Type certificate

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 (STC) 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 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.

The concept of a 'type certificate' was introduced in the UK by the "Air Navigation Regulations" published in May 1919 by one of the UK's Secretaries of State, Winston Churchill.

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.

...
Wikipedia