The ailing carrier was forced to cancel more than 100 flights Tuesday, causing chaos at several German airports, after 200 pilots called in sick at short notice.
As Air Berlin teeters on the brink of collapse, thousands of its passengers have had their travel plans wrecked by a "sick-out" involving flight crew.
Air Berlin continues to operate due to a 150 million euro ($180 milliom) credit loan from the German government.
Air Berlin has been forced to cancel more than 110 flights today after pilots called in sick.
Air Berlin filed for bankruptcy protection last month after its biggest shareholder, Etihad Airways, withdrew funding following years of losses.
Bidders have until September 15 to submit offers.
Lufthansa's low-priced division Eurowings - which is wet-leasing Airbus A320-family jets from Air Berlin - has been affected.
The flight cancellations represent a "massive threat" to Air Berlin's insolvency proceedings, in the view of the carrier's court-appointed executive director Frank Kebekus.
The carrier said Monday it will cancel numerous routes to the Caribbean and some United States flights from September 25, and only customers who bought tickets after the company filed for insolvency on August 15 will be reimbursed.
"I think Air Berlin is in a decidedly hard situation at the moment and the pilots, with their behaviour, are putting at risk a reasonable handover or sale". The allowance may reportedly keep the carrier afloat until mid-November with its German jobs put at risk. At the end of the process, Air Berlin is expected to cease operating and exit the market.
One source has told Reuters that Lufthansa is interested in as many as 90 of Air Berlin's planes.
Vereinigung Cockpit, a collective bargaining group for German pilots and flight engineers, has expressed concern that the airline is planning to offload its long-haul flights branch, which pays staff higher wages.
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