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Service Bulletin (SB)
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A Service Bulletin is the document used by manufacturers of aircraft, their engines or their components to communicate details of modifications which can be embodied in aircraft. If an available modification is judged by the manufacturer to be a matter of safety rather than simply product improvement, then these would be issued as an Alert SB in which case a corresponding Airworthiness Directive (AD) would usually then be issued by the appropriate NAA.