A type certificate is issued to signify the airworthiness of an aircraft manufacturing design or "type". The certificate is issued by a regulating body, and once issued, the design cannot be changed. The certificate reflects a determination made by the regulating body that the aircraft is manufactured according to an approved design, and that the design ensures compliance with airworthiness requirements. The regulating body compares design documents and processes to determine if the design meets requirements established for the type of equipment. Requirements established by a regulating body typically refer to Minimum Operating Performance Standards (MOPS) and related documents (such as DO-178 series, DO-160 series and DO-254 series), which are developed jointly by RTCA, Inc. and EUROCAE. Once issued, the aircraft "type" meets appropriate requirements. The determination process includes a step called "First Article Inspection", for it and for each of its subassemblies. This is a quality control assessment whereas those prior to it are part of quality assurance.
With respect to "cannot be changed": When an airframer wants to change something it has two options. One is to request a Supplemental Type Certificate (STC), the other is to create an entirely different design. The choice is determined by considering whether or not the change constitutes a new design (i.e. introduces risk not considered in the first design). If the manufacturer believes the change doesn't introduce new risk the manufacturer typically requests an STC. This is less expensive. If the regulatory authority agrees with the rationale for choosing STC, the STC is granted.
A supplemental type certificate is issued following a modification. The STC defines the product design change, states how the modification affects the existing type design, and lists serial numbers affected. 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.
Initially, the applicant firm submits documents to their local aviation regulating body, detailing how the proposed design, i.e., the 'Type', would fulfill the airworthiness requirements. After investigations by the regulator, the final approval of such documents (after the required comments and amendments in order to fulfill the laws), becomes the basis of the certification. The firm follows it and draws a proposed timetable of actions required for certification tests. With the application, the regulations to be applied will usually be frozen for this application for a given amount of time in order to avoid a situation where the applicant would have to change the design as a result of changed regulation.
An initial design sample known as a prototype is built. This refers to either the aircraft, the engines or the propeller, depending on the basis of the certification. For the purpose of illustration, the discussion shall be limited to the aircraft. Normally a few prototypes are built, each subject to different tests. The prototypes are first used for ground and system tests. One of the prototypes (known as the "static airframe") is subject to destructive testing, i.e., the prototype is subject to stress beyond normal and abnormal operations until destruction. The test-results are compared with initial submitted calculations to establish the ultimate structural strength.
Other prototypes will undergo other systems tests until the satisfaction of the regulators. With all ground tests completed, prototypes are made ready for flight tests. The flight tests are flown by specially approved flight test pilots who will fly the prototypes to establish the ultimate flight limits which should be within the airworthiness rules. If a long range airliner is tested, the flight tests may cover the whole world.
In parallel with aircraft testing, the applicant firm also draws up maintenance program to support continuous airworthiness after approval of the design. The program is drawn with inputs from tests results and also from initial customers' engineering departments. The proposed maintenance program is submitted to the regulators for comment and approval.
After successful completion of ground and flight tests, along with an approved maintenance program, the prototype is approved, and the firm is granted the type certificate for the prototype (as understood that it should include all furnished equipment for its intended role). The legal term for the firm is now the "type certificate holder". Subsequently the prototype now serves as a template for aircraft production. Hence the aircraft rolling out of the factory should be identical to the prototype, and each given a serial number (a "series aircraft").
As the aircraft enters into service, it is subject to operational wear and tear which may cause performance degradations. A maintenance program is issued by the aircraft manufacturer and approved by the regulatory authorities. It serves to maintain the airworthiness of all aircraft of the type. Owners and operators of the aircraft must comply in order for the airworthiness certificate of their aircraft to remain valid.
Sometimes during service, the aircraft may encounter problems that may compromise the aircraft's safety, which are not anticipated or detected in prototype testing stages. The aircraft design is thus compromised. The regulators will now issue an airworthiness directive to the type certificate holder and to all owners globally. The directives normally consists of additional maintenance or design actions that are necessary to restore the type's airworthiness. Compliance is mandatory and thus if an operator does not comply with an AD, then the datum aircraft is not considered airworthy and further operation of the affected aircraft type would be unlawful and make the operator liable to legal action by the relevant national aviation authority, as well as making any of the operator's insurance policies relating to the type, e.g., hull loss, accident third party, etc., null-and-void. ADs may also be raised with changes of the local or global aviation rules and requirements, e.g., the requirement to fit armored cockpit doors for all airliners past 9-11.
The certifying authority issues an AD when an unsafe condition is found to exist in a product (aircraft, aircraft engine, propeller, or appliance) of a particular type design. ADs are used by the certifying authority to notify aircraft owners and operators of unsafe conditions and to require their correction. ADs prescribe the conditions and limitations, including inspection, repair, or alteration under which the product may continue to be operated.
With increasing in-service experience, the type certificate holder may find ways to improve the original design resulting in either lower maintenance costs or increased performance. These improvements (normally involving some alterations) are suggested through service bulletins to their customers as optional (and may be extra cost) items. The customers may exercise their discretion whether or not to incorporate the bulletins. Sometimes SBs can become mandated by relevant ADs.
Often the basic design is enhanced further by the type certificate holder. Major changes beyond the authority of the service bulletins require amendments to the type certificate. For example, increasing (or decreasing) an aircraft's flight performance, range and load carrying capacity by altering its systems, fuselage, wings or engines resulting in a new variant may require re-certification. Again the basic process of type certifications is repeated (including maintenance programs). However, unaltered items from the basic design need not be retested. Normally, one or two of the original prototype fleet are remanufactured to the new proposed design. As long as the new design does not deviate too much from the original, static airframes do not need to be built. The resultant new prototypes are again subjected to flight tests.
Upon successful completion of the certification program, the original type certificate is amended to include the new variant (normally denoted by a new model number additional to the original type designation). Typical examples are; the Boeing 737NG (737-600, 737-700, 737-800 and 737-900) which replaced the 737 Original family (737-100 and 737-200) and the 737 Classic family (737-300, 737-400 and 737-500) and the Airbus A340-500 and the A340-600 which is based on the Airbus A340-200 and the A340-300.
Any additions, omissions or alterations to the aircraft's certified layout, built-in equipment, airframe and engines, initiated by any party other than the type certificate holder, need an approved supplementary ("supplemental" in FAA terminology) type certificate, or STC. The scope of an STC can be extremely narrow or broad. It could include minor modifications to passenger cabin items or installed instruments. More substantial modifications may involve engine replacement, as in the Blackhawk modifications to Cessna Conquest and Beechcraft King Air turboprops, or a complete role change for the aircraft, such as converting a B-17 or Stearman into an agricultural aircraft. STCs are applied due to either the type certificate holder's refusal (frequently due to economics) or its inability to meet some owners' requirements. STCs are frequently raised for out-of-production aircraft types conversions to fit new roles. Before STCs are issued, procedures similar to type certificate changes for new variants are followed, likely including thorough flight tests. STCs belong to the STC holder and are generally more restrictive than type certificate changes.
The type certificate holder keeps the type certificate valid by continuously following airworthiness directives, issuing service bulletins and as well as providing spares and technical support to keep the aircraft current with the prevailing rules, even after the production of the type has stopped. This is what is meant by supporting the type and in this manner many out-of-production aircraft continue useful lives. STCs are also bound by the same rules. When the holder decides to stop supporting the aircraft type, the type certificate is returned to the regulators and the remaining aircraft fleet permanently grounded. In this manner the whole Concorde fleet was finally grounded when Airbus SAS surrendered its type certificate.