A Camera Pod, a Borrowed Field Approval, and a Hole in the Skin
A Cessna 182Q owner wants a belly-mounted camera pod installed. The pod weighs 14 pounds, mounts with six 3/16-inch screws into fuselage skin and two bolts to a seat-rail frame member, and requires a 4-inch hole cut in the skin for the wiring pass-through. There is no STC for this installation on the 182. The owner's shop has a copy of an FAA Form 337 with block 3 signed in 2009 for the same pod on a Cessna 172N, and proposes to use that document as the approved data to install and sign off the 182 with a new Form 337. The airplane is operated privately under Part 91.
1. Is this a major alteration or a minor alteration? Reason from the definition, not from the paperwork.
Read the worked answer
Apply the Part 1 definition of major alteration to the facts. A major alteration is one not listed in the aircraft specifications (TCDS and equipment list) that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or one that cannot be accomplished by elementary operations.
Elements here: 14 pounds added well below the CG changes weight and balance; six screws plus two bolts into skin and a seat-rail frame member introduce concentrated loads into primary structure; the 4-inch hole is a structural penetration in pressurized? no, but still a skin penetration requiring reinforcement judgment; the added mass and external shape raise flutter and handling questions. Any one of these can make an alteration major, and here several are present.
Conclusion: major alteration. It does not become minor because the component is small or because the shop calls it minor.
2. Can the 172N field approval be used as approved data for the 182Q?
Read the worked answer
No. A field approval is a finding of compliance for the specific aircraft and configuration described in that Form 337. It is not an STC and confers no rights on another make or model. Using it directly on the 182Q would be substituting paperwork for substantiation.
At most the 172 package is acceptable data - useful technical support the applicant may cite in a new field approval for the 182, provided the differences are evaluated: skin thickness and material, frame spacing, attach detail, operating speeds, weight and balance effect, and the structural substantiation for the skin cut.
The path is therefore: obtain approved data for the 182Q - an STC or other approved design data, or a new field approval supported by substantiating data - then install to that data. You, as the IA approving for return to service, do not create the approval; you verify the installation conforms to approved data and is at least equal to the original condition under Section 43.13(b).
3. Assuming the data problem is solved and the pod is installed, what must be in place before you approve the airplane for return to service?
Read the worked answer
Sequence it: (1) approved data covering this installation on this make and model; (2) installation accomplished per that data using the manufacturer's methods or other methods acceptable to the Administrator under Section 43.13(a), resulting in a condition at least equal to the original under Section 43.13(b); (3) FAA Form 337 completed and signed, referencing the STC or carrying the field approval in block 3, describing the work and any weight and balance change; (4) a Section 43.9 maintenance entry; (5) revised weight and balance computed from an actual weighing or by calculation supported by the data, entered into the aircraft records; (6) equipment list and TCDS-based documents updated where affected; (7) any instructions for continued airworthiness for the pod, such as fastener and wiring inspection, folded into the operator's maintenance program.
Finally, Section 91.407(b): no person may carry passengers in an aircraft that has been altered in a manner that may have appreciably changed its flight characteristics or substantially affected its operation in flight until it has been flight tested and shown to be capable of safe operation. Fourteen pounds on the belly and a skin hole is exactly that case. Test fly it before passengers go aboard.