Compared to many CEOs, Apple boss Tim Cook keeps a low profile. But in 2017 he hit a milestone that may be the mark of true celebrity.

Apple's board implemented a policy in 2017 requiring Cook to "use private aircraft for all business and personal travel" for his safety, according to regulatory filings published on Wednesday.

It's quite common for a CEO to travel in private jets — GE's past CEO, Jeff Immelt, apparently had . But even as the leader of the world's most profitable company, Cook has held out until this year, as last year's proxy filing doesn't mention any private flights, just the occasional chartered plane.