The Federal Aviation Administration (FAA) has discovered a new flaw in Boeing's 737 Max that could further delay its return to the skies. All 737 Max models were grounded after two crashes overseas that killed 346 people.

During flight tests in a Boeing 737 Max simulator in Seattle, FAA test pilots discovered a computer issue that could slow down a pilot's ability to quickly respond to a scenario like the ones seen in the two Max crashes.

The issue occurred when the pilots were evaluating failure scenarios where the nose of the plane was being pushed down. The test pilots felt it remained down longer than it should, leaving them concerned it would increase the chances of a crash.

Boeing is now working on a second software fix to address this issue — if that fails, the component would have to be replaced on all Max airplanes likely extending the global grounding.

"We will spend time understanding every dimension of these accidents and making every possible improvement we can make as a result," Boeing CEO Dennis Muilenburg said.

The FAA will not schedule the required airborne test flight of the software updates until this issue is resolved. United Airlines joined American and Southwest on Wednesday in removing the 737 Max from its flight schedule until at least September 3.