PART VS. INSTALLATION


As hinted above, you will see STCs listed together with PMAs and TSOs covering the same type parts or installations. So, which approval to use? Well, it depends.


You may find that installing a PMA or TSO part in one aircraft is a major alteration, but in another similar aircraft, it’s not. Sometimes it’s not the actual part that triggers the requirement. A number of relatively benign or simple part installations can, in fact, be major alterations especially when the aircraft is modified as part of the installation.


For example, specific tasks like cutting a hole in a cowling or disabling a warning system in order to complete the installation, would qualify as major alterations per 43 Appendix A(a)(vii) and (xii), respectively, even when the actual part installation does not qualify.


In the end, an STC can be either a major alteration or a minor alteration. And we didn’t even slip down the bunny hole! Now, for those who don’t believe in rabbit holes, give FAA AC 23-27 a read sometime. But just be careful you don’t fall through the looking glass and end up back in the psychedelic ’60s.


About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full-time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money. He can be contacted at tekaviationllc@gmail.com.


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