Speaking ahead of the Paris Air Show, chief executive Dennis Muilenburg said Boeing’s communications were “not consistent”, The Guardian reports. He added the company had not been sufficiently open with aviation regulators and its airline customers.

The 737 Max was grounded in March following two fatal crashes in just five months.

In October, Lion Air flight 610 came down in the Java Sea shortly after taking off from Jakarta. All 189 people onboard were killed. Then in March, Ethiopian Airlines flight 302 crashed en route to Nairobi. All 157 people onboard died.

Boeing is working on a software fix for a deep-seated control system, understood to have been active when both crashes occurred. It is also planning new pilot training.

However, a number of airlines have now further pushed back plans to reintroduce the 737 Max so far as September. Neither Boeing nor the US Federal Aviation Administration has yet set a date for the 737 Max to return to service.

A number of European airlines operate the aircraft, most notably Norwegian and Tui. Tui says if the 737 Max is not able to return to service this summer, it will dent its balance sheet to the tune of approximately €300 million, around 25% of its forecasted full-year profit.

Muilenburg did, however, state he expected the 737 Max to return to service this year.