intercom installation

[diverrick]

Hi all
I am new here. I am installing a 6 OZ sigtronics SPA400 intercom in a 172, 4 head phone jacks in the rear and the front.
I am unclear as to if it will be considered a Major alteration, or a Minor alteration in the FAA's eyes. The weight and balance issue is so small ( 6 OZ.) it is insignificant. I have been given advice that goes both ways. I want to make sure the paperwork is correct when I sign it off. Anyone know for certain which way it goes? The owner wants to make it a minor alteration, I am leaning towards the 337/Major alteration, but that is much more complicated way to go for such a small alteration.

[Denny of Oakland]

I can say for sure the installation is a major install and will require a FAA 337 field approval. The good part is this is simple, draw a drawing showing type of wires and plugs, locations and how it is wired into the electical system. I have field approved these before.

Denny of Oakland

[diverrick]

I spoke with both the FAA and the Manufacturer (Sigtronics) and they both were very vague about it. they both said "it would depend on who you asked" What it all came down to was if the airframe was modified to the point of changing the airframe structures strength or shape, anywhere. On our installation we didn't change or modify, except for drilling the mike jack holes, which accordig to the FAA did not qualify. Weight was less than 3/4 Lb so no significant changes to weight and balance. Feds and Sigtronics said it should be a Minor Alteration. Sigtronincs did say they would have a inspector look at it anyway, but that is not required by the FAA. Thought an update would be nice, if anyone else was thinking of doing something similar.
Thanks

[Denny of Oakland]

What is a minor alteration? The rule says:

FAR 21.93 Classification of changes in type design.
(a) In addition to changes in type design specified in paragraph (b) of this section, changes in type design are classified as minor and major. A "minor change" is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are "major changes" (except as provided in paragraph (b) of this section).

FAR 43 Appendix A to Part 43 -- Major Alterations, Major Repairs, and Preventive Maintenance:
(xii) Changes to the basic design of the fuel, oil, cooling, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust systems.
(4) Appliance major alterations. Alterations of the basic design not made in accordance with recommendations of the appliance manufacturer or in accordance with an FAA Airworthiness Directive are appliance major alterations. In addition, changes in the basic design of radio communication and navigation equipment approved under type certification or a Technical Standard Order that have an effect on frequency stability, noise level, sensitivity, selectivity, distortion, spurious radiation, AVC characteristics, or ability to meet environmental test conditions and other changes that have an effect on the performance of the equipment are also major alterations.

The FAA has determined that the field approval data package must include Instructions for Continued Airworthiness (ICA). The purpose of the ICA is to provide instructions on how to maintain aircraft that are altered and appliances that are installed IAW a field-approved major alteration. I assume the manufacture instructions have an ICA for the mechanic to inspect the installation during the inspections.

If the installation is part of the Avionics systems performing critical functions or involving complex interfaces to other systems it is a MAJOR installation. This could if it for example is tied into the stall warning horn or is tied into the electrical system (used electrical power) and requires circuit protection. If not then it is a minor alteration.

This guidance is found in the FAA Order 8300.10 Vol.2 chapter 1.

Denny of Oakland