Ryanair pilots are striking in five European countries, forcing the cancellation of a sixth of the firm's flights during the holiday season peak.
The 24-hour walk-out involves staff in Germany, Sweden, Ireland, Belgium and the Netherlands. About 50,000 passengers are understood to have been told of cancellations on 400 flights.
The action is the largest in a series of strikes over pay and conditions.
Ryanair says it has made every effort to resolve the dispute.