Please accept that the following treatise is not official or to be used as a guide in implementation, and rather an introductory overview of what is a very complex subject. In it the terms Civil Aeronautics Administration and (CAA) Federal Aviation Administration (FAA) are used interchangeably since there was much revision taking place during the of spans of the two agencies.

An Approved Type Certificate (ATC), or the post-WW2 renamed Type Certificate (TC), was/is awarded by CAA/FAA to aircraft (and now aerospace) manufacturers after a particular design of a civil aircraft, engine, or propeller met prevailing airworthiness requirements for safe conduct of flight under all normally conceivable conditions. The prize therein was an "NC" or, later, an "N" plus numerals on the tail and wings.

    A Type Certificate (TC) is a design approval by the Civil Aviation Authority (CAA), aka FAA, when an applicant demonstrates that a product—airframe, engine, propeller—complies with applicable regulations. The TC normally includes type design, operating limitations, a Type Certificate Data Sheet (TCDS), relative regulations and other CAA conditions or limitations, and is basis for other approvals, including production and airworthiness approvals. An airworthiness certificate is only issued to an aircraft properly registered and found to conform to its TCDS and ensure safe operations. The certificate is valid and the aircraft may be operated as long as it is maintained in accordance with CAA rules and regulations.

    A Supplemental Type Certificate (STC) is issued by the FAA approving a product modification. The STC defines product design change, how the modification affects an existing type design, and lists serial number effectivity. It also identifies the certification basis listing specific regulatory compliance for the design change. Information in the certification basis is helpful for applicants proposing subsequent modifications and evaluating certification compatibility with other STC modifications.

    A Group 2 Memo, aka Letter of Approval, predated post-war restructuring until its vitals gradually merged into TCs and such. It was a special-purpose CAA approval with eased or simplified requirements for individuals and small operations with only one or few airplanes as subjects, for builders who could not afford a full ATC and its costly requirements, and for minor modifications to existing ATCs. While there was some snobbery by ATC-holders who looked down on the Group-2 planes as their lessers, most "dash-twos" were built equally well, and in many cases better, with the hand of personal construction challenging assembly-line creations. From an Alexander A-1 bearing the distinctive "#2-1" approval to a surplus Taylorcraft TG-6 powered glider as the omega "#2-609," it served its purpose well.

    Additionally, there were two short-lived approvals: Limited Type Certificate (LTC) specifically for surplus military aircraft put into civilian clothes (eg: LTC-21-3, a Bell P-63C as a racer), and Approval Restricted (AR) for special-use conversions to, mainly, WW2 planes (eg: AR-29, a Convair P4Y-2 converted as borate bomber).

Prototype

Initially an applicant submits documents to FAA detailing a proposed design and, after investigations by FAA, final approval of such documents becomes the basis for certification. The firm then draws a proposed timetable of actions required for certification tests. An initial design sample—a prototype—is built of an aircraft (or engine or propeller). Normally multiple prototypes are built, each for specific tests, one of which is subjected to stress beyond normal and abnormal operations, until destruction, to establish ultimate structural strength.

    With all ground tests completed, prototypes are used for flight tests flown by approved test pilots to establish flight limits to fit within the airworthiness rules. Along with aircraft testing, an applicant also devises a maintenance program using input from tests results to support continuous airworthiness after design approval.

    Once approved, a TC for the prototype is issued and it serves as a template for identical production aircraft, which are assigned a "constructor's number" (c/n), more familiarly known as a serial number.

Continuing airworthiness

As an aircraft enters into service, it is subjected to operational wear which may cause performance degradations. An approved maintenance program serves to maintain the aircraft airworthiness, and owners must comply in order to maintain their aircraft's airworthiness certificate. Maintenance may be light or heavy as dictated by the schedules and tasks in the aircraft's maintenance program.

Airworthiness Directives

Sometimes during service an aircraft might encounter problems not anticipated or detected in prototypes and which could compromise the aircraft's safety. In that case FAA issues an Airworthiness Directive (AD) to the TC holder and to all owners, globally. ADs can provide added maintenance/design actions necessary to restore the type's airworthiness, and compliance is mandatory.

    An AD is also issued, when an unsafe condition is found in a product or component of a particular type design, to notify aircraft owners and operators of the condition and to require correction.

Service Bulletins

With experience a TC holder may find ways to improve an original design resulting in lower maintenance costs or increased performance. Such improvements/alterations are suggested through Service Bulletins as optional items, and a recipient can use discretion whether or not to incorporate them.

    When a basic design is further enhanced by the TC holder with major changes beyond the authority of the Service Bulletins—affecting an aircraft's flight performance, altering major components/structures, etc—an amendment to the TC requiring testing and recertification is issued.

Supplemental Type Certificate (STC)

STCs are unique to their holder and are generally more restrictive than TC changes. Additions or alterations to an aircraft's certified layout by anyone other than the TC holder call for an approved STC, the range of which could be anything from a minor modification of instrumentation to an engine replacement. STCs are also issued for out-of-production aircraft conversions to fit new roles, as converting a surplus PT-13 into a cropduster. Before STCs are issued, procedures similar to TC changes for new variants are followed, even up to flight tests.

NOTE: ATC, TC, and Group 2 Memos, if any, are shown in Aerofiles as parenthetical numbers after the year of an entry.