Genghis,
It is an EASA aircraft, its a mod to reduce the cabin noise by replacing the cabin insulation/lining, to reduce cabin noise by 28%. Its STC'd in the States on other types, but not ours. They said they can do it, but only on an engineering sign-off. Hence where my confusion began. Where do I stand over in EASA land?
As for the US, we may be going over there for a while anyhow, so its not too much of a problem. As a matter of interest, and excusing my lack of knowledge, but who/what is a DER?
Kind Regards, SKP