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First post. Yes, I did search but this specific question has not been asked before. This answer is close, however:

How does a mechanic determine if a repair or alteration requires a Supplemental Type Certificate?

I do not own an aircraft and do not have a PPL. BUT, I love the Piaggio P180. This question pertains to the difference between getting a new Type Certificate and getting a STC (Supplemental Type Certificate). So, which of these alterations would require a new TC of the aircraft vs a STC?

  1. Lengthening or shortening the fuselage?
  2. Changing the engine(s) from say piston to turbine or turbine to electric fans?
  3. Adding extra components to the airframe, like batteries, controllers, etc. to make a hybrid aircraft?
  4. Changing the wings to newer ones from another certified aircraft (re-winging)?
  5. Changing the landing gear to a newer model with higher load capabilities?
  6. Expanding the fuselage diameter by 10%?

Would implementing one of these changes require STCs or after a certain number of alterations, does the FAA require a new TC?

I'm really trying to understand when STCs are used and when the FAA requires a new Type Certificate.

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  • $\begingroup$ Are you doing these modifications to sell kits or upgrades, or just modifying your personal aircraft? If it's personal, it will probably just require a "337", if you want to do it as a business, it needs an STC. $\endgroup$ – Ron Beyer Oct 16 at 3:48
  • $\begingroup$ @Ron, changes would be for personal use. I never heard of a "337". Will read up on it. Thank you. $\endgroup$ – Paranoidone Oct 16 at 9:04
  • $\begingroup$ Forgot to add that the aircraft would be flown under Part 23. $\endgroup$ – Paranoidone Oct 16 at 9:06
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Since this is an aircraft for personal use, is under 19,000lbs gross weight, and less than 19 seats it will fall under Part 23.

If you don't plan on selling "kits" to do these modifications, then an STC is not required. STC's are only for modifications intended for sale to aircraft for installation by an A&P mechanic. STC's are required not for the kit itself, but for each aircraft type (and sometimes variant) that it is to be installed on. This is why the STC process can be very expensive.

You would not need an entirely new type certificate either, again which would be required if you were doing these modifications as a business for sale as another aircraft type, or if the original manufacturer modified the aircraft in some significant way.

What you would need in almost all the cases you described above is an FAA Form 337, which describes "major repair & alterations to airframe or power plant". This document will require you (and your A&P) to make detailed descriptions and drawings of the modification and submit it to an FSDO. The FSDO will review the modifications and approve or disapprove of the changes. It is advisable to work with the FSDO from the beginning, not the end.


So, which of these alterations would require a new TC of the aircraft vs a STC?

  • Lengthening or shortening the fuselage?
  • Changing the engine(s) from say piston to turbine or turbine to electric fans?
  • Adding extra components to the airframe, like batteries, controllers, etc. to make a hybrid aircraft?
  • Changing the wings to newer ones from another certified aircraft (re-winging)?
  • Changing the landing gear to a newer model with higher load capabilities?
  • Expanding the fuselage diameter by 10%?

None of them require a new TC or STC, however your first question would probably be difficult at best. The second question doesn't make a lot of sense for the Piaggio since it already has turbine engines, and there are no electric fans of the size that would be required. Changing the wing or modifying the landing gear are pretty common things, usually in the form of STOL kits or FOD deflectors. Some of them would require extensive testing from the FAA before issuing the airworthiness paperwork.

The last one though will nearly be impossible. It would be a lot cheaper to just buy an aircraft that fit your needs, like the Pilatus PC-24 than try to make that modification.

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To start, I would suggest a read through 14 CFR Part 21.

But to summarize: a TC is necessary to manufacture a new aircraft, so TCs are the property of the manufacturer of the aircraft. The TC confers the following privileges:

§21.45 Privileges.

