Trade union GdF said on Thursday it was calling for a second day of walkouts, asking about 200 apron controllers to walk off the job for 14 hours on Friday in addition to a seven-hour strike planned from 1400 GMT on Thursday.

The move comes after the union failed to reach a wage agreement for the workers with airport operator Fraport (FRAG.DE).

Fraport has trained additional staff to replace striking workers and hopes that at least half of scheduled flights will take place during the walkout.

European air traffic agency Eurocontrol said it currently expected capacity to be reduced by a third on Thursday afternoon.

"If the strike starts up again tomorrow, the situation will be as bad," head of network operations unit Brian Flynn told Reuters.

Frankfurt airport is Europe's third-busiest after London-Heathrow and Paris Charles de Gaulle, with about 1,300 flight movements per day, more than half of them by German flagship carrier Lufthansa (LHAG.DE).

Lufthansa has cancelled 89 flights to and from Frankfurt on Thursday, all of them within Europe, meaning long-haul passengers should escape severe disruption.

Others, such as Air Berlin (AB1.DE), started making plans to reroute planes to Cologne/Bonn, about 140 kilometres away from Frankfurt, or to Hahn, 100 km from the city, which said it could accept both passengers and freight.

German train stations were meanwhile gearing up for an expected influx of passengers by bringing in extra staff, rail operator Deutsche Bahn said.

GdF has said apron controllers' pay needed to reflect extra complexity resulting from the recent opening of a fourth runway at the airport. Fraport, meanwhile, has said GdF's demands are too high.

The company has so far not taken any legal steps against the planned strike but reserves the right to do so, a spokesman for Fraport said.

Courts have previously helped fend off aviation-related strikes in Germany.

Last year, the GdF union and Germany's air safety authority DFS reached a deal in court averting a strike by air traffic controllers that would have disrupted thousands of flights across Europe.

During that bitter dispute, the German transport ministry stepped in to encourage the parties back to the negotiating table. The ministry declined to comment on the latest row on Thursday.

In 2010, pilots at Lufthansa and the country's No.2 carrier Air Berlin were forced by judges to call off or curtail strikes.

(Reporting by Peter Maushagen, Sabine Wollrab, Victoria Bryan and Markus Wacket; Writing by Maria Sheahan; Editing by Hans-Juergen Peters and Will Waterman)

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