Appendix A to GM 21.A.91 Examples of Major Changes per discipline

ED Decision 2021/007/R

The information below is intended to provide a few major change examples per discipline, resulting from application of 21.A.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 21.A.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)      changes such as a cargo door cut-out, fuselage plugs, change of dihedral, addition of floats;

(ii)      changes to materials, processes or methods of manufacture of primary structural elements, such as spars, frames and critical parts;

(iii)     changes that adversely affect fatigue or damage tolerance or life limit characteristics;

(iv)     changes that adversely affect aeroelastic characteristics.

2.       Cabin Safety

(i)      changes which introduce a new cabin layout of sufficient change to require a re-assessment 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.

(ii)      changes to the pressurisation control system which adversely affect previously approved limitations.

3.       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.

4.       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.

(ii)      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 the latest edition of AMC 20-115 (see AMC-20 document) the change should be classified as major if either of the following apply, and the failure effect is Catastrophic, Hazardous or Major:

(i)      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

(ii)      the software is upgraded to or downgraded from Level A, Level B or Level C; or

(iii)     the executable code, determined to be level C, is deeply changed, e.g., after a software re-engineering process accompanying a change of processor.

For software developed to guidelines other than the latest edition of AMC 20-115, 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 certification specifications/interpretations.

In the context of a product information security risk assessment (PISRA), a change that may introduce the potential for unauthorised electronic access to product systems should be considered to be ‘major’ if there is a need to mitigate the risks for an identified unsafe condition. The following examples do not provide a complete list of conditions to classify a modification as major, but rather they present the general interactions between security domains. Examples of modifications that should be classified as ‘major’ are when any of the following changes occur:

—         A new digital communication means, logical or physical, is established between a more closed, controlled information security domain, and a more open, less controlled security domain.

—    For example, in the context of large aircraft, a communication means is established between the aircraft control domain (ACD) and the airline information services domain (AISD), or between the AISD and the passenger information and entertainment services domain (PIESD) (see ARINC 811).

As an exception, new simplex digital communication means (e.g. ARINC 429) from a controlled domain to a more open domain is not considered as major modification, if it has been verified that the simplex control cannot be reversed by any known intentional unauthorised electronic interaction (IUEI).

—    A new service is introduced between a system of a more closed, controlled information security domain and a system of a more open, less controlled security domain, which allows the exploitation of a vulnerability of the service that has been introduced, creating a new attack path.

For example:

—    opening and listening on a User Datagram Protocol (UDP) port in an end system of an already certified topology;

—    activating a protocol in a point-to-point communication channel.

—    The modification of a service between a system of a more closed, controlled security domain and a system of a more open, less controlled security domain.

—    The modification of a security control between a system of a more closed, controlled information security domain and a system of a more open, less controlled security domain.

5.       Propellers

Changes to:

—    diameter

—    airfoil

—    planform

—    material

—    blade retention system, etc.

6.       Engines

Changes:

(i)      that adversely affect operating speeds, temperatures, and other limitations.

(ii)      that affect or introduce parts identified by CS E-510 where the failure effect has been shown to be hazardous.

(iii)     that affect or introduce engine critical parts (CS E-515) or their life limits.

(iv)     to a structural part which requires a re-substantiation of the fatigue and static load determination used during certification.

(v)      to any part of the engine which adversely affects the existing  containment capability of the structure.

(vi)     that adversely affect the fuel, oil and air systems, which alter the method of operation, or require reinvestigation against the type-certification basis.

(vii)    that introduce new materials or processes, particularly on critical components.

7.       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

(ii)      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)     adversely affect the results of the shafting critical speed analysis required by CS 27/29.931.

8.       Environment

The introductory text to Appendix A to GM 21.A.91 describes how in Part 21 a negative definition is given of minor changes only. This philosophy is similar to the manner in which the ICAO Standards and Recommended Practices for environmental protection (ICAO Annex 16) and the associated Guidance Material (ICAO Environmental Technical Manual) define changes affecting a product’s environmental characteristics in terms of ‘no-acoustical changes’, ‘no-emissions changes’ and ‘no-CO₂ changes’ (i.e. changes which do not appreciably affect the product’s environmental characteristics).

