Appendix A to GM1 21L.A.103 Classification of changes to the design of an aircraft for which design compliance has been declared

EXAMPLES OF MAJOR CHANGES PER DISCIPLINE

The information below is intended to provide a few examples of major changes per discipline, resulting from the application of point 21L.A.103. 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, 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).

The persons that assess the change for its 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.).

Specific rules may exist which override the guidance of these examples.

In Part 21 Light, 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 should always be understood as being the opposite of ‘no appreciable effect’ as in the definition of minor change in point 21L.A.63. Strictly speaking, the phrase ‘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 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 reassessment of the emergency evacuation capability, or changes 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;

—       changes to cabin layouts that affect evacuation path or access to exits;

—       changes to the cabin area in striking distance of the occupant’s head or torso introducing potentially injurious objects.

3.       Flight

Changes which adversely affect the approved performance or 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 23.2510, the classification process is based on the functional aspects of the change and its potential effects on safety.

(i)      Where the failure effect is ‘catastrophic’ or ‘hazardous’, the change should be classified as ‘major’.

(ii)      Where the failure effect is ‘major’, the change should be classified as ‘major’ if:

—       aspects of the compliance demonstration will use a means that has not been previously utilised 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 should also 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 applies, 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 declarant should assess the 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.

5.       Propellers

Changes to:

(i)      diameter,

(ii)      aerofoil,

(iii)     planform,

(iv)     material,

(v)      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 resubstantiation of the fatigue and static load determination used during the original compliance demonstration;

(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 original detailed technical specifications;

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

7.       Noise

The examples provided below are not exhaustive and will not, in every case, result in an appreciable effect on the declared noise levels and, therefore, will not per se and in every case result in a major change classification.

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

(i)      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 V_Y (best rate of climb speed);

(ii)      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.);

(iii)     a change of engine or propeller type;

(iv)     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;

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

(vi)     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;

(vii)    a change to the engine inlet or exhaust including, if fitted, the inlet or exhaust muffler;

(viii)   a change in propeller diameter, tip shape, blade thickness or the number of blades;

(ix)     the installation of a variable or adjustable pitch propeller in place of a fixed pitch propeller and vice versa;

(x)      a change that causes a change to the angle at which air flows into the propeller.

8.       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.

9.       Stand-alone changes to non-ALS ICAs that require additional work to demonstrate compliance with the applicable detailed technical specifications as follows:

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

(ii)      changes that adversely affect the assumptions made during the original demonstration of compliance: 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 limit loads; such criteria, and adverse changes, should be taken into consideration.

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