Mastering AD Compliance: Unveiling the Dynamics of Airworthiness Directives

February 14, 2024

To effectively control or implement an Airworthiness Directive (AD), there are terms whose nature, meaning, and/or purpose we must understand.

Firstly, it's important to note that a directive is issued to address an unsafe condition in the aircraft or other aeronautical product that compromises airworthiness. Non-compliance with this directive results in a violation of the standard. There are typically two types:

  • Emergency ADs
  • Final Rule ADs

These concepts help us comprehend the reason behind an AD, including why some are labeled for immediate compliance ("before further flight") and others allow operators to plan for aircraft downtime to fulfill the directive. Another crucial point is that an AD is often supported by a Service Bulletin (SB) for its maintenance actions.

Moreover, it's essential to recognize that the applicability of an AD extends to products that are not airworthy or installed in an aircraft, such as components in stock, workshops, shops, etc. Hence, careful attention must be given to AD control affecting Line Replaceable Units (LRUs) to avoid incorporating them into the fleet, especially when the AD action involves equipment disposal or significant monetary investment in configuration or maintenance that could incur additional and unnecessary expenses for the organization.

Now, let's delve into specific terms that are standard in the industry, even when dealing with different authorities such as the FAA and EASA:

  • WOF – whichever occurs first: This term is associated with the compliance interval of the AD when there is more than one, and the product must comply with the interval that occurs first.
  • WOL – whichever occurs later: This term is linked to the compliance interval of the AD when there is more than one, and the product must comply with the interval that occurs later.
  • Effective Date: The date from which the AD compliance in the product should be measured. The application of an AD is not solely tied to the effective date; various cases exist. For instance, when the AD affects a component, the effectiveness may depend on the Total Service Life (TSN) / Cycle Service Life (CSN) of that product, or it may be tied to the airframe's life, among other scenarios. Another common case is dependency on compliance instructions outlined in the associated SB.

For example, in the image, a print AD from the FAA includes application instructions in one of its paragraphs. In this example, paragraph (g)(1) takes effect from the directive's effective date (there are cases where an AD refers to being effective from another; we must pay attention to these details).

  • AD States: The directive can have six states:
  • Open: Published AD not yet complied with in the aeronautical product, usually within the authority's specified threshold for completion.
  • Repetitive: AD with established recurrence intervals.
  • Close: Terminal action fulfilled.
  • Not Applicable (N/A): After the open state, it undergoes an applicability analysis to support this state.
  • Superseded: AD replaced or superseded by a more recent one ensuring an adequate level of operational safety for the affected product. The superseded AD is no longer valid and typically enters this state when a new AD is more restrictive in compliance procedures, application times, etc.
  • Canceled: AD removed from publications.
  • AD Revision: In this case, the AD retains the same number but adds Rn, where n is the number of times as many revisions have been made. This is done when the AD change is in wording, requirements, reduction of applicability, but does not influence the terms of application for compliance.

For example, in the image, an FAA AD with instructions of application in one of its paragraphs, for example, paragraph (g)(1) from the effective date of the directive (there are cases where an AD refers to being effective from another, we must pay attention to these details).

  • AD Numbering: Here is how the AD number is composed for the two most well-known authorities:
  • The FAA format may resemble a date but is actually a location on the calendar and is denoted as follows (example provided).
  • On the other hand, the EASA AD numbering is more concise.

After reviewing standard terms for AD control, we can visualize the items we would find in an AD Status Report or Airworthiness Directive Status Control of an aeronautical product.

From the image above, we may encounter various formats in the industry; the one shown is an example proposed by the aeronautical software SOMA ( for more details). The crucial aspect is the content, and it is important to interpret any report encountered when reviewing an aircraft or another aeronautical product. Additionally, if you are responsible for updating such a report when incorporating a new AD, it is important to be detailed and comply with the standard and local regulation. I particularly recommend reading 14 CFR Part 39, AC 39-7D, as well as the local AD section.

With this, I conclude my post, hoping to have contributed a bit to our community's understanding of the extensive world of Airworthiness Directive control.