NASA , Boeing and Lockheed engineers determined that the probable cause for the accident was in-flight separation of the right wing while cruising at due to flutter caused by unexplained reduced stiffness of the engine mounts . This was subsequently defined as " whirl mode . " Six months earlier , a Braniff International Airways L-188 Electra , Flight 542 , disintegrated over Buffalo , Texas at , killing all on board . This second similar crash moved the Federal Aviation Administration to immediately issue a reduced cruise speed directive while investigators tried to determine the cause of the fatal crashes
Flight 542
Braniff Flight 542
Northwest Orient Airlines Flight 710