Design Change Categorization – Major or Minor

Effective Date: 16 September 2019

Reference: TAM Part 3, Chapter 2

OPI / Telephone: DTAES 3 / 819-939-4835

1.       Purpose

1.1          The categorization of changes to a type design as minor or major is required to determine the route to be followed for the certification of a design change.  This advisory provides guidance related to the Technical Airworthiness Manual (TAM) (regulatory reference 3.2.a) rules and standards for categorizing design changes as either major or minor, by:

1. identifying the rules and standards in the DND/CAF TAM that are applicable to the categorization of design changes
2. identifying the differences between a major and a minor design change
3. providing the criteria and process for identifying a major design change
4. providing a design change categorization checklist and question set
5. providing checklist advisory material to aid with answering the checklist questions

2.      Applicability

2.1        This TAA Advisory is applicable to TAA regulatory staff and TAA-acceptable design and technical organizations, as well as TAA-Authorized Individuals (AIs) who have been assigned authority and responsibility for making a major-minor categorization determination.

2.2       Exceptions.  This advisory only addresses design changes that are considered to be modifications or alterations to the type design.  Therefore, the following types of design changes are to be managed by the separate processes described below:

1. Changes to the Approved Maintenance Program.  Changes to the maintenance program are reviewed and approved using the process defined in the TAM (regulatory reference 3.2.a.), Part 5, Chapter 3.
2. Master Minimum Equipment List (MMEL).  Changes to a MMEL are to follow the process defined in TAA Advisory 2013-07 (regulatory reference 3.2.b).
3. Changes to the Flight Manual (FM).  This advisory does address the categorization changes to the FM in Section M. However, changes to the FM follow the rules and standards provided in the TAM (regulatory reference 3.2.a), Part 2, Chapter 7 – Canadian Armed Forces Flight Publications.
4. Structural Repairs and Rectifications.  TAA Advisory 2019-07 – Structural Repair Categorization (regulatory reference 3.2.c) provides the procedure for categorizing design changes arising from structural repairs and rectifications.  

3.      Related Material

3.1       Definitions

The formal definitions for most of the airworthiness-related terms in this document can be found in the Glossary of regulatory reference 3.2.a. The following definitions are not in the TAM Glossary:

1. Repair. A rectification of a deficiency in an aeronautical product, or the restoration of an aeronautical product to an airworthy condition. If the repair does not require a change to the type design, then it is a restoration. If the repair requires a change to the type design, then it is a rectification.
2. Rectification. A repair leading to a change to the type design, which requires an authorized individual to approve the repair design. If an existing approved repair scheme is available and applicable, then this ‘standard repair’ can be used to perform the rectification.

3.2      Regulatory References

1. C-05-005-001/AG-001 – Technical Airworthiness Manual (TAM)
2. TAA Advisory 2013-07 – Joint TAA-OAA Guidance on the Development of an Initial Master Minimum Equipment List
3. TAA Advisory 2019-07 (Draft) – Structural Repair Categorization (available internally, within DND, at AEPM RDIMS Library #1832041)
4. TAA Advisory 2006-04 – Installation of Miscellaneous Non-Required Equipment
5. AF9000 procedure EMT09.052 – Aircraft Flight Manual and Aircraft Operating Instructions Amendment (available internally, within DND, on DGAEPM MAP Online intranet website)

4.      TAA Regulatory Requirements

4.1       TAM Rule 3.2.2.R1 – Part 3, Chapter 2, Section 2 – Design Change Categorization

This rule states the following:

1. Where a proposed design change is assessed for its potential effects on airworthiness and categorized as minor by an AI, in accordance with TAM Standard 3.2.2.S1, no further airworthiness certification activity is required. Sufficient technical data must be available to substantiate and formally document the categorization.
2. Where a proposed design change is assessed and categorized as major by an AI, in accordance with TAM Standard 3.2.2.S1, design change certification shall be conducted in accordance with the TAM rules and standards.

4.2       TAM Standard 3.2.2.S1 – Part 3, Chapter 2 – Design Change Categorization

This standard states the following:

1. Design Change Category.  A proposed design change shall:
   1. be assessed for its effect on the airworthiness of the approved type design of an aeronautical product into which incorporation is intended and categorized as major or minor
   2. have the assessment and subsequent categorization of the proposed design change conducted by an AI.
2. Design change categories are defined as follows:
   1. Minor. A design change that has a negligible effect on the airworthiness of the approved type design of an aeronautical product
   2. Major. A design change that has, or may have, more than a negligible effect on the airworthiness of the approved type design of an aeronautical product

5.       Discussion

5.1          As described in the TAM, Part 3, Chapter 2, a design change is the act or outcome of making a change to the approved type design of an aeronautical product. The definition of a design change also includes modifications, alterations, changes to the approved maintenance program, as well as changes to the aircraft’s approved roles and missions defined in the Statement of Operating Intent (SOI).

5.2         The focus of this advisory is to provide guidance on how to distinguish between a minor and major design change, by providing specific differentiation criteria, in a set of questions that are designed to establish if the change has an appreciable effect on the airworthiness of the aircraft. If the answer to the question is ‘Yes’, there is an appreciable effect, then the change must be categorized as ‘Major’. The question set is supported by advisory material that provides additional criteria and examples to assist in answering the questions.

