitled to compensation of €250 for a journey of 1,500km or less within the EU, and €400 for journeys of over 1,500km.
Between an EU airport and one outside the EU, the rules also allow for compensation of €600 for flights over 3,500km that are more than three hours late.