The Federal Aviation Administration has issued a new airworthiness directive (AD) requiring inspections of more CFM International CFM56-7B engines, including ongoing inspections.

The directive applies to engines not already subject to an emergency AD the FAA issued 20 April, and it more-closely mirrors a service bulletin issued by CFM International.

The move marks continued response by the FAA to the deadly inflight failure of a CFM56-7B on a Southwest Airlines Boeing 737-700 on 17 April.

The latest order, which takes effect around 10 May, requires airlines to inspect "concave and convex sides" of some CFM56-7B fan blades before the blades accumulate 20,000 cycles, or within 113 days, the order says.

The 113-day period ends 31 August, says CFM.

Operators must then repeat inspections within 3,000 cycles since the last inspection, it says.

The directive applies to 3,716 engines installed on US-registered aircraft, it says.

Unlike the earlier emergency AD, the latest directive comes as an "immediately adopted rule", meaning the public can submit comments and the FAA could make changes.

The FAA's 20 April emergency AD required airlines to inspect within 20 days blades on some CFM56-7Bs with at least 30,000 cycles.

A related CFM service bulletin issued 20 April went further, recommending that airlines inspect engines with more than 20,000 cycles.

That bulletin also called on airlines to perform ongoing inspections on those engines every 3,000 cycles, which equates to about two years of average airline service, CFM said.

"This latest action mirrors the CFM service bulletin and the [European Aviation Safety Agency] airworthiness directive issued on April 20," says CFM in a statement.

Investigators traced the 17 April failure of a Southwest Airlines' 737-700 CFM56-7B engine to a fan blade separation. The blade apparently struck the cowling, causing shrapnel that broke a window, killing one passenger, they say.