How does a mechanic determine if a repair or alteration requires a Supplemental Type Certificate? How does a mechanic know when they have exceeded what 14 CFR Part 43 allows for a repair or alteration to an aircraft? They have thus entered an area where the repair/alteration is so significant that the Type Design has been changed, requiring a Supplemental Type Certificate (STC). Do aircraft maintenance manuals give the mechanic some limits to what he can do, or is AC 21.101 all they have to go on?
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.