It is not necessary to have permission from the manufacturer. For a major mod you need permission from the FAA by means of a DER/DAR. The manufacturer will do this for you, but it might be more expensive than finding a decent engineering company to do the mod for you.
As for major or minor, all flight critical structure is major (Flight controls, large skin repairs, spars, etc), As well as anything affected by an AD.
I know it's not much, but I hope this helps.
TB

Major/Minor is a continuous topic. Some SRM's have a decision tree chart to help you along to make the decision. Maintenance shops and reps will try to turn themselves inside out to keep it in the "minor" category, using existing approved data. Major repairs can be "Major activities" when it comes to getting them approved. If you visit and browse the FAA netsite you will find a number of presentations made on major-minor by different regions. Basically, if you don't have access to ALL data, including OEM, the decision of major-minor can be very, very difficult to make when you're an engineer, because according to 14CFR part 145, engineers apparently don't exist, so they don't have to listen to you, and this is not meant as a joke..

Hi Baja, if you are referring to minor/major modification in terms of incorporating new design, I believe you are referring to an alteration to the original type certificate. Thus, it is required per FAR 23 to obtain approval from the OEM. The only reason to obtain approval from the OEM is that they have all the loads on the parts. An alternative method to incorporating of an STC can be performed by reverse engineering substantiations for the new design and approval from independent DER/DAR (very costly).

I hope that helps.

Cheers
AEU

baja,

I am assuming you talking about modifying an aircraft that is already FAA type certificated. You do not need OEM permission, you need FAA approval. The following is from 14 C.F.R. 1.1 definitions:

Major alteration means an alteration not listed in the aircraft, aircraft engine, or propeller specifications--
(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or
(2) That is not done according to accepted practices or cannot be done by elementary operations.

Minor alteration means an alteration other than a major alteration.

For more detailed information try the following link to an FAA website:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/CurrentFARPart/AC9BED30F1D032B9852566AB006BC89C?OpenDocument

There are some grey areas and individual interpretation on whether an alteration is major/minor. A simple approach would be to speak with your local FAA FSDO (Flight Standards District Office)and discuss your modification. If your modification is simple, you/your company can normally put together a data package that is acceptable to the FAA. More complicated alterations can require substantial amounts of data. That is where a helpful OEM can/might provide data that the FAA will most likely accept as part of your data package. A DER can also be used to supply data that the FAA can choose to accept.

And of course in order to be allowed by the FAA to alter an aircraft you have to be an FAA certified repair station, airframe/powerplant mechanic or manufacturer depending on the type of alteration you are performing.

Good luck,

jstolp

A good understanding of this is available from the powerpoint presentation available from this link
http://www1.faa.gov/fsdo/orl/files/pp/mrma.ppt

I hope that this assists
Ian