The Irish budget carrier slammed the demands as "unacceptable" and "unnecessary."
Thousands of Ryanair passengers have been affected by the walkout, which started at 03:00 am (0100 UTC). Germany's Cockpit pilots' federation and the Verdi service workers' union said their talks with the Ryanair management had failed to break the impasse.
Last month, Ryanair pilots in five European countries, including Germany, held their first-ever simultaneous walkout, causing some 400 flight cancellations and travel chaos for 55,000 passengers.
Union leaders are expected to announce Thursday details of another coordinated walkout by Ryanair cabin crew in Belgium, Italy, the Netherlands, Portugal and Spain. They have vowed to stage "the biggest strike action the company has ever seen."
VC has been demanding better pay and working conditions for Ryanair workers for months.
The Irish airline insists the German pilots' dispute should be settled by an Irish mediator, but VC maintains he or she "could not possibly have enough knowledge of German law and therefore already is not qualified to act as an arbitrator in this case."
The union has called the strike from Wednesday, September 12, 3:01 a.m. (0101 UTC) to Thursday, September 13, 2:59 a.m. (0059 UTC).
Ryanair is currently experiencing the worst wave of strikes in its history. While the low-cost carrier has begun recognizing pilot and cabin crew unions across Europe for the first time, workers are unhappy that improvements have yet to be made on pay and other conditions.
"It is unacceptable that a union representing Ryanair's German pilots, who earn up to 190,000 euros ($220,000) a year and work a five-day week followed by a four-day weekend, is now threatening customers travel plans at short notice," said Jacobs.
The majority of Ryanair pilots based in Germany are direct employees, unlike the situation in other Ryanair markets, and in theory, a widely implemented strike of German-based pilots could significantly affect the airline's operations.
Ryanair said it would would be forced to cancel 150 out of 400 scheduled flights to and from Germany on Wednesday, but it would try to minimise the disruption for customers.
Separately, pilots unions across Europe on Friday threatened "the biggest strike action the company has ever seen" over their members' working conditions. The strike, so far organised by unions in Belgium, Holland, Italy and Spain, is scheduled for the last week in September. The walk-out could be called off should an annual general meeting of Ryanair shareholders on September 20 meet union demands. Ryanair has banned the media from the AGM and there will be no news briefing afterwards, according to a statement cited by Irish public broadcaster RTE.