FAA Changed Product Guidance Available for Comment

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.

A change is significant if it affects general configuration, principles of construction, or the assumptions used for certification, but not to the extent that the change would be considered a substantial change. It may be easier to understand the situations where a change is not significant. A change is not significant and the STC applicant does not need to comply with the latest airworthiness requirements:

1. when the change (including the cumulative effect of other changes) is not significant pursuant to the guidance in the AC;

2. for areas, systems, components, equipment, or appliances of the product not affected by the change;

3. when compliance with the latest standards would not contribute materially to the level of safety of the changed product; or

4. when compliance with the latest standards would be impractical.

New Guidance

The above provides only a basic overview of the decision making process behind the determination of whether a change is “significant” and whether an applicant need apply for a new type certificate. The FAA’s draft AC 21.101A provides an extensive appendix of typical type design changes and whether the design changes are significant, as well as a description of the steps an applicant should take in determining whether a change is significant. It is important for members of the PMA community to note the differences from existing guidance in AC 21.101.

The new draft guidance proposes that the applicant must arrange changes into related & unrelated groups in order to better analyze cumulative effects. In essence, this could make the applicant responsible for prior STCs for which the applicant was not responsible and for which the applicant has no data. Where done well, this could help prevent problems associated with cumulative effects, but there is a great deal of discretion left to the FAA to reject an applicant’s plan and to add to the analysis systems that the applicant may feel are unrelated.

The new draft guidance also seems to expand and reorganize somewhat the tables of examples of significant and substantial changes.

Comments are due by February 19, 2010. They should be emailed to 9-AWA-AVS-AC-21-101A-Comments@faa.gov. MARPA members filing comments are asked to forward copies of their comments to MARPA for our files.