Air Berlin filed for bankruptcy protection last month after its biggest shareholder, Etihad Airways, withdrew funding following years of losses.
Parties interested in buying its assets have until Friday to submit binding offers.
On Tuesday, about 100 flights were cancelled after 200 pilots called in sick.
Some short-haul flights at Lufthansa's budget airline Eurowings were also hit because it leases 33 planes with crews from Air Berlin.
About 150 pilots called in sick on Wednesday, Air Berlin said.
The airline has said the cancellations threatened its existence and could force it to shut down, jeopardising negotiations with potential investors.
Germany's biggest airline, Lufthansa, is seen in pole position to acquire large parts of its rival and a decision on the bids come could as early as Sept. 21, three days before a national election.
Thomas Cook's German carrier Condor is preparing a bid for parts of Air Berlin, another source said, while media reports said easyJet may want up to 40 planes.
Aviation investor Hans Rudolf Woehrl said he had submitted a bid for the whole of Air Berlin while German family-owned logistics company Zeitfracht has also expressed interest.
Air Berlin has also attracted buyer interest from China's LinkGlobal Logistics. German newspaper Bild reported on Wednesday.
© Thomson Reuters 2017
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