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Timestamp: 2020-01-28 20:55:54
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part21A 91 | Aviation | Aerospace Engineering
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SECTION A/Subpart D
AMC & GM for PART 21
Subpart D Changes to type-certificates
GM 21A.91 Classification of changes to a type design 1. PURPOSE OF CLASSIFICATION
Classification of changes to a type design into MAJOR or MINOR is to determine the approval route to be followed in Part 21 Subpart D, i.e., either 21A.95 or 21A.97, or alternatively whether application and approval has to be made in accordance with Part 21 Subpart E. 2. 2.1 INTRODUCTION 21A.91 proposes criteria for the classification of changes to a type design as minor and major. (i) This GM is intended to provide guidance on the term appreciable effect affecting the airworthiness of the product from 21A.91, where airworthiness is interpreted in the context of a product in conformity with type design and in condition for safe operation.. It provides complementary guidelines to assess a design change in order to fulfil the requirements of 21A.91 and 21A.117 where classification is the first step of a procedure. For classification of Repairs see GM 21A.435.
Although this GM provides guidance on the classification of major changes, as opposed to minor changes as defined in 21A.91, the GM and 21A.91are deemed entirely compatible.
For an ETSO authorisation, 21A.611 gives specific additional requirements for design changes to ETSO articles. For APU, this GM should be used.
ASSESSMENT OF A DESIGN CHANGE FOR CLASSIFICATION Changes to the type design 21A.31 defines what constitutes the type design. Alteration to any of the data included within the scope of 21A.31 is considered a change to the type design.
Classification Process (see attached diagram) 21A.91 requires all changes to be classified as either major or minor, using the criteria of 21A.91 and the complementary guidance of paragraph 3.3. On some occasions, the classification process is initiated at a time when some data necessary to make a classification decision are not yet available. Therefore, the applicant should wait for availability of data before making a decision. Wherever there is doubt as to the classification of a change, the Agency should be consulted for clarification. When the strict application of the paragraph 3.3 criteria results in a major classification, the applicant may request re-classification, if justified, and Agency could take the responsibility in re-classifying the change. A simple design change planned to be mandated by an airworthiness directive may be reclassified minor due to the involvement of the Agency in the continued airworthiness process. Reasons for a classification decision should be recorded. 33
Complementary guidance for classification of changes. A change to the type design is judged to have an appreciable effect on other characteristics affecting the airworthiness of the product and therefore should be classified major, in particular but not only, when one or more of the following conditions are met: (i) Where the change requires an adjustment of the type-certification basis (such as special condition, equivalent safety finding, elect to comply, exemption, reversion, later requirements). Where the applicant proposes a new interpretation of the requirements used for the type type-certification basis, that has not been published as AMC material or otherwise agreed with the Agency. Where the demonstration of compliance uses methods that have not been previously accepted as appropriate for the nature of the change to the product or for similar changes to other products designed by the applicant. Where the extent of new substantiation data necessary to comply with the applicable airworthiness requirements and the degree to which the original substantiation data has to be re-assessed and re-evaluated is considerable. The change alters the Airworthiness Limitations or the Operating Limitations. The change is made mandatory by an airworthiness directive or the change is the terminating action of an airworthiness directive (ref. 21A.3B). See note 1. Where the change introduces or affects functions where the failure effect is classified catastrophic or hazardous.
Note 1: The design change previously classified minor and approved prior to the airworthiness directive issuance decision needs no re-classification. However, the Agency retains the right to review the change and re-classify/re-approve if found necessary. Note 2: These above conditions are an explanation of the criteria noted in 21A.91.
For an understanding of how to apply the above conditions it is useful to take note of the examples given in Appendix A to GM 21A.91.
Appendix A to GM 21A.91: Examples of Major Changes per discipline The information below is intended to provide a few major change examples per discipline, resulting from application of 21A.91 and paragraph 3.3 conditions. It is not intended to present a comprehensive list of all major changes. Examples are categorised per discipline and are applicable to all products (aircraft, engines and propellers). However a particular change may involve more than one discipline, e.g., a change to engine controls may be covered in engines and systems (software). Those involved with classification should always be aware of the interaction between disciplines and the consequences this will have when assessing the effects of a change (i.e., operations and structures, systems and structures, systems and systems, etc.; see example in paragraph 2 (ii). Specific rules may exist which override the guidance of these examples. In the Part 21 a negative definition is given of minor changes only. However in the following list of examples it was preferred to give examples of major changes. Where in this list of examples the words has effect or affect(s) are used, they have always to be understood as being the opposite of no appreciable effect as in the definition of minor change in 21A.91. Strictly speaking the words has appreciable effect and appreciably affect(s) should have been used, but this has not been done to improve readability. 1 Structure (i) (ii) (iii) changes such as a cargo door cut-out, fuselage plugs, change of dihedral, addition of floats; changes to materials, processes or methods of manufacture of primary structural elements, such as spars, frames and critical parts; changes that adversely affect fatigue or damage tolerance or life limit characteristics; changes that adversely affect aeroelastic characteristics.
changes which introduce a new cabin layout of sufficient change to require a reassessment of emergency evacuation capability or which adversely affect other aspects of passenger or crew safety. Items to consider include, but are not limited to, : changes to or introduction of dynamically tested seats. change to the pitch between seat rows. change of distance between seat and adjacent obstacle like a divider. changes to cabin lay outs that affect evacuation path or access to exits. installation of new galleys, toilets, wardrobes, etc. installation of new type of electrically powered galley insert.
changes to the pressurisation control system which adversely affect previously approved limitations.
