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Major Alteration or STC ?

Major Alteration or STC ?

Major Alteration or STC ?

Coming from the EASA world in terms of certification activity, I have trouble understanding the difference between major alterations and STCs (EASA major mod).

I thought at first that a major alteration had the same meaning as an STC and thus that the process was the same (approval by a DER or ODA mandatory).

However, when I opened the 8110-3 form, I realized that there was a distinction between the two STC or Major Alteration.

Can you explain to me the difference in process in terms of certification/engineering?

Is form 337 applicable to Major Alteration and STC?

A Major Alteration is a modification not in the aircraft, aircraft engine or propeller specifications.
(1) likely to materially affect the weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or
(2) not performed in accordance with accepted practices or not capable of being performed by elementary operations.

A supplemental type certificate ( STC ) is a type certificate ( TC ) issued when an applicant has received FAA approval to modify an aeronautical product from its original design.

Thank you very much

RE: Major Alteration or STC ?

Looks like you have done some research, which will help to fend off the trolls.

This document delineates what MUST be STC'd:


There is still a lot of art in "interpreting" this document (only the FAA is allowed to interpret).

Major Alterations only require data approvals from the applicable disciplines (electrical systems, power plant, structures, etc.). STC's require an application, Certification Plan, conformity inspections, witnessed tests - all in all much more drama. Especially if Issue Papers are involved.

ODA's can also get approval to do Major Alterations, but some make it just about as burdensome as an STC.

A 337 is filled out to return an individual airplane to service, and would list the Major Alteration data or the STC installed. Or both, if there are deviations to the STC treated as Major Alterations. The 337 is also used or a Field Approval - something you'll never see in the EASA world. For Field Approvals, an FAA Inspector approves the modification (what could go wrong with that?), sometimes using DER-approved data or consulting with an ACO.

Your definitions are spot-on. Also be aware that Part 43 Major-Minor is different than 21.93 Major-Minor.

STC's are different than TC's, perhaps the distinction you seek is that after the TC is issued, the Manufacturer can apply for an Amended TC or STC, any other party has to do an STC.

Finally - DER's use form 8110-3, ODA's (and their minions) use form 8100-9.

RE: Major Alteration or STC ?

Not quite on topic, but in response to this:

Quote (der8110)

ODA's can also get approval to do Major Alterations, but some make it just about as burdensome as an STC.

ODAs can also be granted the authority to approve STCs. And I would rather have it the way you describe than the opposite... I have known ODAs who treat approving an STC with far too little scrutiny.

You can see the ODA list and their authorities here:


I'm not going to name any names but I have seen companies made ODAs who had absolutely no business being so. I have seen ODA analysis supporting STCs which is literally garbage.

Not to be too bleak, but ODAs are granted by ACO, and the relationships between ODAs and ACOs can be HIGHLY political. ODA status does NOT indicate expertise or ability. I have seen a lot of politically motivated authority, and even ethically questionable authority granted (a company who is an ODA owns several STCs for a certain modification... the parent company that owns the ODA company manufactures a direct competitor to this modification. Basically the FAA granted ODA authority to a company holding STCs of their direct competitor...weird).

So yeah, just be careful out there.

Keep em' Flying
//Fight Corrosion!

RE: Major Alteration or STC ?

Hi @der8110,

Thank you very much for your answer.
After reading the document "Major Repair and Alteration Data Approval Online Job Aid" as well as "8300.16A" I think I am even more lost ahah

In the chronological order when a modification must be carried out, I understand that the AME will establish an application process with the FAA (FSDO) to determine the classification of the modification here we are always at the level of the part 43. I understand that there are two levels of Major Alteration, the EVL level and the ENG level.
- The EVL level requires the involvement of an ASI
- ENG level which requires the involvement of a DER

These two levels are managed via form 337 in which the 8110-3 of DERs can be mentioned in the context of an alteration classified as "ENG".

If the modification is considered to be an STC then it is the ACO that will take over and not the FSDO (maintenance). And the STC process will be launched with either a DER group (8110-3) or an ODA (8100-9) and thus the classification will be in accordance with Part ยง21.93. Part 21.93

Am I completely wrong or is my understanding close to reality ?

RE: Major Alteration or STC ?

On a similar 'vein', I've seen a recent Boeing Service Letter dedicated to elements close-to Your inquiry... specifically regarding repair-level classification

707-SL-51-028 757-SL-51-021
727-SL-51-040 767-SL-51-035
737-SL-51-041 777-SL-51-012

Some useful Info associated with the SL...

b) FEDERAL AVIATION REGULATION (FAR) 43, APPENDIX A, Major Alterations, Major Repairs and Preventative Maintenance
c) FAA ADVISORY CIRCULAR 120-77, Maintenance and Alteration Data
d) EUROPEAN AVIATION SAFETY AGENCY (EASA) REGULATION 21A.91, Classification of changes in type design
e) EASA GUIDANCE MATERIAL (GM) 21A.435(a), Classification of repairs

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

RE: Major Alteration or STC ?

Voodoo, you are a quick learner! One detail I did not mention before and you found, is that it is whoever returns the airplane to service - IA, Repair Station - or "AME" as in EASA terminology, determines whether a Repair or Alteration is Major or Minor. Another "what could go wrong with that" scenario but it does work for the most part. In most cases they will defer to the engineer's opinion (DER, or not), a few will make it very clear they are in charge, recently even had an ASI approve what was a very involved alteration without any external input, just to throw their weight around. A good mod, yet another example of "they know not what they do".

Yes you will drive yourself nuts with the Job Aid. There are items in there directly next to each other which have been used to disallow a modification, despite very clear language that describes and permits. Never mind actual service history. Two issues: (1) the FAA has given up on consistency and standardization efforts, and (2) the document is maintained by Flight Standards so if there is ever an opportunity to play "topper" with an ACO or DER, that is what will happen.

For 21.93, that actually comes into play more so AFTER an STC is issued (or TC). Simple things like typo's, are approved under a method acceptable to the FAA, Major changes are approved by Amended TC, Amended STC, 8110-3 and so on. The key distinction being the difference between Type Design data and modification data. The same drawing could be both.

BTW, the FAA AIR (Certification) is reorganizing this week. If there is anything interesting out of it, it will be copied and pasted here.

RE: Major Alteration or STC ?

I don't think it makes sense to take a one size fits all approach to aviation. Theres the physics, theres the operating environment technical and business wise.

The processes are probably different because when you do something to one airplane operated for a specific purpose in a specific well defined operational environment you really have a smaller universe of well defined requirements to address

When you make a supplement to a type or multiple types the universe of physics and operational requirements and number of unknowns is much larger.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

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