An airworthiness directive is a notice by the FAA that defines an unsafe condition that commonly occurs in an aircraft type and prescribes appropriate corrective actions. Before every flight, the pilot-in-command (PIC) must ensure that aircraft is in compliance with all applicable ADs.
ADs are divided into two categories: normal and emergency issuance.
Under normal conditions, when the FAA recognizes a safety problem that may require an AD, they must issue a Notice to Proposed Rulemaking (NPRM). As part of the NPRM process, the FAA sets forth its case for why the AD is needed as well as the proposed corrective action. At this point, the rule is open for public comments. Many owners, operators, and related organizations (such as AOPA) will provide input so that the AD can be refined to best serve safety while imposing as little of a burden on owners and operators as possible. Once an AD is finalized, the FAA will provide a reasonable amount of time for all owners and operators to comply with the rule.
When the FAA recognizes the need to issue an Emergency AD (EAD), an aircraft must immediately comply with the requirements of the AD before it can fly again. Due to the emergency nature of the AD, there is no time for an NPRM process. As a result, EADs are uncommon and only reserved for situations in which there is significant risk associated with further operations of a particular type of aircraft.
In AD 2008-05-09, the FAA responded to a pattern of failures of the mechanism that locked the seat's forward and aft travel in the Cessna 172 Skyhawk, Cessna 182 Skylane, and Cessna 206 Stationair. As the locking mechanism failed during takeoff, the seat would lean to the full aft position. When this occured, the pilot could lose control and crash the aircraft. On page 2 of the AD, notice that the FAA responded to the public comments in its final rule. Of particular interest is Comment No. 3. The proposed rule required the new bracket to be installed on all seats with the failing mechanism. However, as AOPA noted, in the Cessna 206, though passenger seats 3 and 4 did have the bad part, they would not affect the safety of the aircraft if they failed. The FAA agreed with the proposed change and removed the original requirement from the final AD.
In Emergency AD 2013-02-51, the FAA responded to battery thermal run-aways that resulted in fires on two two separate Boeing 787s. The 787 uses relatively-new technology in the form of lithium-ion batteries as opposed to the long-time standard nickel-cadmium batteries. The FAA determined that the new battery technology needed to demonstrate a significant improvement of safety prior to being permitted to resume normal service in the airlines.
An extensive list of all active ADs is found on the FAA's website. In the interest of safety, knowledge, and legality, it is prudent for a pilot to be familiar with all ADs associated with any aircraft they own or operate. By being aware of the ADs associated with an aircraft, a pilot is able to increase their knowledge of the aircraft's systems and mitigate risk factors associated with their operation.