STC question

I was always under the impression that an engine related STC had to be
approved for both the engine and the airframe on which it was mounted.
I've seen this in applications like autogas STCs and STCs that bump a
150 hp O-320 to 160 hp.

At a recent Cessna fly-in a saw something that made me look twice.
It was a Powerflow exhaust system on a Cessna 170. I know that the
Powerflow is STCed only for the Lycoming powered Cessnas (later model
172 & 177s). Even assuming that the 170 owner had an STC for the 180
hp Lycoming, wouldn't it be illegal to mount that exhaust on the 170
airframe? I can't find any sign that the Powerflow STC covers a 170.

I've heard of people sneaking in a "not-quite-fully-approved" STC
here and there, but this one sticks out like a sore thumb
(literally!).

Notes : I didn't see any "experimental" signs on the plane. I'm
sure it wasn't a 172 taildragger conversion (obvious by the tail).
Unfortunately, the owner wasn't around to ask.

So, whaddya think? Can this be legal?

John Galban=====N4BQ (PA28-180)

In article ,
(John Galban) wrote:

[snip]
Even assuming that the 170 owner had an STC for the 180
hp Lycoming, wouldn't it be illegal to mount that exhaust on the 170
airframe? I can't find any sign that the Powerflow STC covers a 170.

I've heard of people sneaking in a "not-quite-fully-approved" STC
here and there, but this one sticks out like a sore thumb
(literally!).
[snip]
So, whaddya think? Can this be legal?

one guess: field approval rather than STC.

--
Bob Noel