The holder or licensee of a type certificate for a product may—

(a) In the case of aircraft, upon compliance with §§21.173 through 21.189, obtain airworthiness certificates;

(b) In the case of aircraft engines or propellers, obtain approval for installation on certificated aircraft;

(c) In the case of any product, upon compliance with subpart G of this part, obtain a production certificate for the type certificated product;

(d) Obtain approval of replacement parts for that product.

[Doc. No. 5085, 29 FR 14564, Oct. 24, 1964, as amended by Amdt. 21-92, 74 FR 53386, Oct. 16, 2009]

OTOH, An STC can be obtained by anyone including the TC holder. The big difference is that it is a 'supplement' to a TC. That is, it modifies the design specified by the TC.

So we can look to the following section of Part 21:

§21.113 Requirement for supplemental type certificate.

(a) If a person holds the TC for a product and alters that product by introducing a major change in type design that does not require an application for a new TC under §21.19, that person must apply to the FAA either for an STC, or to amend the original type certificate under subpart D of this part.

(b) If a person does not hold the TC for a product and alters that product by introducing a major change in type design that does not require an application for a new TC under §21.19, that person must apply to the FAA for an STC.

(c) The application for an STC must be made in the form and manner prescribed by the FAA.

[Doc. No. FAA-2006-25877, Amdt. 21-92, 74 FR 53387, Oct. 16, 2009; Doc. No. FAA-2018-0119, Amdt. 21-101, 83 FR 9169, Mar. 5, 2018]

As stated here, an STC applies to any major modification that doesn't require an new TC under §21.19 which states:

§21.19 Changes requiring a new type certificate.

Each person who proposes to change a product must apply for a new type certificate if the FAA finds that the proposed change in design, power, thrust, or weight is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.

[Doc. No. 28903, 65 FR 36265, June 7, 2000]

So your question as to TC vs. STC really only applies to the manufacturer. And as stated here, it's at the discretion of the FAA. But as a point of reference, every Boeing 737 ever built has been built under TC Data Sheet A16WE (currently Rev 64).

Typically manufacturers will continue to modify their TC to support continuing production. They will use an STC when the changes are to modify previously manufactured aircraft.

Third parties that modify aircraft will also get STCs. A large percentage of STCs are obtained by third parties. The STCs only cover the changes from the aircraft's existing certification basis (which is the original TC plus any previously installed STCs.) An STC may be for a single aircraft, a single aircraft type, or for multiple types (an "Approved Model List STC" or AML STC). The last is common for general aviation avionics.

So back to your original list of mods. All of them are major mods and would require at least an STC, though the last one would likely require an modification of the TC if not a new one. Replacing the entire fuselage is a huge change. If Piaggio wanted to make those changes to their design going forward, they would likely modify their existing TC. If the FAA decreed the modification was extreme enough to warrant a new TC, they would have to apply for a new TC.

If someone other than Piaggio wants to make the mods, they would have to get an STC and use it to modify an aircraft that was manufactured by Piaggio. Once the STC is obtained, the STC holder has the following privileges:

§21.119 Privileges.

The holder of a supplemental type certificate may—

(a) In the case of aircraft, obtain airworthiness certificates;

(b) In the case of other products, obtain approval for installation on certificated aircraft; and

(c) Obtain a production certificate in accordance with the requirements of subpart G of this part for the change in the type design approved by the supplemental type certificate.

[Doc. No. 5085, 29 FR 14568, Oct. 24, 1964, as amended by Amdt. 21-92, 74 FR 53387, Oct. 16, 2009]

One advantage to an STC, is that the owner can sell the rights to use the STC to another. So the following 'permission' typically involves the exchange of money:

§21.120 Responsibility of supplemental type certificate holders to provide written permission for alterations.

A supplemental type certificate holder who allows a person to use the supplemental type certificate to alter an aircraft, aircraft engine, or propeller must provide that person with written permission acceptable to the FAA.

[Doc. No. FAA-2003-14825, 71 FR 52258, Sept. 1, 2006]

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