Boeing's 737 Max aircraft, grounded since March after two fatal crashes in five months, should be back in the air by December, a top US regulator said.

It's not possible to give an exact date as work progresses on safety fixes to the aircraft, Ali Bahrami, the Federal Aviation Administration's associate administrator for aviation safety, said in an interview Wednesday at a conference in Cologne, Germany. While the FAA is "under a lot of pressure," he said the Max will be returned to service "when we believe it will be safe," following reviews of the design, flight testing and other checks.

Bahrami was reluctant to provide a timeline, but asked whether the plane would resume service this year or next, he said remarks by Boeing Chief Executive Officer Dennis Muilenburg projecting a return by the end of 2019 sounded correct.

Knowing when the latest version of the 737 will fly again would help airlines contend with the disruption caused by the grounding of the narrow-body, Boeing's most popular model. The FAA has said that there's no time frame to sign off on Boeing's proposed fix for the jet.

Muilenburg said last week on CNBC that he expected that the Max would be back in the air by year-end.

American Airlines has kept the plane off its schedule through September 3, while Southwest Airlines and United Continental Holdings are looking at resuming Max flights in early August.

Software Fix

Boeing is finalising a software fix for a flight-control system malfunction linked to the accidents involving Lion Air and Ethiopian Airlines, as well as proposed new pilot training. A combined 346 people were killed in the crashes five months apart.

The manufacturer has cut its production rate for the model by 10 planes a month to 42. The company had earlier aimed to increase output to 57 monthly in the second half of the year.

The Chicago-based planemaker faces an estimated $1.4 billion bill for canceled flights and lost operating profit at airlines if the Max is still grounded by the end of September, said Bloomberg Intelligence analyst George Ferguson.

The FAA isn't the only regulator that holds sway over returning the Max to the skies. The European Aviation Safety Agency plans to examine the entire flight-control system, including the plane's displays, alerts and air-data systems, as well as the aircraft's autopilot function.