At least one TIE occurred at Southwest Airlines, the largest U.S. operator of 737NG aircraft. According to a memo issued to crews in January, one crew experienced a TIE during cruise flight and was able to restart the engine after its performance stabilized at sub-idle speed as they shut it down.

Boeing says it has made a software change to the CFM56-7B electronic engine control module, part of the full authority digital engine controller, to lessen the chances of the problem occurring.

“We are doing a root cause analysis though because it’s not just [the] fuel control unit,” says Boeing. “We are working with CFM and with the fuel supply chain to understand the issue and build a fix. It might be fuel-based. It’s one of the areas we are looking at.”

Boeing says there have not been any new incidents since the software change.