FAA Changed Product Guidance Available for Comment

The FAA has released a draft update to the changed product rule guidance (draft AC 21.101A).

The FAA has released a draft update to the changed product rule guidance (draft AC 21.101A). PMA holders who may be affected should review the document, and do it soon, in order to offer their thoughts to the FAA before the policies become final.


Major changes to an aircraft or engine must be approved by the FAA under either a supplemental type certificate (STC) or a new or amended type certificate. Major changes to an aircraft can fit into three categories. The first is changes that are so significant that they require a new TC. The second is major changes that require a supplemental STC but that must be approved under the latest airworthiness standards found in the regulations. The third is major changes that require a supplemental STC but that may be approved under the same certification standards that were applied to the original aircraft or engine.

The FAA’s “Changed Product Rule” (found in 14 C.F.R. § 21.101 and 21.19) explains that someone who seeks to make a change to a product (such as through the introduction of an alternative part), must meet the latest certification basis in certain cases. This can impose significant new burdens on the person introducing the change that were not imposed on the type certificate holder. The reason is because if the airworthiness standards that are affected by the change have changed significantly since the original certification, then it is possible that the new standards might be overly burdensome on the party seeking to make a change. There is an exception that applies to insignificant changes.

When is a New TC Necessary?

Whether a new type certificate application is necessary depends upon a determination of whether the changes made to the product are “substantial” or “significant.” If the changes are substantial, then the applicant must apply for a new type certificate. 14 C.F.R. § 21.19 requires the applicant to obtain a new type certificate for a changed product if the change in design, power, thrust, or weight is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Very few PMA parts are going to result in changes in design, power, thrust, or weight that are so extensive as to require a substantially complete investigation of compliance.

The FAA’s regulations provide three automatic criteria where changes are substantial:

1. Changes where the general configuration is not retained. For example, the resulting product differs in performance or interchangeability of major components from the old product.

2. Changes where the principles of construction are not retained. For example, where the resulting product contains changes in the materials or construction methods that affect a product’s operating characteristics or inherent strengths.

3. Changes that invalidate the assumptions used in certification. For example, any change that creates assumptions so different from the original assumptions that the methodologies of demonstrating original compliance are invalidated.

In each of these cases, a new TC would be appropriate.

STCs and the Changed Product Rule

Where a PMA part introduces a major change to the design of an aircraft or engine,the applicant will need to seek an STC. But whether the applicant is permitted to comply with the original type certication basis, or is required to meet the airworthiness standards that were applicable on the date of application, is going to depend on whether the change is significant.

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