A new walkout by Lufthansa pilots on Tuesday forced the cancellation of 50 flights in Frankfurt, Germany’s busiest airport, affecting roughly 20,000 passengers, the airline said.

Flights in and out of the hub were being affected but a company spokesman said the situation was calm and that Lufthansa was confident it would operate all 32 flights it had promised would take off.

In its fourth labour action in recent weeks, the pilots’ union Vereinigung Cockpit called on its members to strike on long-haul flights departing from the airport between 8:00 am and 11:00 pm (0600-2100 GMT).

Affected were flights operated on Airbus A380, A330 and A340 aircraft, as well as Boeing 747 jets.

“The situation in Frankfurt is calm. It seems like a normal day,” the spokesman told AFP.

Lufthansa’s pilots are striking over changes to their retirement benefits being introdcued as part of deep restructuring in recent years to bring down the German carrier’s costs.

Pilots can currently take paid early retirement from the age of 55. They are fighting a plan by the airline to raise the minimum age and to involve pilots in the financing of their pensions.

Tuesday’s work stoppage was the fourth by the pilots’ union in recent series of bottom-line-hitting operations.