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.
The company says 85% of its scheduled flights will operate as normal on Saturday and the majority of customers affected have been given places on other Ryanair flights.
The Irish airline, Europe's largest low-cost carrier, averted widespread strikes before Christmas by agreeing to recognise unions for the first time in its 30-year history.
But there have been protests ever since over the negotiating of collective labour agreements.
About 300 Ryanair flights were cancelled in July when cabin crews in Belgium, Portugal and Spain went on strike for 48 hours.