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 airline said that over 2,000 flights, or 85 percent of the schedule, would operate as normal and that the majority of passengers affected have been re-booked on other Ryanair services.
"What I find unjustified is that the pilots draw the short straw, because people want to fly cheaply", said Daniel Flamman, one of several passengers Reuters spoke to at Frankfurt airport who said they sympathised with the pilots.
A statement from CEO Michael O'Leary said: "Ryanair fully complies with all EU261 legislation, however as these flight cancellations were caused by extraordinary circumstances, no compensation is due".
Germany's powerful Cockpit union accused Ryanair of "categorically" ruling out higher personnel costs for cockpit crew, leaving no room for a compromise.
It already suffered a round of strikes by cockpit and cabin crew last month that disrupted 600 flights in Belgium, Ireland, Italy, Portugal and Spain, affecting 100,000 travellers.
But there have been protests ever since over the negotiating of collective labour agreements. But a court said yesterday Ryanair pilots in the Netherlands could not be prevented from doing so.
French news source France Info dubbed the strike "historic", specifying that it is the biggest walkout in the history of Ryanair.
The unions are demanding a raise in salaries, unions say, alongside the issuing of more secure working contracts that are valid in each pilot's country of residence (which they claim does not always happen).
A Dutch court also rejected a case from Ryanair seeking to block pilots in the Netherlands from joining Friday's strike, but the Irish airline said all of its flights there would run as scheduled.
It has already threatened to move part of its Dublin fleet to Poland, which could cost 300 jobs, including 100 pilot positions.
When is the Ryanair strike taking place?
At Charleroi Airport, Belgium's second largest and a major Ryanair hub in the region, striking staff gathered in the departure hall and held up banners reading "Ryanair must change- Respect us".
Unions have strongly condemned what they see as Ryanair's attempts to play countries off against each other.