Apron controllers at Frankfurt Airport widened their plans to strike, which could disrupt more than 1,000 flights at one of Europe's busiest airports.
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.
Fraport has trained additional staff to replace workers going on strike and hopes that at least half of scheduled flights will take place during the walkout.
Frankfurt is Europe's third-busiest after London Heathrow and Paris Charles de Gaulle, with about 1,300 flight movements per day. Lufthansa accounts for more than half of those.
GdF has said that 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.