# 14 CFR 39.3: What an Airworthiness Directive is

> An Airworthiness Directive is a legally enforceable FAA rule, not a recommendation. ADs apply to four kinds of product: aircraft, aircraft engines, propellers and appliances.

Source: https://www.aerscheduler.com/regulations/39-3
Official text: https://www.ecfr.gov/current/title-14/section-39.3 (eCFR, current as of 2026-10-15; section last amended 2016-12-30)

## In plain English

- ADs are legally enforceable rules.
- They apply to aircraft, engines, propellers and appliances, so an aircraft can be subject to ADs on its airframe and on every part installed on it.

## Common questions

**Is a manufacturer's service bulletin the same as an AD?**

No. A service bulletin is the manufacturer's recommendation. It becomes mandatory for Part 91 operators only when an AD requires it, which many ADs do by reference.

**Why does an engine AD apply to my airplane?**

Because 39.3 makes ADs apply to engines, propellers and appliances as products in their own right. An AD on your engine model or your magnetos applies wherever that product is installed.

## Official text of 14 CFR 39.3

Definition of airworthiness directives.

FAA's airworthiness directives are legally enforceable rules that apply to the following products: aircraft, aircraft engines, propellers, and appliances.

## Related

- [14 CFR 39.7: Flying without complying with an AD](https://www.aerscheduler.com/regulations/39-7.md)
- [14 CFR 39.11: What an AD can require](https://www.aerscheduler.com/regulations/39-11.md)
- [14 CFR 91.403: The owner is responsible for airworthiness](https://www.aerscheduler.com/regulations/91-403.md)
- [AD library: search FAA Airworthiness Directives](https://www.aerscheduler.com/airworthiness-directives)
- [Airworthiness Directive tracking](https://www.aerscheduler.com/resources/airworthiness-directive-tracking)

AerScheduler lists the ADs that name each of your aircraft, records who decided each one, and tracks the repeat inspections that follow. [https://www.aerscheduler.com/features/inspections](https://www.aerscheduler.com/features/inspections)

_The plain-English summary is AerScheduler's reading of the rule, not legal advice. The official text above is the eCFR's, published by the Office of the Federal Register; the eCFR is the current text._