Originally Posted by
Deltasierra010
What is the approval process for a conversion like this that is intended to be used for commercial fare paying passengers.
It'll be a Supplemental Type Certificate (STC). Commonly used for aircraft modifications - some more major than others. While some STC projects are done via DER approvals, I'm guessing that for a completely new powerplant installation the local regulatory office would be involved (maybe even some involvement from the national level due to the novel aspect of the change and it's high profile).