* Air Berlin says around 150 pilots call in sick
* Cancels 32 flights on Wednesday, after 100 on Tuesday
* Bids for Air Berlin assets due on Friday (Adds union comments, call for staff to return to work)
FRANKFURT, Sept 13 (Reuters) - Air Berlin was forced to cancel flights for a second day on Wednesday after pilots again called in sick in unusually high numbers, potentially complicating efforts to rescue the insolvent carrier.
Air Berlin, Germany's second-biggest airline, is set to be carved up, most likely among several buyers, with binding offers due this Friday.
The airline filed for bankruptcy protection last month after its biggest shareholder, Etihad Airways, withdrew funding following years of losses.
However, those losses are mounting as more plans are grounded. Air Berlin said it had cancelled 32 flights on Wednesday after around 150 pilots called in sick. On Tuesday, Air Berlin had scrapped about 100 flights.
Some short-haul flights at Lufthansa's budget airline Eurowings are also affected because it leases 33 planes with crews from Air Berlin.
Air Berlin has already said the cancellations threatened its existence and could force it to shut down, jeopardising negotiations with potential investors and costing it several million euros a day.
Both management and unions have called on staff to return to work to keep the business going so that talks can succeed.
LUFTHANSA LOOMS LARGE
Air Berlin said in an internal memo that after some pilots returned to work on Wednesday, it hoped for stable operations on Thursday.
A 150 million euro ($180 million) government loan is helping to keep it going. Should the airline be grounded, its take-off and landing slots will be divided up among rivals and staff will have to apply for jobs on new contracts.
German airline crews have in the past used sick leave to demonstrate concerns over job security and conditions, such as at TUIfly last year.
Pilots' union Vereinigung Cockpit (VC) said it did not ask staff to call in sick and was surprised by the absences. Representative Markus Wahl said it was understandable members were worried because management had refused to talk about conditions for taking pilots into any new company.
Cancellation of long-haul routes after a lessor asked for planes to be returned is adding to uncertainty, he added.
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.
One source has told Reuters that Lufthansa is interested in as many as 90 of Air Berlin's planes. That number includes the 33 being used by Eurowings, five already leased to Lufthansa's Austrian Airlines as well as planes used by Air Berlin subsidiary Niki, the source said.
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. ($1 = 0.8346 euros) (Reporting by Sabine Wollrab and Klaus Lauer; Writing by Maria Sheahan and Victoria Bryan; editing by Jason Neely/Keith Weir)