5.3         The traditional definition of a minor change is one that has a negligible effect on the airworthiness of the underlying type design. While this definition remains valid, it presents a problem for categorizing changes to complex, highly-integrated military and civil aircraft designs. In this context, it can be challenging to demonstrate that any given change to an aircraft design has a negligible or ‘zero’ effect. To solve this problem, most civil and military airworthiness authorities have chosen to define a major design change as one that has an ‘appreciable’ effect on airworthiness. Under this construct, a minor change becomes one that has ‘less than an appreciable’ effect. This interpretation allows for a minor change to have some effect on airworthiness, provided that the effect does not significantly reduce aircraft safety.

5.4         The TAA has adopted the ‘appreciable effect’ criteria to distinguish between a major and minor design change for the following reasons:

1. Align the TAA’s definition with that being adopted by other airworthiness authorities
2. Align with the system safety definition of minor hazard / failure effect: ‘A slight reduction in safety margins’ (source: SAE-ARP 4761)
3. Align with the definition of a minor failure condition that is related to a software error or complex electronic hardware malfunction: ‘Failure conditions which would not significantly reduce aircraft safety, and which would involve crew actions that are well within their capabilities. For example, minor failure conditions may include a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, such as, routine flight plan changes, or some inconvenience to occupants.’ (Source: RTCA DO-178B)
4. Although these criteria are subject to interpretation, the risk of misidentifying a major change as minor is mitigated by the fact that all of the design changes to DND/CAF aircraft are managed and executed by TAA-acceptable organizations. These organizations have TAA-acceptable system engineering processes that are applied to all design changes, regardless of their categorization (major or minor).

5.5        The revised TAA definitions for major and minor are as follows:

1. Minor. A design change that has less than an appreciable effect on the airworthiness of the approved type design of an aeronautical product.
2. Major. A design change that has, or may have, an appreciable effect on the airworthiness of the approved type design of an aeronautical product.

5.6        The term ‘appreciable’ is defined as an effect that is large enough to be important or clearly noticed. Synonyms include: significant, discernable, measurable, serious and substantial. In the airworthiness context, a design change that has less than an appreciable effect on airworthiness can be defined as follows:

1. the change does not significantly reduce aircraft safety
2. the change does not involve more than a slight reduction in safety margins, or a slight increase in crew workload

5.7        The TAA process for categorizing a design change as major or minor is shown in the flowchart provided in Figure 1 – Design Change Categorization Process.

5.8     Design Change Categorization Checklist. The categorization checklist that is provided in Annex A to this advisory consists of the following three parts:

1. Part 1 – Design Change Information. This section of the checklist identifies the information, or the appropriate reference documents, that will be required by the persons who will be completing Parts 2 and 3 of the checklist. In addition to the design change identification data, a description of the change is required, including the aircraft systems affected and the scope of the proposed design change. The design organization is responsible for providing the required information to the persons who will be completing Parts 2 and 3 of the checklist.
2. Part 2 – Design Change Categorization Question Set. This part of the checklist provides questions that are used to determine if a change has an appreciable effect on airworthiness. Answer all of the questions in Part 2. If the response to any of the checklist questions is ‘Yes’, then the design change has an ‘appreciable effect’ and must be categorized as ‘Major’. If all of the responses are ‘No’ then it is categorized as minor. Although only a single ‘Yes’ answer is needed to categorize a change as major, a response must still be provided for all of the questions.
3. Part 3 – Design Change Categorization – Review and Approval. Once the Part 2 questions have been answered, an AI who has not been involved in answering the Part 2 questions will review the question set answers and, if in agreement, approve the results.

5.9      Question Set – Advisory Material. Advisory material that may be used to develop answers to the question set is provided in Annex B to this advisory. This material is intended to assist with formulating a ‘Yes’ or ‘No’ response to the questions presented in Annex A. Also provided are examples of major design changes and additional criteria that may assist in determining if the effect of the change is appreciable.

5.10     Early in the design change development, there may not be sufficient information about the scope of the design change and its effects on the underlying type design to support the categorization process. Normally, the Applicant should wait for the required data before making a categorization decision. If this is not practicable, then the alternative is to default to the treatment of the change as major. The categorization can subsequently be changed, if new information becomes available to support the downgrading to a minor categorization.

5.11      TAA Specialist Advice. Whenever there is any doubt surrounding the interpretation of the checklist, or the design change categorization, the TAA (staff of the Directorate of Technical Airworthiness and Engineering Support (DTAES) 3 section), or the engineering support staff of DTAES 6, 7 and 8 should be consulted. Note that seeking advice from the TAA staff, or even requesting a recommendation Tech Note from the DTAES engineering specialists, does not imply that the design change must subsequently be classified as major. In cases where DTAES 3 or the DTAES Engineering Support staff do not agree with the Applicant regarding the categorization of a design change, the DTAES 3 decision will be binding.

5.12     Design Changes Certified by another Authority.  A major design change that has been previously approved by another military or civilian airworthiness authority will continue to be treated as a major design change. The fact that the design has been approved by another authority does not mean that the incorporation of the change in a DND/CAF aircraft can be treated as a minor change. In this situation, the Type Design Examination (TDE) process will be applied to complete the DND approval process.