The FAA has issued another safety inspection directive in the aftermath of last month's fatality on a Southwest flight.

Under the order, to be published in the Federal Register on Wednesday, airlines will be required to perform ultrasonic tests by the second half of August on all CFM56-7B fan blades that have been used at least 20,000 times. Further tests will be required after each additional 3,000 flights. The order impacts an estimated 3,700 aircraft registered in the U.S., the FAA said.

The directive complements the April 20 emergency FAA airworthiness directive requiring all airlines to inspect fan blades on CFM56-7B engines with more than 30,000 uses within 20 days.

The CFM56 is made by CFM International, a joint venture between GE Aviation and Safran Aircraft Engines.

On April 17, an engine blade on Southwest flight 1380 broke due to what the National Transportation Safety Board believes was metal fatigue, sending shrapnel into the plane and breaking a window. Passenger Jennifer Riordan died of head trauma as she was partially sucked out that window.

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