3.1 Introduction
3.1.1 This paragraph should be read in close conjunction with Part III, Chapter 1 “Type Certification”. In particular, all of the certification aspects (design requirements and CAA approval)for modifications and repairs are dealt with in Part III.
3.1.2 All modifications and repairs must comply with airworthiness requirements acceptable to the State of Registry which must approve these modifications and repairs. Procedures must be established to ensure that the substantiating data supporting compliance with the airworthiness requirements are retained. In this regard the structural repair manual (SRM) of the manufacturer of the aeronautical product is approved by the State of Design directly or by delegation. Repairs incorporated in accordance with such a manual may be deemed to be in accordance with approved data.
3.1.3 The objective of this part is to provide guidance to aircraft operators by setting out acceptable means for showing that modifications and repairs to aircraft comply with appropriate airworthiness requirements. Guidance is also provided concerning acceptable procedures for retention of substantiating data supporting compliance with the airworthiness requirements.
3.1.4 The information in this part applies to all types and classes of aircraft for which a Type Certificate or equivalent document has been issued and includes all components of the aircraft.
3.2 Compatibility of modifications and repairs
3.2.1. Introduction
3.2.1.1 When any modification or repair is installed on an aircraft, care should be taken to ensure that it is compatible with all other design changes installed on that aircraft. Modifications or repairs designed separately may conflict or interfere with each other, despite having been individually shown to comply with all applicable standards of airworthiness. Interaction between different modifications or repairs may be of a physical, aerodynamic, structural or fatigue strength, electromagnetic or any other nature. Such interaction may jeopardize the airworthiness of the aircraft.
3.2.1.2 An example of potential incompatibility would be a repair installed in close proximity to an existing repair. While the two repairs individually may be completely satisfactory if separately installed on an aircraft, the combination in close proximity may introduce additional stress concentrations which cause fatigue cracks to occur after a period of time in service. The designer of a repair scheme should survey the aircraft to be repaired to establish whether there are any other design changes in the vicinity which may interfere. In the case of an existing repair in close proximity to the new damage, it may be necessary to remove the old repair and install a new repair encompassing both damaged areas, designed in a manner to reduce any stress concentrations to a level that will not produce fatigue cracking.
3.2.1.3 In a more general situation, modifications may be separately designed for the same basic aircraft type by different organizations with no knowledge of the other’s work. The modifications may be shown separately to comply with all applicable airworthiness standards; however, they may physically interfere with each other. Alternatively, no problems may be encountered with the installations, but it may be found in service that the combination causes aerodynamic buffeting, stability or control problems, fatigue cracking, structural failure, electromagnetic interference, or other problems. If the concurrent installations of different modifications are not rigorously assessed for compatibility, there exists the possibility that in combination they may cause serious airworthiness hazards.
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