Aircraft which have been in service for a long time often have a lot of entries in their logbooks for Airworthiness Directives (ADs). What are they, and why are they important?
Airworthiness Directives (ADs) are legally enforceable rules issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. 14 CFR part 39 defines a product as an aircraft, aircraft engine, propeller, or appliance.
When an aircraft or other component is found to have flaws or other issues, the FAA will issue an AD specifying a solution to be applied to the item. The AD must be complied with, often by a date or total time on the airframe. The action required could simply be an inspection (often recurring after, say, 100 hours of time in service), while in other cases requires modification, strengthening, or replacement of parts.
One of the more prominent cases requiring ADs to be issued was the V-tail Bonanza, which had issues with the tail's structural integrity under high loads.
The cost of complying with an AD falls on the owner/operator of the aircraft. Failing to comply with an AD renders the aircraft unairworthy. It also could incur FAA penalties, but more importantly, it could result in structural damage or failure, and in some cases ultimately cause a fatal accident.
An Airworthiness Directive is a legal notice, issued by either the manufacturer and/or the FAA, alerting an owner to an issue with the aircraft or system which presents a hazard to flight safety as well as instructions on how to remedy this problem. It's similar to a recall notice on an automobile with the added mandate by the government that the aircraft is not airworthy until the AD is complied with.
It is very important that the owner retain records in the aircraft's maintenance logbooks that affective ADs have been complied with by an authorized AMT. This is one reason that logbooks are so valuable to a prospective buyer and a big red flag and loss of value to a potential seller if they are missing, as any new owner would then have to spend their own time and money to ensure that the airframe complies with all affective ADs and would not be considered airworthy by the FAA until this was accomplished.