My plane had a Telex semi-portable intercom in it that wasn't installed properly. Anyway I just bought a Flightcom 403 MC 4 place intercom. It didn't come with any paperwork for approved installation. It had all of the install info but nothing to say it was approved for the plane. How is it that you install an intercom on the paperwork side of things? I had a heard a 337, any ideas?

Permanant install? 337 from an avionics shop is best/easiest.

What if the intercom DOESN'T HAVE an STC? Am I right the intercom either needs to ALREADY BY ON THE TYPE CERTIFICATE for our aircraft, OR it needs an STC of it's own?

If I'm right, what intercoms ARE ACTUALLY APPROVED without an STC and a 337 for our M5?

-Brendan

Another possibility is the Maule Optional Equipment List available online. If the intercom is listed as optional equipment, it's considered part of the TC and doesn't need a STC. If it's not in the REL/OEL (on the TC) and there's no STC, then you're left with getting an A&P to sign off on it. A 337 is required for a major airframe modification. A logbook entry is all that's required for a minor modification. There's no clear list of what is major or minor in Part 43. It's up to the A&P signing off in the logbook. There's a wide variation in how individual A&Ps view major vs. minor modifications. Most owners who install a panel mounted intercom system would use a FAA certified avionics shop.

Intercoms are too easy to install to only use an avionics shop.

The part I’m hung up on is this - do all 337 forms require an
STC to be available for the product ?

-B

The 337 is for major airframe alterations. Depending on what the A&P who signs off thinks, they might consider it a major airframe modification just to be on the safe side. 337 forms do not require a STC but most of them have one since the alternative is a field approval, which might be hard to get if you have an overworked or uncooperative FSDO.