The Federal Aviation Administration issued an order Wednesday that paved the way for the troubled Boeing 737 Max to carry passengers again, ending the jet's 20-month grounding.

The plane, the best-selling jet in Boeing's (BA) fleet, was grounded in March 2019 after two fatal crashes that killed 346 people. It was discovered that a safety feature meant to stop the plane from climbing too fast and stalling had improperly forced the nose of the plane down, causing the crashes.

The process of approving the plane to carry passengers has stretched on far longer than originally expected and cost Boeing more than $20 billion, according to the company. Lost orders for the jet during that time could make it among the most expensive mistakes ever made by a company.

Although approval was expected this week, Boeing (BA) shares nonetheless rose 6% in premarket trading Wednesday.