The Federal Aviation Administration released a directive Thursday requiring airlines to do regular maintenance checks of the flight-control software on Boeing’s 737 MAX and to periodically test the operation of cutoff switches the pilots use if system failures occur.

All the MAXs currently flying had these checks done before the planes returned to the sky after a 21-month grounding of the worldwide fleet.

Boeing sent details of the maintenance regime to all airline operators of the MAX in December, just before the first planes re-entered service.

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The FAA said in a statement that all MAX operators in the U.S. have already included these inspections in their maintenance programs and that it issued the directive “to highlight the importance of these inspections to other international regulators and to operators outside the United States.”

Boeing said it “fully supports” the directive, which affects approximately 72 MAXs registered in the U.S. and 389 of the jets worldwide.

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New flight-control software on the MAX that wasn’t on previous 737 models — known as the Maneuvering Characteristics Augmentation System (MCAS) — activated erroneously on the two fatal crash flights in 2018 and 2019 that killed 346 people in Indonesia and in Ethiopia.

Boeing updated that software to fix its flawed design and the FAA recertified it in November, approving the MAX for its return to service.

The new FAA directive requires a check of the entire flight-control software system every 6,000 flight hours, which could be about every two years for a jet that’s used intensively on domestic routes.

The system test is straightforward and can be scheduled during routine maintenance. It’s conducted using built-in electronic test equipment on the airplane.

An FAA official explained that it checks the health of the entire automated flight control system, “including MCAS.”

The directive states that regular checks are deemed necessary because of the possibility of a “latent failure” of some element of the flight-control system — meaning a failure that might not be immediately apparent but could manifest later.

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The FAA official said the directive demands relatively frequent inspections because “if it’s potentially latent, you go conservative.”

The directive also requires airline maintenance staff to check that the switches on the pilot-control console that cut electrical power to the moving surfaces on the horizontal tail are functioning properly.

If MCAS malfunctioned, the pilots would use this pair of cutoff switches to stop the system from pushing the airplane’s nose down.

A third newly mandated maintenance check requires technicians to periodically check an autopilot cutoff switch as well as the electrical ground path that controls the moving part of the horizontal tail — known as the stabilizer — and therefore the pitch of the airplane.