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a)  Identification of the proposed modification. The applicant should identify/describe the proposed modification to the aeronautical product. The application for approval could involve a single modification or a collection of modifications. Changes to a product can include physical design changes, changes to an operating envelope, and/or performance changes. An applicant for a change to a type design should consider all previously installed modifications to the affected aeronautical product that are relevant to the proposed modification. It is important that the effects of the proposed modification on other systems, components, equipment, or appliances of the affected product are properly identified. The intent is to encompass all aspects where there is a need for re-evaluation, that is, where the substantiation presented for the product being modified should be reviewed, updated, or rewritten.
b)  Determination if the proposed modification is considered a substantial change. The question of whether a proposed modification is considered a Substantial change should be addressed at the beginning of the process. By definition, if the proposed change in design, power, thrust, or mass is so extensive that a substantially complete investigation of compliance with the applicable airworthiness standards is required, the ACD should require the applicant to apply for a new Type Certificate instead. A “substantially complete investigation” of compliance is warranted when it is determined that most of the existing substantiation is no longer applicable to the modified product. A new Type Certificate could be required for either an extensive modification, or for a new design that evolved or was derived through a series of previous relevant modifications, to a previously type certificated product. The need for a new Type Certificate may or may not be obvious when the proposed modification is first considered, and may need a more extensive evaluation by the ACD. If at any point, while developing the certification basis, it becomes clear that the proposed modification is a substantial change, then the application ceases to be a modification approval process and becomes a new type certification process under Part III, Chapter 1 – Type Certification of this manual.
c)  Determination if the proposed modification is a Significant change.
1)  Following a determination in b) above that the proposed modification is not a substantial change, the next step is to segregate Significant changes from other types of modifications considered excluded from the application of the airworthiness policy. Most States have determined that a significant change is an automatic candidate for requiring compliance with the latest design standards. Significant changes are typically product level changes and, by their very nature, distinct from the vast majority of major modifications. In general, a significant change is either the result of an accumulation of previous modifications or occurs through an isolated but extensive major modification that rises to a product level. A modification to a single area, system, or component of an aircraft, engine, or propeller will not likely result in a product level change. 
2)  When assessing the proposed modification, the cumulative effect of previous relevant modifications in the areas related to the current proposal should be considered. For example, previous relevant aircraft design changes may address incremental increases in mass or thrust that, while individually not significant (for example, 2 per cent, 4 per cent, 5 per cent discrete increases) can, through a series of modifications, achieve a significant product level change. The collective and cumulative effects of previous modifications, along with the proposed modification, may result in the modified product being considerably different from the latest product or model. If this is the case, the proposed modification should be categorized as a significant change. Typically, significant product level changes result in a model change necessitating an amendment to the Type Certificate or an STC that rises to a level similar to that of an amended Type Certificate. Applications for a new model designation that are not associated with hardware changes (that is commercial considerations) are not an indication of a Significant change. In cases of doubt, and to ensure a consistent outcome, the ACD should work closely with the applicant during this important step of categorizing the proposed modification.
　
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