AD / SB (airworthiness directive / service bulletin)
An airworthiness directive is a legally binding order from a regulator such as the FAA or EASA requiring an inspection, modification or repair to correct an unsafe condition on a particular aircraft or engine type, with a deadline. A service bulletin is a manufacturer's recommendation for an improvement or fix, which is optional unless a regulator adopts it as an AD or an operator's programme mandates it. For buyers, the status of ADs and major SBs is a core part of the records review: an aircraft with outstanding ADs cannot legally fly, and one behind on important SBs may need expensive catch-up work before it can go on a charter certificate.