Ryanair was forced to cancel nearly 400 flights on Friday because of pilot strikes in some of its biggest markets.
The discount carrier said it had canceled flights in Germany, Ireland, Belgium and Sweden, affecting thousands of passengers. About 15% of the carrier's scheduled flights were affected.
Ryanair pilots in the Netherlands were also striking on Friday, but their action did not result in flight cancellations.
The airline has been rocked by a series of staff strikes in recent months that have forced it to cancel hundreds of flights and sullied its reputation for reliable service. Separately, strikes by French air traffic controllers forced Ryanair to cancel about 1,500 flights and delayed 7,000 more.
Ryanair reversed decades of policy in late 2017 when a pilot shortage and looming holiday strikes forced it to recognize unions for the first time.
While it has recognized unions, the airline has yet to sign any labor contracts. And it has responded to worker demands with its characteristically pugnacious approach.
Ryanair announced in July that it would slash its fleet in Dublin by 20% this winter because of rolling strikes by pilots in Ireland. It warned 100 pilots and 200 cabin crew that they could lose their jobs, and posted pilot payslips on its website to show it's paying generous wages.
Kenny Jacobs, the chief marketing officer at Ryanair, said recently that the airline was "not prepared to concede to unreasonable demands that will compromise either our low fares or our highly efficient model."
The turmoil shows that Ryanair is no longer an industry upstart with a huge competitive advantage. It now faces the same pressures as traditional airlines, many of which have sought in recent years to emulate its low-cost tactics.
Still, clashes with unions are unlikely to unseat CEO Michael O'Leary, who has led the airline since 1994 and is under contract until late 2019.
He did give up a bonus of up to €1 million ($1.2 million) after scheduling failures led to mass flight cancellations in late 2017.
Shares in Ryanair dipped 1% on Friday. The stock is down roughly 30% from a record high set last year.