Boeing Co.’s 737 Max aircraft, grounded since March after two fatal crashes in five months, will be back in the air by December, according to a top Federal Aviation Administration safety official.

It’s not possible to give an exact date as work progresses on safety fixes to the aircraft, Ali Bahrami, the U.S. regulator’s associate administrator for aviation safety, said Wednesday in interview at an aviation safety 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.”

Knowing when the aircraft may return to service would help airlines contend with the disruption caused by the grounding of the narrow-body Max, Boeing’s most popular model. The FAA has said that there’s no time frame to sign off on Boeing’s proposed software fix for the jet.