So one TSO'd belt, and the other can be high quality, like Silver Parachute?
That's a good question. I think the way it's written in the IAC Technical Tips is ambiguous.
The FAA requires each seat have an "approved" seat belt. The IAC requires a second seat belt which is not an FAA requirement, and on any ramp check the FAA inspector would just look for an installed and approved seat belt for each installed seat. As long as one of them is approved, the seat belt requirement is met.
The Technical Tips article indicates that the FAA requires a TSO approved belt be installed to the seat attachment bolt with a suitable AN bolt, if a STC is to be avoided.
The IAC summary doesn't say you couldn't remove the original approved seat belt, but it's just a summary, and I have not seen the actual guidance letter. If the guidance letter isn't explicit on the subject, it would not violate the specific wording of the guidance (since any ambiguity goes against the author), but it would probably violate the
intent of the guidance.
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A related issue is missing TSO tags. I know Air Tran got dumped on as some of their seat belts were missing TSO tags or had illegible tags and the Feds insisted the tags be replaced. The FAA approved Air Tran doing their own maintenance on them so they could replace their own tags. For most GA aircraft owners however that isn't an option and the odds of finding a shop that will put a TSO tag on a belt that comes in with a missing or illegible tag is slim to none.
I'm also not clear how this all affects old aircraft - like a 1967 7KCAB with what appear to be 3" military belts. When did the TSO requirement come in for lap belts? "Approved" shoulder harnesses became a requirement in 1986 and my impression is the FAA views anything as an improvement for a pre-1986 aircraft. How do they view lap belts?