After 20 months of being grounded due to serious safety issues, Boeing’s 737 Max has received the all-clear from the Federal Aviation Administration (FAA) to carry passengers again.

The aircraft was grounded in March 2019 after two crashes that killed 346 people due to a malfunctioning safety feature. Since fixing the issue, Boeing has undergone an extensive (and expensive) approval process to get the plane in the air again.

The FAA order only covers United States domestic flights operated by American, United, and Southwest airlines, i.e. 72 aircrafts. Aircraft to or within other countries — 315 planes in total — require the approval of those nations’ aviation authorities, and could still take longer. Even in the US, individual planes must still be inspected by the FAA before being allowed to fly, and pilots are required to undergo additional training.

Right now, only American Airlines has reintroduced the 737 Max to its flight schedule, using it for several flights between Miami and New York in late December and January. Southwest isn’t expected to fly the 737 Max until spring, while United may start using it in the first quarter of 2021.