Air-traffic controllers at Europe\'s third busiest hub in Frankfurt, who are due to stage a walkout over pay later on Thursday, will extend their strike to Friday, their union said. Some 200 personnel -- including workers in the actual air traffic control centre, as well as those working on the airport tarmac -- are already stopping work from 14:00-2100 GMT on Thursday. But the GdF union of air traffic controllers said the walkout was not having a big enough effect and a further warning strike would therefore be held again from 0700-2100 GMT on Friday. Frankfurt is Germany\'s biggest airport and the third-busiest in Europe and so industrial action there could potentially cause severe disruptions. The dispute concerns the airport\'s so-called \"apron control\" staff -- in charge of directing aircraft in and out of their parking positions -- who decided to take limited industrial action after Fraport, the company which owns and operates the airport, turned down mediation proposals in their wage dispute. The union argues that apron controllers\' pay does not take into account the additional complexity resulting from the opening late last year of a fourth runway in Frankfurt. Fraport has denounced the action as \"irresponsible\" and \"incomprehensible\", but pledged to limit disruption as much as possible.