The Federal Aviation Administration (FAA) on Wednesday issued a directive for Boeing requiring operators of 737 MAX airplanes to conduct additional inspections for the plane's automated flight control system. 

The directive makes mandatory instructions released by Boeing in December that recommend planes with more than 6,000 flight hours be subject to specific electronic checks. MCAS, an automated flight control system on the 737 MAX, was tied to two fatal 737 MAX crashes that led to the plane's 20-month grounding that was lifted in November. 

The three repetitive inspections are to be done during existing maintenance programs, the FAA said. 

The FAA also issued a notice on Wednesday called a Continued Airworthiness Notification to the International Community "to highlight the importance of these inspections to other international regulators and to operators outside the United States." The directive impacts about 72 U.S.-registered airplanes and 389 airplanes worldwide, the FAA said.

Boeing did not immediately comment. 

The FAA said the directive is necessary because a "potential latent failure of a flight control system function" if combined with "unusual flight maneuvers or with another flight control system failure" could result in reduced controllability of the airplane. 

The FAA said all operators of U.S.-registered 737 MAX airplanes have already included these inspections in their maintenance programs. 

The 737 MAX was grounded in March 2019 worldwide after two fatal crashes in five months killed 346 people. The grounding was not lifted until November 2020 by the FAA after Boeing made significant safety upgrades and improvements in pilot training as well as adding new safeguards to MCAS.