Around 200 workers at Frankfurt airport, Europe's third-busiest, went on strike over pay Thursday forcing 150 flights to be cancelled and risking Europe-wide air travel disruption.
In the first of two planned days of warning strikes, the airport's so-called "apron control" staff -- who direct aircraft in and out of their parking positions -- halted work at 3:00 pm (1400 GMT).
Of the 1,250 flights planned for Thursday, 150 were cancelled, according to an official of Fraport, the company which owns and operates Germany's biggest airport.
Initially, the GdF air traffic workers union had planned only one day of action, until 10:00 pm (2100 GMT) on Thursday.
But it was decided to repeat the strike again from 8:00 am until 10:00 pm (0700-2100 GMT) on Friday to increase the pressure on Fraport.
The move was heavily criticised not only by Fraport itself, but also industry federations BDL and BDI.
"Our contingency plans for tomorrow are ready and we've sufficient personnel to handle more than 50 percent -- at the very least -- of flight operations," Fraport board member Peter Schmitz told reporters.
"And I'm optimistic that, following the learning curve that our employees surmounted today, we'll succeed again tomorrow," Schmitz said.
A Fraport spokesman advised passengers to contact airlines directly, since it was primarily up to them to decide which flights would take off or land.
The company had trained up other staff to take over the duties and tasks of the striking workers, spokesman Mike Schweitzer said.
"And former colleagues who used to work in this area have also said they would jump in," Schweitzer added. "We will ensure that flight safety is guaranteed."
German carrier Lufthansa, which has its main hub in Frankfurt, said it cancelled 100 flights out of a total 316 taking off or landing on Thursday, and 250 flights were expected to be cancelled on Friday.
So far, only flights within Europe had been affected, with all long-haul flights going ahead, a spokeswoman said.
Travellers seemed resigned to having their travel plans disrupted with some voicing sympathy over the strike action.