In the Netherlands, the Dutch Airline Pilots Association said Ryanair pilots based there would go also on strike Friday.
"This European pilot strike should be a wake up call for the Ryanair management," it said.
The strike call followed a move by Ryanair to block a walkout by Dutch pilots with a court near Amsterdam's Schiphol due to consider an injunction on Thursday.
The walkout by German and Dutch pilots piles pressure on the Irish budget airline after pilots in Ireland, Sweden and Belgium also vowed to strike on August 10, promising major disruptions for thousands of holidaymakers.
Ryanair said before the announcement of the Dutch strike it would have to axe around 400 out of 2,400 European flights scheduled for Friday, affecting some 55,000 passengers.
Germany will be worst hit with 250 flight cancellations.
Europe's second biggest airline has been grappling with staff unrest since it recognised trade unions for the first time in December 2017, in a bid to ward off widespread strikes over the Christmas period.
But unions say little progress has been made on their demands for better wages and fairer contracts despite months of talks.
Germany's Cockpit union said Ryanair management had failed to respond to this week's deadline for an improved offer, leaving them with no choice but to strike. "There can be no improvements without increasing staff costs," Locher said, but added that this has been "categorically" rejected by Ryanair.
"At the same time, Ryanair has at no stage signalled where there might be leeway to find solutions." But Ryanair's chief marketing officer Kenny Jacobs accused the German union of calling "an unjustified and unnecessary strike", and urged Cockpit to return to the negotiating table to discuss a revised offer the airline proposed last week. Speaking at a Frankfurt press conference, Jacobs said German pilots enjoyed "excellent working conditions" with an average salary of around 150,000 euros ($173,000) a year, more than their peers at low-cost rival Eurowings.
He added that Ryanair had already committed to a 20-percent pay increase, and that 80 percent of its pilots in Germany were now on permanent contracts. Passengers caught up in Friday's stoppages will be contacted by email or text message later for information about refunds and alternative routes, Jacobs said.
The German strike announcement had been widely expected and investors appeared sanguine about the upcoming disruptions, sending Ryanair shares climbing 1.2 percent in London.
Founded 33 years ago in Dublin, Ryanair boasts lower costs per passenger than its competitors.
But Ryanair pilots have long claimed that they earn less than their counterparts at budget airlines like EasyJet.
Another key complaint of workers based in countries other than Ireland is the fact that Ryanair employs them under Irish legislation. Staff say this creates huge insecurity for them, blocking their access to state benefits in their country.
Unions also want the airline to give contractors the same work conditions as staff employees.
Ryanair, which flies in 37 countries and carried 130 million passengers last year, has repeatedly said it remains open to further talks with pilot representatives. But its combative chief executive Michael O'Leary has also warned the airline may shift jobs and planes to more profitable areas if the turmoil continues.
The carrier was already hit by a round of strikes last month disrupting 600 flights in Belgium, Ireland, Italy, Portugal and Spain, affecting 100,000 travellers. After those stoppages Ryanair threatened to move part of its Dublin fleet to Poland, which could cost 300 jobs, including 100 pilot positions.
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