Flight Changes which adversely affect the approved performance, such as high altitude operation, brake changes that affect braking performance.
Changes which adversely affect the flight envelope. Changes which adversely affect the handling qualities of the product including changes to the flight controls function (gains adjustments, functional modification to software) or changes to the flight protection or warning system.
Systems For systems assessed under CS 25.1309, the classification process is based on the functional aspects of the change and its potential effects on safety. (i) Where failure effect is 'Catastrophic' or 'Hazardous', the change should be classified as major.
Where failure effect is 'major', the change should be classified as major if: aspects of the compliance demonstration use means that have not been previously accepted for the nature of the change to the system; or the change affects the pilot/system interface (displays, controls, approved procedures); or the change introduces new types of functions/systems such as GPS primary, TCAS, Predictive windshear, HUD.
The assessment of the criteria for software changes to systems also needs to be performed. When software is involved, account should be taken also of the following guidelines: Where a change is made to software produced in accordance with the guidelines of EUROCAE ED12B/RTCA DO-178B "Software Considerations in Airborne Systems and Equipment Certification", the change should be classified as major if either of the following apply, and the failure effect is Catastrophic, Hazardous or Major: (1) the executable code for software, determined to be Level A or Level B in accordance with the guidelines, is changed unless that change involves only a variation of a parameter value within a range already verified for the previous certification standard; or the software is upgraded to or downgraded from Level A, Level B or Level C; or the executable code, determined to be level C, is deeply changed, e.g., after a software reengineering process accompanying a change of processor.
For software developed to guidelines other than ED-12B/DO-178B, the applicant should assess changes in accordance with the foregoing principles. For other codes the principles noted above may be used. However, due consideration should be given to specific requirements/interpretations.
Propellers Changes to: (i) (ii) diameter airfoil
planform material blade retention system, etc.
Engines Changes:
that adversely affect operating speeds, temperatures, and other limitations. that affect or introduce parts identified by CS E-510 where the failure effect has been shown to be hazardous. that affect or introduce engine critical parts (CS E-515) or their life limits. to a structural part which requires a resubstantiation of the fatigue and static load determination used during certification. to any part of the engine which adversely affects the existing containment capability of the structure. that adversely affect the fuel, oil and air systems, which alter the method of operation, or require reinvestigation against the type-certification basis. that introduce new materials or processes, particularly on critical components.
Rotors and drive systems Changes that: (i) adversely affect fatigue evaluation unless the service life or inspection interval are unchanged. This includes changes to materials, processes or methods of manufacture of parts, such as rotor blades rotor hubs including dampers and controls gears drive shafts couplings
affect systems the failure of which may have hazardous or catastrophic effects. The design assessment will include: cooling system lubrication system rotor controls
(iii) adversely affect the results of the rotor drive system endurance test, the rotor drive system being defined in CS 27/29-917.
(iv) 931.
adversely affect the results of the shafting critical speed analysis required by CS 27/29-
Environment A change that introduces an increase in noise or emissions.
Power plant Installation Changes which include: (i) (ii) control system changes which affect the engine/propeller/airframe interface; new instrumentation displaying operating limits; modifications to the fuel system and tanks (number, size and configuration); change of engine/propeller type.
Change in Type Design
Classification of Design Change acc. 21A.91 Goals: - determine approval route - assess effect on airworthiness
Any of 21A.91 following criteria met ? - appreciable effect on weight - appreciable efect on balance - appreciable effect on structural strength - appreciable effect on reliability - appreciable effect on operational characteristics of the product
no Any of following criteria met ? (i) adjustment of certification basis (ii) new interpretation of the requirements used for the TC basis (iii) aspects of compliance demonstration not previously accepted (iv) extent of new substantiation data and degree of reassessment and reevaluation considerable (v) alters the limitations directly approved by the Agency (vi) mandated by AD or terminating action of AD (vii) introduces or affects function where failure condition is catastrophic or hazardous
Wherever there is doubt as to the classification of a change, the Agency should be consulted for clarification
See also Appendix A : Examples: 1. Structure 2. Cabin Safety 3. Flight 4. Systems 5. Propellers 6. Engines 7. Rotors and Drive Systems 8.Environment 9. Powerplant Instalation
Agency decides classification
Request for reclassification yes
Any good reason to reclassify minor ?
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