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5.3.1.3 An applicant seeking foreign approval of their modification should request their local CAA to consult foreign CAAs to clarify potential differences in the modification category, and consequently their approval requirements. In addition to airworthiness considerations, an assessment on the environmental standards of the aircraft or engine should be conducted to ensure that the approved noise and exhaust emissions levels remain within the approved limitations.
5.3.2   Major modification category
By definition, a major modification has an appreciable, or other than negligible, effect on the airworthiness of an aeronautical product. The CAA should evaluate the technical merit of each modification proposal and establish a clear understanding of the intended and/or consequential effect on the affected product. The intensity of such effect will vary with the complexity and extent of the proposed design change, but is generally recognized as falling under one the following three levels, presented in order of decreasing effect:
a)  Substantial Change. A proposed change in design, configuration, power, thrust, speed limitations, or mass is so extensive that a substantially complete investigation of compliance with the applicable airworthiness standards is required. A design change at this level is generally viewed as having a technical scope and nature that the affected product, when modified, can be regarded as essentially a new product, i.e. there are differences in major design and/or production elements. Further, due to the extensiveness of the proposed modification, most of the existing substantiation of the product will no longer be applicable. Therefore, there is a need for a substantially complete, or complete, re-investigation of compliance of the new substantiating data with the applicable airworthiness requirements. For this reason, some States may consider this level of design change as enough to warrant an application as a new Type Certificate, rather than as a modification. The need for a new Type Certificate may not be obvious when the proposed modification is first submitted to the CAA. A substantial change to an aeronautical product may evolve from single extensive design change proposal, or from previous relevant design changes that incrementally evolved an aircraft, engine or propeller over a period of time. If at some point, during the application and/or approval process, a proposed modification is evolving into a substantial change, the CAA should cease the modification approval process, and require the application to become an application for a new Type Certificate (see Part III, Chapter 1 of this manual: Type Certification). Some examples of modifications that are generally regarded as substantial change are:
1)  in the case of aircraft, the modification involves change in the number or location of engines, change in the number of rotors, increase from subsonic to supersonic flight regime, change from high wing to low wing configuration, or change from an all metal aircraft to an all composite primary structure (fuselage, wing, empennage);
2)  in the case of an aircraft engine, the modification involves change in the principle of operation or use of different principles for propulsion; or
3)  in the case of propellers, the modification involves change in the number of blades or the principle of pitch change operation.
b)  Significant Change. A proposed change in the general configuration, principles of construction, assumptions used for the certification, or a combination of these, of a type certificated product but not to the extent to be considered a substantial change. A significant change in the general configuration are design changes that are likely to require a new product model designation to distinguish it from other product models. A significant change to the principles of construction are changes to the materials and/or construction methods that affect the overall product’s operating characteristics or inherent strength. A significant change to the assumptions used for certification are changes to the product level assumptions associated with the compliance demonstration, performance, or operating envelope so different that they invalidate the original assumptions. The assessment of the effect of a significant change is made on the overall aircraft, engine, or propeller, rather than at the level of a part, component or system. A significant change usually results in a modified product that is distinct from other models of the same product, while still retaining common major design or production elements. Some examples of modifications that are generally regarded as significant changes are:
　
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