Following the general philosophy of this Appendix, however, it is preferred to give examples of changes which might have an appreciable effect on a product’s environmental characteristics (i.e. the effect might be greater than the no-acoustic change, no-emissions change and no-CO₂ change criteria) and might therefore lead to a ‘major change’ classification.

Where a change is made to an aircraft or aircraft engine, the effect of the change on the product’s environmental characteristics should be taken into account. Examples of changes that might have an appreciable effect on the product’s environmental characteristics, and might therefore be classified as major changes, are listed below. The examples are not exhaustive and will not, in every case, result in an appreciable change to the product’s environmental characteristics, and therefore, will not per se and in every case result in a ‘major change’ classification.

An appreciable effect is considered to be one which exceeds the ICAO criteria for a no-acoustical change, a no-emissions change or a no-CO₂ change. For the definition of a no-acoustical change refer to the section of the ICAO Environmental Technical Manual, Volume I (ICAO Doc 9501, Volume I – Procedures for the Noise Certification of Aircraft) concerning changes to aircraft type designs involving no-acoustical changes (see also the definitions of a ‘derived version’ in ICAO Annex 16, Volume I). For the definition of a no-emissions change, refer to the section of the ICAO Environmental Technical Manual, Volume II (ICAO Doc 9501, Volume II – Procedures for the Emissions Certification of Aircraft Engines) concerning no-emissions changes. For the definition of a no-CO₂ change, refer to ICAO Doc 9501 ‘Environmental Technical Manual’, Volume III ‘Procedures for the CO₂ Emissions Certification of Aeroplanes’, 1st Edition 2018, concerning no-CO₂ changes.

(i)      Noise: A change that introduces either:

—       an increase in the noise certification level(s); or

—       a reduction in the noise certification level(s) for which the applicant wishes to take credit.

Examples of noise-related changes that might lead to a major change classification are:

(1)     For jet and heavy (maximum take-off mass greater than 8 618 kg) propeller-driven aeroplanes:

—    A change that might affect the aircraft’s take-off performance including:

—    a change to the maximum take-off mass;

—    a change to V2 (‘take-off safety speed’); or

—    a change to the lift augmentation devices, including their configuration under normal take-off operating conditions.

—    A change that might affect the aircraft’s landing performance including:

—    a change to the maximum landing mass;

—    a change to VREF (reference landing speed); or

—    a change to the lift augmentation devices, including their  deployment under normal landing operating conditions.

—    A change to the Centre of Gravity (CG) limits;

—    A change that increases the aircraft’s drag;

—    A change that alters the external profile of the aircraft, including the installation or change of shape or size of any item on the external surface of the aircraft that might protrude into the airflow such as winglets and vortex generators; generally the installation of small antennas does not represent an acoustical change;

—    A change that introduces an open-ended hollow cavity at more or less right angles to the airflow (e.g. hollow pins in undercarriage assemblies);

—    A change of engine or, if fitted, propeller type;

—    A change in engine thrust rating;

—    A change to the engine rotating parts or stators, such as geometry, blade profile or blade number;

—    A change to the aerodynamic flow lines through the engine;

—    A change that affects the engine thermodynamic cycle, including a change to the engine’s bypass ratio;

—    A change to the engine nacelle, including a change to the acoustic liners;

—    A change to the engine exhaust;

—    A change to the engine bleed valves, including bleed valve scheduling;

—    A change in the operation of engine power off-takes (e.g. the operation of the Environmental Control System (ECS) during a normal take-off or approach);

—    A change to the Auxiliary Power Unit (APU), including associated operating limitations (e.g. a change that allows the APU to be operated during a normal approach when previously it was not allowed);

—    A change to the propeller pitch and/or propeller speed during a normal take-off or approach;

—    A change that causes a change to the angle at which air flows into the propeller.

(2)     For light (maximum take-off mass 8 618 kg or less) propeller-driven aeroplanes:

—    A change that might affect the aircraft’s take-off performance including:

—    a change to the maximum take-off mass;

—    a change to the take-off distance;

—    a change to the rate of climb; or

—    a change to Vy (best rate of climb speed).

