Is a cracked vent SCAT tube an urgent problem?

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.

--
Ben Jackson

http://www.ben.com/

yes

On Thu, 18 Sep 2003 04:53:09 GMT, (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.

"Stu Gotts" wrote in message
...
yes


Why? This is heated air, not exhaust.

If I'm reading it correctly, it's in the engine compartment where
exhaust gasses may leak (or could be blown back on the ground) and
enter the tubing.

On Thu, 18 Sep 2003 08:22:43 -0500, "CriticalMass"
wrote:


"Stu Gotts" wrote in message
.. .
yes


Why? This is heated air, not exhaust.

"Stu Gotts" wrote in message
...
If I'm reading it correctly, it's in the engine compartment where
exhaust gasses may leak (or could be blown back on the ground) and
enter the tubing.

If that's the case you have problems way beyond a cracked Scat tube.

On Thu, 18 Sep 2003 08:22:43 -0500, "CriticalMass"
wrote:


"Stu Gotts" wrote in message
.. .
yes


Why? This is heated air, not exhaust.

yes

Why?

If it's just the flexible tubing that carries the heated air from the heat
exchanger, there should be no danger of carbon monoxide getting into the
cabin.

Or do you think that carbon monoxide could get into the cabin from the
engine compartment through the tubing from some other leak in the engine
compartment? If that's the worry, I'd say there are other issues in his
engine compartment.

In the short term, BTW, duct tape works just fine as a "fix".
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"
"Stu Gotts" wrote in message
...

On Thu, 18 Sep 2003 04:53:09 GMT, (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.

Agree on the duct tape. Let's not think that the exhaust systems on
our aeroplanes are as sealed as on our autos. Exhaust gasses, if
leaking, could enter the cabin through small firewall holes if they
are not sealed properly.

On Thu, 18 Sep 2003 13:51:51 GMT, "Jay Honeck"
wrote:

yes

Why?

If it's just the flexible tubing that carries the heated air from the heat
exchanger, there should be no danger of carbon monoxide getting into the
cabin.

Or do you think that carbon monoxide could get into the cabin from the
engine compartment through the tubing from some other leak in the engine
compartment? If that's the worry, I'd say there are other issues in his
engine compartment.

In the short term, BTW, duct tape works just fine as a "fix".

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

Agreed, but you are assuming that the exhaust system is sealed well.
Sorry, but I can't and won't make that assumption, even on my own
plane.

On Thu, 18 Sep 2003 09:35:31 -0500, Russell Kent
wrote:

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

Stu Gotts wrote:

Agreed, but you are assuming that the exhaust system is sealed well.

Actually, I think I'm assuming that the heater system is very slightly
pressurized (versus the engine cowl space). If the input fresh air SCAT tube for
the heater is kaput (obstructed, disconnected, missing), then the heater system
may not be so pressurized, and exhaust gases may enter the cabin if the under
cowl pressure is higher than the cabin pressure.

Sorry, but I can't and won't make that assumption, even on my own plane.

I wouldn't dream of trying to convince you to be less conservative, *BUT* IMHO
the risk of an otherwise intact heating system (as described by Ben Jackson)
being a source of CO poisoning is no worse a risk than CO poisoning via exhaust
fumes entering the cabin via other routes (leaky firewall throughways, leaky door
seals, ...). Were I in Ben's shoes, I'd patch the tear in the SCAT tube (Home
Depot HVAC repair tape), verify the integrity of the remainder of the under cowl
portion of the heating system, order a replacement SCAT, and schedule replacement
at the next convenient downtime (oil change, snowstorm, whatever).

Russell Kent