The National Transportation Safety Board urged Boeing on Tuesday to redesign the engine casing on some of its planes after a mid-air engine explosion caused debris to blow out a window, killed a passenger and forced an emergency landing in Philadelphia. The April 2018 incident on board Southwest Airlines Flight 1380 involved a Boeing 737 New Generation jet. Thousands are in service across the U.S. and some 6,800 are in service worldwide.

NTSB noted the probable cause of the accident was a cracked fan blade impacting the fan casing at a critical location, turning fragments of the engine into deadly shrapnel that struck a window, causing it to blow out. The cabin rapidly depressurized, triggering a rapid descent from 32,000 feet, with one passenger getting partially sucked out the blown out window. That passenger later died.

NTSB issued a new recommendation to the Federal Aviation Administration to require Boeing to determine which areas of the fan case of certain planes would be most vulnerable in the event of a fan blade becoming loose and require a redesign of the housing for stronger structural integrity. They also recommended airlines be required to retrofit existing Boeing 737 NG planes with the redesign.

Boeing began manufacturing the 737 NG in the early 1990s, but the redesign requirement would not be applied to Boeing's subsequent model — the 737 Max — as it has a different engine and casing. The 737 Max is currently grounded following two crashes in October 2018 and March 2019.

If the FAA were to accept the NTSB's recommendations, it would likely be a lengthy and phased process. Boeing would need to determine design changes, test the changes and the FAA would have to then certify them. Airlines are typically given given a window of time within which they need to adopt new regulations — in this case swapping out the engine housings. It is unlikely this would result in any grounding of the planes. NTSB recommendations are nonbinding and it is not clear if the FAA will follow through.

Through its investigation, NTSB did not find fault with the Southwest Airlines flight crew, air worthiness of the plane before the engine failure or the plane's maintenance.