The US Federal Aviation Administration has no specific timeline for the return to service of Boeing’s 737 MAX, which has been grounded after two deadly crashes, the agency’s acting administrator said on Thursday. Acting Administrator Dan Elwell, speaking to reporters at an event in Wisconsin, said the timeline would be driven by one criteria: safety. Elwell’s comments came a day after Boeing Chief Executive Dennis Muilenburg told analysts on a conference call he was confident the MAX would be back in service as early as October, after a certification flight with regulators in September. Muilenburg has also said that the timing could shift as Boeing works to win approval for reprogrammed stall-prevention software and related training materials required in the wake of crashes in Ethiopia and Indonesia that together killed nearly 350 people in the span of five months. “We don’t have a timeline. Don’t have October. Don’t have August. Don’t have 2021,” Elwell told reporters at an annual air show in Oshkosh. “We have one criteria. When the 737 MAX has been, when the complications to it have been satisfactorily assessed, and the MAX is safe to return to service, that’s the only criteria,” Elwell said.