Air Berlin posted an operating loss of €9.3 million ($13 million) in 2010, compared with a profit of €28.5 million in 2009, figures released late Thursday showed.
Sales rose almost 15 percent to €3.72 billion but "the increase remained below expectations," the carrier said in a statement.
It pointed to a harsh winter last year in Europe, where most of its flights are located, the eruption of the Icelandic volcano in April, and strikes by Spanish air traffic controllers and by its own pilots.
Air Berlin also had to book a depreciation on derivative financial products, and its net loss grew to €97.2 million from €9.5 million in 2009.
As of end-December, the group's debt totalled €489.2 million, down 15 percent.
Air Berlin did not give a forecast for the current year, which could be difficult owing to political uncertainty in Egypt and Tunisia, two major destinations for German tourists.
Detailed results are to be released on March 24.
Shares in the carrier showed a gain of 2.72 percent to €3.17 in morning trading on the Frankfurt stock exchange, while the SDAX index on which they are listed was up by 1.39 percent.