Before installing something new on an aircraft, consider what it means for the aircraft’s safety – its airworthiness – and if the installation can be done in accordance with Civil Aviation Rules.

Design changes to aircraft require acceptable technical data and acceptable parts. A list of what is acceptable technical data can be found in Part 21 Appendix D. Rule 21.303 outlines the requirements for replacement and modification materials, parts and appliances.

Here is a diagram showing the process, rule references and options for modifying an aircraft: Modification and repair process diagram [PDF 238 KB] 

Data that is not listed in Part 21 Appendix D but has been accepted by the Director: List of technical data accepted by the Director under the provisions of CAR 21.503(a) [PDF 237 KB]

To apply for technical data such as an EASA STC to be accepted by the Director, email with the data and evidence of permission from the data owner (e.g. STC holder).

Who can approve a design change?

The Director has delegated the approval of minor design changes to individuals who are employed by Part 146 Design Organisations. Those design delegation holders may also approve one-off major design changes with prior authorisation from the Director.

A “Major Design Change” is defined in the Schedule One [PDF 745 KB] attachment to a Design Delegation.

It is different to a “Major Modification” as defined in rule part 1 which relates to the decision of whether an IA is required to sign conformity per part 43.

NZ STCs, which can be either minor or major, can only be approved by the CAA, but require a Statement of Compliance from a Part 146 Design Organisation. Advisory circular AC43-9 explains much of the differences between a modification and an STC, the main one being STCs can be sold as a package to be installed by another party but also that STCs typically encompass major design changes for a series of aircraft rather than a one-off. NZ STCs are also recognised by some foreign regulators which can be an advantage.

Use List of Part 146 Design Organisations to find an organisation who can approve design changes and support STCs.

Design delegation seminars and advice includes presentations given at seminars for design delegation holders and advice to design delegates and Part 146s.

While the CAA can approve minor modifications which do not require any flight testing or compliance inspections, there will be a minimum application period of 60 days before they are first reviewed. The actual time to review will vary but will in most cases will take 4-6 weeks at a minimum. CAA form 337 [PDF 402 KB] should be used to apply.

How long does it take to approve a design change?

Each modification will vary depending on the size and complexity of the change. Even small modifications may have a large effect on the aircraft handling, performance, electrical, structural and safety systems.

Also, the time may depend on resourcing at the available Part 146 design organisations. A Part 146 design organisation should be contacted early to ensure expectations of timing are managed.

Advisory Circular references:

AC43-9 Modifications, Repairs, and the Form CAA337

AC00-1 Acceptability of Parts

AC21-8 Design Changes - Supplemental Type Certificate

AC21-5 Approval of modifications covering aircraft ferry fuel systems and overweight operation

AC146-1 Aircraft design organisations