Pilots at Lufthansa's Eurowings began a three-day strike over working hours, their union said, affecting tens of thousands of the budget airline's passengers on Monday.
"It's begun. And so far there is no new offer," said a spokesperson for the Vereinigung Cockpit (VC) pilots' union.
The union wants the carrier to ease pilot workloads through measures such as increased rest periods, but Eurowings warned over the weekend that the demands put jobs at risk.
The airline said about 240 out of 488 flights have been cancelled at airports in Cologne, Hamburg, Berlin and Stuttgart, affecting about 25,000 passengers on the strike's first day.
It expects to cancel a similar number of flights on Tuesday and Wednesday. The walkout is due to end at 11:59 pm local time (2159 GMT) on Wednesday.
Eurowings pilots had held a one-day strike for improved working conditions at the start of October, affecting about 30,000 passengers and the cancellation of about 250 flights.