—    A change that increases the aircraft’s drag (e.g. the installation of external cargo pods, external fuel tanks, larger tyres to a fixed undercarriage, floats etc.);

—    A change of engine or propeller type;

—    A change in take-off power including a change in engine speed (tachometer ‘red line’) or, for piston engines, a change to the manifold pressure limitations;

—    A change to the highest power in the normal operating range (‘top of green arc’);

—    In the case of an aircraft where take-off power/engine speed is time limited, a change in the period over which take-off power/engine speed may be applied;

—    A change to the engine inlet or exhaust including, if fitted, the inlet or exhaust muffler;

—    A change in propeller diameter, tip shape, blade thickness or the number of blades;

—    The installation of a variable or adjustable pitch propeller in place of a fixed pitch propeller and vice versa;

—    A change that causes a change to the angle at which air flows into the propeller.

(3)     For helicopters:

—    A change that might affect the take-off and/or landing performance, including a change in take-off mass and VY (best rate of climb speed);

—    A change to VNE (never-exceed airspeed) or to VH (airspeed in level flight obtained using the torque corresponding to minimum engine installed, maximum continuous power available for sea level pressure, 25°C ambient conditions at the relevant maximum certificated mass);

—    A change to the maximum take-off engine power or maximum continuous power;

—    A change to the gearbox torque limits;

—    A change of engine type;

—    A change to the engine intake or exhaust;

—    A change to the maximum normal operating rpm of the main or tail rotors;

—    A change to the main or tail rotors, including a change in diameter, blade thickness or blade tip profile.

Note: The effect on the helicopter’s noise characteristics of either carrying external loads or the installation of external equipment need not be considered.

(ii)      Emissions: A change that introduces an increase or decrease in the emissions certification levels. Examples of smoke and gaseous engine emission-related changes that might lead to a major change classification are:

—    A change in engine thrust rating;

—    A change to the aerodynamic flow lines through the engine;

—    A change that affects the engine thermodynamic cycle, specifically relevant engine cycle parameters (e.g. combustor pressure P3, combustor entry temperature T3, Air Fuel Ratio (AFR));

—    A change to the compressor that might influence the combustor inlet conditions and engine overall pressure ratio;

—    A change to the combustor design (geometry);

—    A change to the cooling of the combustor;

—    A change to the air mass flow through the combustor;

—    A change that affects the fuel spray characteristics.

(iii)     CO₂: a change that introduces either:

—    an increase in the CO₂ emissions certification level; or

—    a decrease in the CO₂ emissions certification level for which an applicant wishes to take credit.

Examples of CO₂ emission-related changes that may lead to a ‘major change’ classification are:

—    a change to the maximum take-off mass;

—    a change that may affect the aeroplane’s specific air range performance, including one or several of the following:

—    a change that increases the aircraft’s drag;

—    a change of engine or, if fitted, propeller type;

—    a change in the engine design that affects the engine specific fuel consumption in cruise.

—    a change to the aeroplane’s reference geometric factor (RGF).

9.       Power plant Installation

Changes which include:

(i)      control system changes which affect the engine/propeller/airframe interface;

(ii)      new instrumentation displaying operating limits;

(iii)     modifications to the fuel system and tanks (number, size and configuration);

(iv)     change of engine/propeller type.

10.     Stand-alone changes to non-ALS ICA that require additional work to demonstrate compliance with the applicable certification basis as follows:

(i)      changes related to accomplishment instructions (e.g. to the aircraft maintenance manual) related to the CDCCL, or the EWIS ICA, for which the technical content (e.g. gaps, steps) of the procedures is changed;

(ii)      the introduction of novel technology for inspection purposes related to an ALS task;

(iii)     changes that adversely affect the certification assumptions: e.g. some specific inspection procedures, such as inspection procedures for use after a hard landing, may include a decision-making chart based on the level of exceedance of the load in comparison with the certified limit loads; such criteria, and adverse changes, need to be agreed with EASA.

Classification Process