In an email to Fortune, Southwest confirmed that overall there were about 40 cancellations out of nearly 4,000 flights on Sunday.

Southwest also explained that the cancellations were due to their “voluntary, accelerated engine fan blade inspection program” announced on Tuesday, and not because of the Federal Aviation Administration orders issued on Friday night.

The emergency directive applies to “owners and operators of CFM International S.A. (CFM) Model CFM56-7B engines,” and includes engines that have been flown 30,000 or more flight cycles. The order stipulates that effective immediately, operators have 20 days to “perform a one-time ultrasonic inspection (USI) of all 24 fan blade dovetail concave and convex sides to detect cracking.” In a service bulletin issued on Friday, CMF also recommended that engines with 20,000 flight cycles be inspected by August.

CFM International is a joint venture between GE and the French company Safran Aircraft Engines, and CFM56-7B engines are among the most common commercial aircraft engines in the world. It is estimated that in the U.S. 352 engines will be impacted by the FAA’s directive. The European Aviation Safety Agency is also adopting a similar policy. There are 681 engines that could be impacted worldwide.

Some 8,000 Boeing 737s – including nearly all of Southwest’s fleet – fly CFM56-7B engines.

This FAA directive comes after Tuesday’s deadly engine explosion on a Southwest Airlines flight from New York to Dallas. The pilot had to make an emergency landing in Philadelphia after the plane experienced “engine failure due to a fractured fan blade, resulting in the engine inlet cowl disintegrating,” according to the FAA.

Jennifer Riordan, an executive at Wells Fargo and mother of two, was partially sucked out of the plane after debris from the explosion cracked the fuselage, she later died in a hospital in Philadelphia.