Boeing and the Federal Aviation Administration issued directives last week telling flight crews about the system, which is designed to provide extra protection against pilots losing control. That prompted aviators, unions and training departments to realize that none of the documentation for the Max aircraft included an explanation of the system, the union leaders said. “We don’t like that we weren’t notified,” Few details have been released about the underlying causes of the Lion Air crash Oct. 29 in the sea near Jakarta, but Indonesian investigators say that an erroneous sensor prompted the plane’s computers to push the aircraft into...