Air Navigation Orders and CAD Circulars: Hierarchy and Practical Use
Where ANOs and CADs fit
Air Navigation Orders are the instruments that give operational effect to the Act in areas such as aircraft registration, airworthiness of aircraft, licensing of personnel, rules of the air, and aerodrome use. In practice an ANO sets out an obligation in relatively short, enforceable terms and cross-refers to schedules and regulations for detail. CAD Circulars (Civil Aviation Department circulars) are the authority's published guidance: they explain acceptable interpretations, announce policy, promulgate airworthiness directives and provide information the industry must act on. A candidate should be able to distinguish an ANO article (binding) from a CAD circular (guidance, though a circular may publicise a mandatory requirement such as an AD).
Concepts to master
Understand how an airworthiness directive becomes mandatory, and how it reaches the maintainer - normally through the authority's publications and the continuing airworthiness management process. Know the role of CAD circulars in communicating change and that organisations must have a system for receiving and acting on them. Understand the registration and airworthiness framework an ANO establishes: an aircraft must be registered and hold a valid certificate of airworthiness (or permit) to fly, and maintenance must preserve that validity. Recognise the language of obligation: shall and must are mandatory; should and may are guidance or permission.
Traps in questions
- Assuming a CAD circular can never create a mandatory obligation - it can publicise one (e.g. an AD), but the obligation's source lies elsewhere.
- Confusing an Air Navigation Order with the Civil Aviation Regulations; they are different instruments with different functions.
- Overlooking that an aircraft with an expired certificate of airworthiness is not airworthy for flight even if recently maintained.
- Missing the requirement for a documented system to receive and act on authority publications.
Rule of thumb
Ask: is this instrument telling me I must, or telling me how? Worked example: a CAD circular announces an AD mandating inspection of a composite spar within 50 hours. The circular is guidance in form, but the AD is mandatory; the maintenance organisation must record receipt, plan the inspection, and certify it before further flight. Ignoring it because "it's only a circular" is a classic wrong answer.