Regulations (Parts 1, 21, 39, 43, 45, 65, 91, 121, 135) and Airworthiness Directives
Start with the definitions
Nearly every scenario question reduces to two questions: which document authorizes this action, and to what standard must it be done? The answers start in Part 1. Know maintenance, preventive maintenance, major repair, major alteration, time in service, and appliance cold - later rules turn on those words.
The parts that matter most
Part 21 covers design and production approval: type certificates, supplemental type certificates (STCs), parts manufacturer approvals (PMA), and the requirement that the design approval holder furnish instructions for continued airworthiness (ICA). Part 39 governs airworthiness directives. Section 39.3 makes operation unlawful unless the directive's requirements are met; the AD itself - not a service bulletin - defines applicability, method and interval of compliance, and terminating action. Part 43 sets performance and recording rules; Part 45 covers marking and identification; Part 65 covers certificates and the inspection authorization; Part 91 supplies operating and inspection rules; Parts 121 and 135 add approved maintenance programs for air carriers.
Recurring traps
ADs apply to a product by make, model, and serial number range - never to an operator or a fleet in general. Service bulletins and manufacturer's recommendations are advisory for Part 91 operators unless an AD, the airworthiness limitations section, or an approved operator program makes them mandatory. An alternate method of compliance requires FAA approval, not inspector approval. An air carrier's approved program may replace the annual/100-hour rules, but no program waives an AD.
Rule of thumb
Read the applicability paragraph line by line first: model designation, serial number range, and any installed component or part-number condition. If the aircraft does not match every element, the AD does not apply - and that conclusion should be documented.