As an aeronautical engineer I am often asked about the correct approval process for a modification i.e. approval under CAR 35, or going the whole hog and pursuing a Supplemental Type Certificate (STC) for the modification.
To this question I normally and simplistically answer that for a modification that does not affect the basic type design as defined by the Type Certificate (TC) and the corresponding Type Certificate Data Sheet (TCDS) a CAR 35 approval may be appropriate, albeit with several caveats.
CASA have issued guidelines regarding the issue of STCs, namely Advisory Circular AC 21.15. The Advisory Circular states that the design of many major modifications can be approved under CAR 35, however, CASA will require the applicant to apply for the issue of an STC in the following circumstances:
(a) where there is any variation or addition to the type design limitations and conditions contained in the TCDS, or the airworthiness limitations; or
(b) where there is any change outside the type design limitations and conditions contained in the approved sections of the Aircraft Flight Manual (AFM).
A major change to a Type Certificate is defined under CASR 21.93 as one that has an appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the aircraft, aircraft engine or propeller. Appendix 1 of the Advisory Circular provides more detailed information and provides a list of what modifications may constitute major modifications and anyone interested should refer to that document, the list is too extensive to reproduce here.
There are, of course, other considerations, some of them commercial rather than regulatory. For instance CAR 35 is a unique Australian regulation that does not have a real equivalent overseas, consequently a modification that has been approved to CAR 35 in Australia will almost certainly not be recognised overseas. If your intention is to sell your modification to the broader market overseas you will need an STC! This will not necessarily guarantee acceptance of the modification by a foreign National Airworthiness Authority (NAA) as many overseas airworthiness authorities do not have a bilateral agreement with Australia that provides for automatic acceptance of STCs issued by CASA. Nevertheless, the approval will be much easier with an STC than without one. Furthermore, CASA will facilitate a request by the holder of an Australian STC for acceptance of that STC by a foreign NAA.
The bottom line is, if the modification is not a modification for which an STC is mandatory, is relatively simple, is only intended to be incorporated on one or two aircraft and is not intended to be made available generally, particularly overseas, a CAR 35 approval may be appropriate.
So what is actually involved in getting an STC? CASR 21.113 states that any person may apply to CASA for an STC but, in reality, the process is somewhat different. A realistic approach would be to seek the assistance of a CAR 35 authorised person in the first instance. The STC process is quite involved and, unlike a normal ‘CAR 35′ approval where you usually only have to deal with the CAR 35 person, the STC process will involve regular consultation with CASA and the active participation of them (this will of course incur CASA ‘fees’). In most instances the appropriate person to deal with CASA is the CAR 35 person as most dealings and discussions with CASA will be technical in nature.
Following the initial application a certification plan must be developed. This plan must be agreed to by CASA and must include items to be considered such as:
• Project description – category, operational requirements etc;
• Product description – product, type and S/Nos to be modified, basic specifications;
• Certification Basis – proposed design standard.
• Who will be nominated to find compliance – CAR 35 authorised person?
• Means of showing compliance – test, formal analysis, inspection?
• Communication with CASA and co-ordination;
• Certification Documentation – test plans;
• Document control;
• Conformity Plan – controlled drawings, inspection to drawings, test equipment;
• Prototype production control;
• Noise – impact (if any) on the noise certification of the aircraft;
• Schedule – plan for when will it all happen.
The process of gaining the STC is then one of following the plan, negotiating with CASA over any points of contention or where any clarification is required. Provided all goes to plan (and it should if the plan is comprehensive enough) and CASA is happy with the outcome, your STC should be issued by CASA as expected.
Just remember, don’t underestimate the complexity of the STC process. Whilst the benefits of being able to freely sell your product are obvious, gaining an STC can be a rather protracted and expensive exercise – not for the faint hearted!