I am hearing more and more questions about whether someone needs a supplemental type certificate (STC) in addition to a Part Manufacturer Approval (PMA) for their part.
In the context of independent replacement and modification parts, an STC is required when the part would introduce a major change in type design. The regulations define major change as anything that is not a minor change, so the definition of minor change becomes important to us:
A “minor change” is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product.
Let’s look at some key words in that definition, starting with the end of the definition: “product.” A product is defined as an aircraft, engine or propeller. So the change is defined based on its effect on the entire product. We’ve often seen disputes where someone believes that the change to the system or to the next higher assembly is major, but this doesn’t matter to the STC question. The question is whether the change has an appreciable effect on characteristics affecting the airworthiness of the complete aircraft, complete engine or complete propeller.
The change that is being tested is a change to type design, so it is also worth looking at the definition of type design:
The type design consists of—
(a) The drawings and specifications, and a listing of those drawings and specifications, necessary to define the configuration and the design features of the product shown to comply with the requirements of that part of this subchapter applicable to the product;
(b) Information on dimensions, materials, and processes necessary to define the structural strength of the product;
(c) The Airworthiness Limitations section of the Instructions for Continued Airworthiness as required by parts 23, 25, 26, 27, 29, 31, 33 and 35 of this subchapter, or as otherwise required by the FAA; and as specified in the applicable airworthiness criteria for special classes of aircraft defined in § 21.17(b); and
(d) For primary category aircraft, if desired, a special inspection and preventive maintenance program designed to be accomplished by an appropriately rated and trained pilot-owner.
(e) Any other data necessary to allow, by comparison, the determination of the airworthiness, noise characteristics, fuel efficiency, fuel venting, and exhaust emissions (where applicable) of later products of the same type.
The minor change definition ultimately lists some characteristics (“weight, balance, structural strength, reliability, operational characteristics”) and then calls out any “other characteristics affecting the airworthiness of the product.” If there is a question about whether a characteristic is an airworthiness characteristic, then the first step in the inquiry should be to identify the regulation (airworthiness standard) that is affected. When you are changing the way that an aircraft complies with an airworthiness standard in an appreciable way, then you are likely to have a major change to type design.
For example, a replacement part that has the same form, fit and function as the part that was replaced is likely to reflect a minor change to type design, because it functions and wears the same way. In such a case there is typically no reasonable possibility of a change to the type design.
Imagine another example where the material of an airframe part is changed to improve reliability (e.g. to minimize corrosion). This would normally be a modification part that is approved under PMA (the applicant will have to demonstrate compliance with the applicable FAA airworthiness standards). The question to be raised vis-a-vis STC is whether the material change in the part reflects an appreciable change in the way that the product’s type design meets its applicable airworthiness conditions. Imagine that the change affects the part’s function within the next higher assembly, but it does not change the interaction between the next higher assembly and any other systems within the aircraft, nor does it change the form, fit or function of the next higher assembly. In such a case there would appear to be no appreciable change to the airworthiness characteristics of the aircraft, which would mean that this is a minor change to the type design, even though the PMA application may need to demonstrate some compliance elements with respect to the new material.
Discussion
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