Airworthiness directives are legally enforceable rules that apply to aircraft, aircraft engines, propellers, and appliances.
The FAA issues an airworthiness directive when there is an unsafe condition that exists with a product, or the condition is likely to develop in other products of the same type design.
Compliance with the requirements of an airworthiness directive is mandatory. Airworthiness directives may include specific inspections that must be completed, conditions or limitations to be adhered to, or any other actions that must be taken to resolve an unsafe condition.
14 CFR 39 Airworthiness Directives
14 CFR 91.403 General (Maintenance)