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This is more serious than people think. Aircraft design is a meticulous business and any nut or bolt that is designed to stay tightened should be designed and specified with a system to prevent it from loosening under the influence of vibration (the sworn enemy of all threaded fixings!). There are half a dozen or more ways of stopping nuts and bolts from loosening, ranging from the use of castellated nuts with a split pin through them to the use of locking compounds like Locktite. One of the primary concerns in aircraft design is the elimination of anything that could go wrong including the loosening of any sort of fixing due to vibration. This fault appears to indicate yet again that Boeing are cutting corners that shouldn't be cut!
