US regulators have called on Boeing to redesign the engine covering on its 737 NG planes after a fatal 2018 accident, in which a woman died after being nearly sucked out of a window.
The National Transportation Safety Board (NTSB) also said that existing planes should be updated with the changes.
The recommendations came at the end of the board's investigation of the deadly Southwest Airlines incident.
Boeing said it was making enhancements.
The death of Jennifer Riordan of New Mexico, a 43-year-old Wells Fargo vice president and mother of two, in April 2018 was the first on a US passenger airline since 2009.
She was killed after an engine exploded and shattered a plane window.
In its final report on the accident, the safety board said that a fan blade in the engine was cracked due to "fatigue". It punctured its case, creating fragments that crashed into the window.
The incident, which caused injuries in eight other passengers and led to an emergency landing in Philadelphia, followed an earlier engine fan issue - known as a "fan blade out" event on a Southwest 737-700 flight.
NTSB chair Robert Sumwalt said the refit of the fan cowl structure could be expensive but was necessary to improve safety in the event of a fan blade issue.
"These recommendations show the way toward greater safety even when a fan blade out event occurs," said NTSB chair Robert Sumwalt.
Boeing has delivered about 7,000 737 NG planes to customers. It is an earlier generation of planes than the Boeing Max series, which has been grounded after deadly crashes in Ethiopia and Indonesia.
The safety board did not call for 737 NG planes to be grounded. Airlines are now inspecting fan blades on a more regular basis, it added.
Engine manufacturer CFM International is a transatlantic joint venture between General Electric Co and France's Safran SA.