Norwegian Air's Bjoern Kjos has today stepped down as chief executive of the airline he co-founded and turned into Europe's third-biggest budget carrier by passenger numbers.

The airline's chief financial officer Geir Karlsen will act as interim CEO while the company recruits a permanent, new CEO. 

Kjos, who is 72, will have a new role as an adviser to the chairman. 

The airline has shaken up the long-haul market with cut-price transatlantic fares.

But its rapid expansion has left it with hefty losses and high debts and it had to raise 3 billion crowns ($350m) from shareholders earlier this year. 

"I am way overdue," Kjos told a news conference, announcing his plans to quit the top job. 

A former fighter pilot, Kjos helped to expand what was a tiny Norwegian airline housed in pre-fabricated barracks on the edge of Oslo airport. 

"Bjorn has been the driving force behind the business - the big question now is whether Norwegian Air can maintain momentum as he takes a less active role," Bernstein said in a note to clients. 

The airline also said it expected its 18 grounded Boeing 737 MAX aircraft to return to service in October, compared with its previous view that they would return to service in August. 

The jets have been grounded worldwide since March following two fatal crashes and Norwegian Air has said the disruption could scupper its plans to return to profitability this year. 

The company, which was set up in 1993, today reported second-quarter earnings that beat expectations. 

Its net profit came in at 82.8 million Norwegian crowns ($9.7 million), down from 300.3 million in the same period last year, but ahead of the average forecast of 76.2 million from five analysts compiled by Refinitiv.

The company cut its target for passenger carrying capacity growth to 0-5%, compared with the previous guidance for 5-10%. 

It said earnings before interest, tax, depreciation and amortisation and restructuring or rent costs (EBITDAR), excluding "other" losses or gains for items such as foreign currency contracts and forward fuel contracts, were expected to reach 67 billion crowns in 2019.

This would be up from 3.2 billion in 2018.