Careful What You Modify
“I am practically industrious — painstaking, a workman to execute with perseverance and labour …” – Robert Walton, Letter 2 (novel)
FAA Advisory Circular (AC) 43-210, Appendix 1, Item 9 states;
Previous Alterations or Repairs that May be Affected by This Alteration. Look at the aircraft and review its records to determine if there are any modifications, Supplemental Type Certificates (STC), alterations, or repairs that could cause a problem or conflict with the proposed alteration or repair …
This might be easier said than done. You might assume that the job of determining a “problem or conflict” is typically left to the FAA Aircraft Certification engineers who review and approve STC applications, but in truth there are an infinite number of modification possibilities for which a person might apply. It just isn’t feasible for a representative to be able to account for every possible combination. Ultimately it comes down to the person you have commissioned to do the work on the aircraft (the installer), and you (the aircraft owner) to determine the interrelationship among multiple STCs.
This begins with the major alteration and repair application process. Appendix 1, Item 8 of the AC mentioned above, warns that “before completing the alteration or repair to your aircraft, [you must] be aware that after it has been altered or repaired, the aircraft must still meet its certification basis,” and then requests documented proof — most typically given in the form of data. This might seem daunting, but your two biggest allies in getting the job done are patience (self-explanatory) and research (read on).
Before you proceed to purchase an STC, first make sure you are clear about the desired outcome of the modification. Then consider everything the modification will affect within the existing system, even if it is a stock airplane, and especially if it has been previously altered. Identify what adding a new system could override in the previous system, what it might overlap with, and what it might complement. This process should be an active dialogue between the installer and the owner, and if the conversation starts to get a bit too “nebulous,” that is the time to include a subject matter expert such as a designated engineering representative or the type manufacturer.
When dealing with surface or structure changes, an FAA engineer reviewing the paperwork will want to consider whether the change affects the structure, creates fatigue points, increases loads, or changes aerodynamics. For powerplant modifications, he or she will want to know how it will affect power output, change fuel consumption, or affect speed controls. For avionics or electrical component STCs, you can be sure that aspects such as how the “boxes” integrate with one another and how much electrical power the system consumes will be scrutinized. Some key “catch-all” questions to consider are whether the change(s) alter gross weight, center of gravity, stability, or control. Any one (or more) of these categories could compromise the airworthiness of your aircraft should the STCs not be compatible.
To start identifying your needs, a great idea is to ask if the STC holder can give you some insight on what to expect of a post-modified aircraft, how they came to the decisions they reached — what ideas worked, and what didn’t (and why). Next, ask your local FSDO representatives what they have been seeing out in the field as they might have more experience dealing with different types of modified aircraft. Lastly, seeking the advice of an experienced flight test pilot could also be very beneficial in determining interrelationship operability. This information, in conjunction with all the technical data you need for the individual STC itself, should get you on the right path to success.