FRANKFURT AM MAIN — Insolvent Air Berlin has cancelled dozens more flights as pilots again called in sick, despite warnings from the airline that the wildcat action could jeopardise rescue talks.

The mass "sick-out" comes ahead of a Friday deadline for potential investors to submit bids for Air Berlin assets.

"More than 30 flights" had to be scrapped early on Wednesday as some 150 pilots handed in sick notices for a second day, an Air Berlin spokeswoman said.

Duesseldorf and Berlin-Tegel airports were worst hit by the cancellations, she said, advising affected passengers "not to come to the airport".

Thousands of travellers were already left stranded on Tuesday when some 200 of Air Berlin’s 1,500 pilots suddenly called in sick, forcing the cancellation of around 100 flights.

Air Berlin has accused the absent pilots of "threatening the existence" of the airline, warning that the turmoil could scare off investors.

Lufthansa’s low-cost subsidiary Eurowings, which leases Air Berlin aircraft and crew, was also affected for a second day.

Air Berlin filed for insolvency in mid-August, after its main shareholder, Gulf carrier Etihad Airways, unexpectedly pulled the plug on its cash lifeline.

The airline has long struggled for survival, and booked losses amounting to 1.2 billion euros (US$1.4 billion) over the past two years.

Germany’s giant services sector union Verdi on Tuesday expressed solidarity with the absent pilots.

"All the conversations surrounding insolvent Air Berlin are always about its economic interests, never about the jobs of its more than 8,000 employees," said Verdi board member Christine Behle.

Meanwhile, German Transport Minister Alexander Dobrindt urged the pilots to end their "risky manoeuvre" and return to work for the sake of Air Berlin’s roughly 8,000 employees.

"I can only appeal to everyone to come to their senses and let the flights take place. This is necessary to make a transition to new owners possible," he said at a press conference.

German flagship carrier Lufthansa -- which already leases 38 of Air Berlin’s 140 planes -- is seen as the favourite to take over the bulk of the stricken carrier’s assets.

Other interested suitors cited in media reports include package holiday firm TUI, British low-cost carrier EasyJet and Thomas Cook subsidiary Condor, as well as Bavarian entrepreneur Hans Rudolf Woehrl.

China’s LinkGlobal, which operates Parchim airport in northern Germany, also plans to join the scramble for Air Berlin assets, Bild reported on Wednesday.

Late Wednesday, Austrian former Formula One champion Niki Lauda was quoted as saying that he too would bid for parts of Air Berlin as well as its Austrian subsidiary Niki, which he founded.

"I am going to make a bid together with Thomas Cook and Condor," Lauda, 68, told the Austrian daily Kurier in an article to be published on Thursday. He will own 51 per cent in the consortium, he said. — AFP