How does a mechanic determine if a repair or alteration requires a Supplemental Type Certificate?
Repairs and alterations are defined at FAR 1.1, FAR 43 App(A)(a)(1-4), FAR 43 App(A)(b)(1-4)

&quot;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.&quot;
Minor alteration means an alteration other than a major alteration.
Major repair means a repair: (1) That, if improperly done, 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 repair means a repair other than a major repair.

In the US, to be airworthy (FAR 21.183) all aircraft must &quot;conform to type design and condition for safe flight&quot;. To &quot;conforms to type design&quot; there must be some type of approved authorization for every piece on the airplane. There are several ways to do this.

The manufacture may have pre-approved the equipment you wish to install. Under CAR 1,2, &amp; 3 (~prior to 1958) regulations a separate document for the aircraft called a &quot;data sheet&quot; showed approved equipment. After 1958 aircraft where certified under FAR's and used a &quot;Equipment List&quot;. The &quot;Equipment List&quot; for light aircraft is usually found at the back of the Pilot Operating Handbook (POH). An example for a 1978 C152 is shown below.



If a manufacture did not approve the equipment than it may be installed under the authority of a &quot;STC&quot; (equipment pre-approved by the FAA).

...or an &quot;alteration&quot; that a AI (Authorized Inspector) gets approval from the local FAA FSDO office by filling out Form 337 and have it field approved.


The owner is required to keep a copy of either a &quot;STC&quot; or Form 337 &quot;alteration&quot; to show the equipment is legal for each annual inspection. If an aircraft has equipment installed that is not authorized by a Data Sheet, Equipment List, STC, or Form 337, the aircraft is un-airworthy and illegal to fly.
There are three small exceptions:

The FAA provides an alternate approval for small common parts (FAR 21.303-4 and Court Order 8110-42A)
such as wheel bearings, chemicals i.e de-iceing fluid, etc via a &quot;PMA&quot; authorization.

The FAA approves replacement of some parts that meet industry
standards (AC 20-62e) without need of authorization for parts such as sheet metal,
electrical components, minor hardware, etc.

In certain circumstances (FAR 21.303-2 and AC 20-62e) the owner may have a part built or manufactured that is no-longer available for purchase. This provision is often used for old bi-planes and war birds.