British Aerospace was quite correct in its interpretation of what an Airworthiness Directive (AD) is. ADs are only ever issued by the aviation safety regulator and these essentially make a manufacturer service bulletin mandatory. ADs are issued where the regulator considers that an unsafe condition exists, is likely to exist or is likely to develop.
http://www.theairlinepilots.com/medical/aircontaminationanda...

An airworthiness directive (commonly abbreviated as AD) is a notification to owners and operators of certified aircraft that a known safety deficiency with a particular model of aircraft, engine, avionics or other system exists and must be corrected.
http://en.wikipedia.org/wiki/Airworthiness_Directive

Dyrektywa Zdatności (Airworthiness Directive, AD) jest dokumentem wydawanym przez...
Bardzo ważnymi elementami są biuletyny obsługowe (Service Bulletin) oraz dyrektywy zdatności (Airworthiness Directive). Oba te dokumenty są integralnymi...
http://tinyurl.com/c37kehm