FAA administrator Mike Whitaker said the agency is requiring immediate inspections of certain planes before they can return to flight. The order impacts 171 airplanes worldwide.

The emergency airworthiness directive will require operators to inspect aircraft before further flight that do not meet the inspection cycles. Required inspections will take around four to eight hours per aircraft.

United Airlines also operates the 737-9 MAX and did not immediately comment.

Alaska Airlines said early Saturday it had voluntarily and temporarily grounded its fleet of 65 737-9 MAX airplanes following the incident. It has resumed operations using about a quarter of the planes following inspections that turned up no concerning findings.

It is not clear if the FAA directive will mirror the Alaska inspections. Alaska cancelled about 100 flights on Saturday, or 13% of scheduled operations, according to FlightAware.

Association of Flight Attendants-CWA (AFA) president Sara Nelson said the union “supports the FAA’s quick and decisive action to ground certain 737-9 MAX Fleet that do not meet the inspection cycles specified in the Emergency Airworthiness Directive. This is a critical move to ensure the safety of all crew and passengers, as well as confidence in aviation safety.”

Senator JD Vance, a Republican on the committee that overseas the FAA, said “the FAA has assured me the 737-MAX is safe – last night’s near catastrophe calls that determination into question.”

He added every American deserves a full explanation from Boeing and the FAA on what has gone wrong and on the steps that are being taken to ensure another incident does not occur in the future.