Good question, with no fully satisfactory answer from us, unfortunately. There appears to be no specific definition of the term “base maintenance”. The best attempt at an answer, crossing between multiple sources, follows.

First, from the CASR “Dictionary”.

“Base Maintenance, for a Part 145 organisation, means maintenance on an aircraft that CASA has approved in the organisation’s exposition as being base maintenance for the organisation.” (Reference: CASR Volume 5, Page 234). 

So, not much help there. Now let us turn to the Part 145 Manual of Standards. 

It states here that an AMO (Approved Maintenance Organisation) exposition must clearly describe its “line maintenance” and “base maintenance” capabilities. (Reference: Part 145 MOS, Page 2). Furthermore, Appendix II gives a list of maintenance tasks which a Category ‘A’ licence holder may hold perform (if given approval to do so by the AMO). (Reference: Part 145 MOS, Page 26)

Again, nothing specific here. Now, the Part 66 Manual of Standards.

The section on “privileges” outlines the respective authorities of Categories A, B and C licence holders. The section on category A licence holders refers to the above-referenced Appendix II from the Part 145 MOS. Furthermore, category C licence holders are associated directly with “base maintenance on large aircraft”. Conversely, category B licence holders can certify where maintenance is “not base maintenance on large aircraft”. (Reference: Part 66 MOS, Pages 6 to 8)

An attempt to draw some conclusions from the above points, although some assumptions are made.

1. Line maintenance comprises those tasks outlined in Appendix II from the Part 145 MOS. (Assumption)
2. Base Maintenance would therefore appear to be any maintenance over and above that mentioned in Appendix II from the Part 145 MOS. (Assumption)
3. Base maintenance and line maintenance are both undertaken by a Part 145 AMO. (Fact)
4. A large aircraft can be subject to either base maintenance or line maintenance. (Fact)
5. A category C licence holder is required to issue a CRS after base maintenance. (Fact)
6. A category A licence holder can issue a CRS after line maintenance. (Fact)
7. A small aircraft can be subject only to base maintenance. (Assumption)

See also the FAQ What is particularly different with a category ‘C’ Part 66 engineer’s licence in relation to a category ‘B’ licence?