Do you obtain STCs for every PMA? Do you need to? The FAA recently published new guidance for its field inspectors, STCs and other forms of alteration design approval. The guidance is meant to to aid FAA employees in analyzing field approvals, but the guidance can be useful to the industry as well.
PMA parts can reflect an alteration to the structure into which they are installed. When this happens, the alteration needs to receive appropriate design approval. Many PMA applicants obtain STC as a normal part of their approval process. Where the STC process represents a significant and regulatorily unnecessary expense, though, it would be useful for PMA applicants to take note of the alteration circumstances where an STC appears unnecessary (before discounting STCs altogether, though, it is important to remember the positive aspects – like the fact that the STC is directly transferable, while the PMA is not).
The Aviation Inspectors’ Handbook includes a “Major Alterations Job Aid,” which lists a variety of different common major alterations and specifies the recommended form of approval for each – ranging from DER data approvals to supplemental type certificates (STCs). Be sure to use it only for major alterations – minor alterations are not meant to be covered in the chart, so efforts to use it for minor alterations will likely lead to overly onerous data approval requirements.
There is room to disagree with the chart in the job aid. For example, you can have a major alteration from one type certificated configuration to another type certificated configuration. Since both configurations are in the type design, no STC should be necessary (it is not a major change to type design if it is already in the type design). But the chart implies that if such an alteration affects a critical engine part then it will require a STC, even though the regulations do not require an STC for such an alteration.
Despite the fact that the chart can ‘miss’ from time-to-time, the chart still reflects the guidance that your FAA inspector will be relying on, so it is a good guideline to use. The chart can be found as figure 4-68 of volume four, chapter nine, section one of FAA Order 8900.1.