If you are an airplane owner, you undoubtedly already know what an AD is. Airworthiness Directives are issued by the FAA to address mechanical safety of flight issues that become evident in the field after an aircraft has been produced and delivered to the customer. The document itself is mailed to the owner of every registered aircraft that may be affected by the AD.
Sometimes an AD may require nothing more than the addition of a placard to the aircraft; other times it may require that the entire engine be torn down. Most of the time, the owner is not a mechanic and simply passes the information on to his/her AMT (aircraft maintenance technician) at annual time.