The standard AD process is to issue an NPRM followed by a Final Rule. After an unsafe condition is discovered, a proposed solution is published as an NPRM and solicits public comment on the proposed action. After the comment period closes, the final rule is prepared, taking into account all substantive comments received with the rule perhaps being changed as warranted by the comments. The preamble to the final rule AD provides response to the substantive comments or states there were no comments received.

In certain cases, the critical nature of an unsafe condition may warrant the immediate adoption of a rule without prior notice and solicitation of comments. This is an exception to the standard process. If time for the terminating action to be accomplished is too short to allow for public comment (that is, less than 60 days), then a finding of impracticability is justified for the terminating action, and it can be issued as an immediately adopted rule. The immediately adopted rule is published in the Federal Register with a request for comments. The Final Rule AD may be changed later if substantive comments are received.

An Emergency AD is issued when an unsafe condition exists that requires immediate action by an owner/operator. The intent of an Emergency AD is to rapidly correct an urgent safety deficiency. An Emergency AD may be distributed by fax, letter, or other methods. It is issued and effective to only the people who actually receive it. This is known as “actual notice.” All known owners and operators of affected U.S.-registered aircraft, or those aircraft that are known to have an affected product installed, are sent a copy of an Emergency AD. To make the AD effective to all persons, a follow up publication of the Final Rule AD in the Federal Register is critical. This Final Rule AD must be identical to the Emergency AD and is normally published in the Federal Register within 30 days of the Emergency AD issue.

Generally, ADs include:

ADs have a three-part number designator. The first part is the calendar year of issuance. The second part is the biweekly period of the year when the number is assigned. The third part is issued sequentially within each biweekly period.

The AD subject line specifically identifies the TC holder of the aircraft or products affected by the AD. The specific models affected and any special considerations, such as specific installed part numbers or modifications, are listed in the AD applicability section. In order to find all applicable ADs for a specific product, you must search for ADs on the product, aircraft, engine(s), propeller, or any installed appliance. If there are multiple series under the aircraft or engine model, you must also search for ADs applicable to the model, as well as the specific series of that model. The final determination of ADs applicable to a particular product can only be made by a thorough examination of the ADs and the product logbooks. No person may operate a product that an AD applies to, except in accordance with the requirements of the AD. Furthermore, the owner or operator of an aircraft is required by 14 CFR part 91, section 91.403 to maintain the aircraft in compliance with all ADs. The AD specifies a compliance time that relates to the effective date of the AD. That compliance time determines when the actions are required.

Different approaches or techniques that are not specified in an AD can, after FAA approval, be used to correct an unsafe condition on an aircraft or aircraft product. Although the alternative was not known at the time the AD was issued, an alternative method may be acceptable to accomplish the intent of the AD. A compliance time that differs from the requirements of the AD can also be approved if the revised time period and approved alternative method provides an acceptable level of safety as the requirements of the AD.

A Special Airworthiness Information Bulletin (SAIB) is an information tool that the FAA uses to alert, educate, and make recommendations to the aviation community. SAIBs contain non-regulatory information and guidance that does not meet the criteria for an AD. [Figure 2-13]

Specifications were originated during implementation of the Air Commerce Act of 1926. Specifications are FAA recordkeeping documents issued for both type-certificated and non-type-certificated products that have been found eligible for U.S. airworthiness certification. Although they are no longer issued, specifications remain in effect and will be further amended. Specifications covering type-certificated products may be converted to a TCDS at the option of the TC holder. However, to do so requires the TC holder to provide an equipment list. A specification is not part of a TC. Specifications are subdivided into five major groups as follows:

When an aircraft is designed and that design is formally approved for manufacturing, the manufacturer is issued a Type Certificate (TC). The TC is issued by the FAA to signify the airworthiness of an aircraft design and may not be changed except by formal authorization of the FAA. This formal authorization supplements the original TC and is called the Supplemental Type Certificate (STC). Therefore, the STC issued by the FAA approves a product (aircraft, engine, or propeller) modification. [Figure 2-14] The STC defines the product design change, states how the modification affects the existing type design, and lists serial number effectivity. It also identifies the certification basis listing specific regulatory compliance for the design change. Information contained in the certification basis is helpful for those applicants proposing subsequent product modifications and evaluating certification basis compatibility with other STC modifications. Refer to Figure 2-15 for a listing of how TCs and STCs are numbered.