Strikes by public sector workers have caused most February 27 flights in and out of Cologne-Bonn and Dusseldorf airports to be cancelled or diverted.
"Large parts of the airport staff are taking part," according to a Cologne-Bonn statement.
Carrier Eurowings echoed Cologne-Bonn's warning of "massive disruptions" due to the strikes, which have also affected public transport day-care facilities and local administration in the region.
Flights are expected to return to normal by Tuesday after the strike, which was called by the Ver.di union, comes to an end.