Boeing (BA) has discovered a software glitch in its 737 MAX planes that could cause navigation system failures and raise safety concerns during landings, the Wall Street Journal reported Monday, citing a document it examined.

The glitch takes place during cockpit software updates, which occur when planes change their flight paths after a missed approach and could force pilots to fly manually, the report said.

Boeing, airlines, and the Federal Aviation Administration, or FAA, are assessing the issue, the Journal reported.

Southwest Airlines (LUV) and United Airlines (UAL) have informed Boeing they don't want new planes with the flawed software and have requested an earlier software version, the report said, citing industry officials.

Boeing, Southwest, United, and the FAA did not immediately reply to MT Newswires' request for comment.

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