Advisory Circulars (AC)
AC refers to a type of publication offered by the FAA to provide guidance for compliance with airworthiness regulations. They provide guidance such as methods, procedures, and practices acceptable to the Administrator for complying with regulations. ACs may also contain explanations of regulations, other guidance material, best practices, or information useful to the aviation community. They do not create or change a regulatory requirement. The AC system became effective in 1962. It provides a single, uniform, agency-wide system that the FAA uses to deliver advisory material to FAA customers, industry, the aviation community, and the public.Unless incorporated into a regulation by reference, the content of ACs are not binding on the public. ACs are issued in a numbered-subject system corresponding to the subject areas of the FARs (14 CFR, Chapter 1, Federal Aviation Administration) and Chapter 3, Commercial Space Transportation, Federal Aviation Administration, Department of Transportation, Parts 400–450. An AC is issued to provide guidance and information in a designated subject area or to show a method acceptable to the Administrator for complying with a related federal aviation regulation.
Because of their close relationship to the regulations, ACs are arranged in a numbered system that corresponds to the subject areas of the CFRs. In some series, consecutive numbers may be missing. These numbers were either assigned to ACs still in preparation that will be issued at a later date or were assigned to ACs that have been canceled.
The AC Numbering System
There are three parts to an AC number, as in 25-42-C.- The first part of the number identifies the subject matter area of the AC. This corresponds to the part of the FAA’s regulations. In the above example, this would be part 25.
- The second part of the number, beginning with the dash, is a sequential number within each subject area. In the above example, this would be the 42nd AC relating to part 25.
- The third part of the number is a letter assigned by the originating office showing the revision sequence if an AC is revised. The first version of an AC does not have a revision letter. In the above example, this is third revision, as designated by the “C.” [Figure 1]
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| Figure 1. List of advisory circular numbers |
Airworthiness Directives (AD)
In accordance with 14 CFR part 39, the FAA issues ADs in response to deficiencies and/or unsafe conditions found in aircraft, engines, propellers, or other aircraft parts. ADs require that the relevant problem must be corrected on all aircraft or aircraft parts using the same design. ADs are initiated as either proposed, corrective, or final (telegraphic) via the Federal Register. The Federal Register is the official daily publication of the United States Government. It is the printed method of informing the public of laws that are enacted or will be enacted. Electronic versions of ADs are available from the Federal Register and from the Regulatory and Guidance Library. You can search by manufacturer, model, or AD number. All ADs are “incorporated by reference” into part 39 and are considered final. ADs must be followed to remain in compliance with the FAA. Once an AD has been issued, a person/company is authorized to use the affected aircraft or part only if it has been corrected in accordance with the AD.Types of Airworthiness Directives (AD)
Three types of ADs are issued:- Notice of Proposed Rulemaking (NPRM), followed by a Final Rule
- Final Rule; Request for Comments
- Emergency ADs
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.
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.
AD Content
Generally, ADs include:
- A description of the unsafe condition
- The product that the AD applies to
- The required corrective action or operating limitations or both
- The AD effective date
- A compliance time
- Where to go for more information
- Information on alternative methods of compliance with the requirements of the AD
AD Number
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.Applicability and Compliance
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.Alternative Method of Compliance
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.Special Airworthiness Information Bulletin (SAIB)
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] |
| Figure 2. Special Airworthiness Information Bulletin (SAIB) |
Aircraft Specifications
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:- Group I—Type Certificate Aircraft, Engines, and Propellers. Covering standard, restricted, and limited types issued for domestic, foreign, and military surplus products.
- Group II—Aircraft, Engine, and Propeller Approvals. Covering domestic, foreign, and military surplus products constructed or modified between October 1, 1927, and August 22, 1938. All have met minimum airworthiness requirements without formal type certification. Such products are eligible for standard airworthiness certification as though they are type-certificated products.
- Group III—Aircraft, Engine, and Propeller Approvals. Covering domestic products manufactured prior to October 1, 1927, foreign products manufactured prior to June 20, 1931, and certain military surplus engines and propellers. All have met minimum airworthiness requirements of the Air Commerce Act of 1926 and implementing Air Commerce Regulations without formal type certification. Such products are eligible for standard airworthiness certification as though they are type-certificated products.
- Group IV—Engine Ratings. Covering unapproved engines rated for maximum power and speed only, their use being limited to specific aircraft with maximum gross weights less than 1,000 pounds. Such engines are not eligible for independent airworthiness certification. These ratings are no longer issued.
- Group V—Engine Approvals. Covering military surplus engines meeting CAR 13 design requirements without formal type certification. Such engines are eligible for airworthiness certification as though they are type-certificated engines.
Cessna; 182T; Locked Rudder Trim Wheel; A4A 2721
A transit customer required help as his rudder trim was stuck in the full-right position. The attending mechanics found the trim indicator pin had jumped free of its positioning track and locked the trim wheel. After adjustment, the system was cycled to full extremes several times. The submitter notes they could replicate the “jammed” trim condition with extreme R/H trim. This aircraft was the second 182 observed by these mechanics having this particular problem.Part total time: 31.9 hours
Supplemental Type Certificates (STC)
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 3] |
| Figure 3. Supplemental type certificate (STC) |
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 4 for a listing of how TCs and STCs are numbered.
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| Figure 4. Numbering system for type certificates (TCs) and supplemental type certificates (STCs) |
Possession of the STC document does not constitute rights to the design data or installation of the modification. The STC and its supporting data (drawings, instructions, specifications, and so forth) are the property of the STC holder. You must contact the STC holder to obtain rights for the use of the STC.
Type Certificate Data Sheets (TCDS)
The TCDS is a formal description of the aircraft, engine, or propeller. It lists limitations and information required for type certification including airspeed limits, weight limits, thrust limitations, and so forth.TCDSs and specifications set forth essential factors and other conditions that are necessary for U.S. airworthiness certification. Aircraft, engines, and propellers that conform to a U.S. TC are eligible for U.S. airworthiness certification when found to be in a condition for safe operation and ownership requisites are fulfilled. [Figure 5]
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| Figure 5. Type certificate |
TCDSs were originated and first published in January 1958. Title 14 of the CFR part 21, section 21.41 indicates they are part of the TC. As such, a TCDS is evidence the product has been type certificated. Generally, TCDSs are compiled from details supplied by the TC holder; however, the FAA may request and incorporate additional details when conditions warrant. [Figure 6]
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| Figure 6. Type Certificate Data Sheet |
Under federal law, no civil aircraft registered in the United States can operate without a valid airworthiness certificate. This certificate must be approved and issued by the FAA; and it is only issued if the aircraft and its engines, propellers, and appliances are found to be airworthy and meet the requirements of an FAA-approved TC. The FAA issues a TC when a new aircraft, engine, propeller, and so forth, is found to meet safety standards set forth by the FAA. The TCDS lists the specifications, conditions, and limitations that the airworthiness requirements were met under for the specified product, such as engine make and model, fuel type, engine limits, airspeed limits, maximum weight, minimum crew, and so forth. TCDSs are issued and revised as necessary to accommodate new models or other major changes in the certified product. TCDSs are categorized by TC holder and product type.