Ben Jackson wrote:

I found the SCAT tube between the muffler shroud and the defrost vent
in the glareshield has some cracks (you can only see them if you flex
the tube a certain way, which is why they've been missed).

Does this present a carbon monoxide danger? It's not the heat exchanger
that's cracked, only the tube that carries the warm air to the cabin.
I plan to get it replaced soon but I need to know if I should ground the
plane in the meantime.

Answer: no

Rationale: It helps to understand how the heated air system works. In the
engine compartment there's another SCAT tube that runs from a cowl opening
to the muffler shroud (aka "heater muff"). That SCAT tube is conducting
very slightly pressurized (small fraction of a PSI) air to the muffler
shroud. The SCAT tube that runs from the muffler to the cabin (for cabin
heat and defroster) is conducting that same "very slightly pressurized air"
(albeit now heated, too) from the muffler shroud to the cabin. If there's a
break in either of the SCAT tubes, then air leaks from inside of the tube to
outside. Thus, a broken SCAT tube cannot be a source of carbon monoxide
(since nothing comes into the SCAT tube via the break).

The reason that a fracture exhaust manifold inside the heater muffler is a
carbon monoxide risk is because the exhaust gases (which contain carbon
monoxide) are more pressurized than the "very slightly pressurized" air in
the heater muffler. Thus, exhaust gases leak from the exhaust system into
the heated air system.

Russell Kent