Is a cracked vent SCAT tube an urgent problem?

[Ben Jackson]

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/

[Stu Gotts]

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.

[CriticalMass]

"Stu Gotts" > wrote in message
...
> yes


Why? This is heated air, not exhaust.

[Jay Honeck]

> 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.
>

[Russell Kent]

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]

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.
>

[Newps]

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?

No.


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.

No.

[Stu Gotts]

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".

[Stu Gotts]

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

[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

[Dave Stadt]

"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.
> >
>

[Darryl Gibbs]

Hi Ben.

I have noted a number of replies saying yes and no to your question.
Unfortunately I cannot comment with authority on the 172's system, but on
CT-4s small pinprick size holes in both the heated and cool air scatting
were normally found to be the cause of higher than allowable levels of
carbon monoxide in the cockpit on post maintenance ground runs.

I tend to be wary with these matters. You can't just pull over to the side
of the road for some fresh air if you start feeling tired. :-)

Darryl Gibbs.


"Ben Jackson" > wrote in message
et...
> 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/

[Stu Gotts]

Thank You, Thank You, Thank You. Now if you can have the pope verify
your post, maybe some of the readers here will believe it.

DON'T MONKEY AROUND WITH EXHAUST GASSES, that includes anything that
may be lingering around the engine compartment.

On Fri, 19 Sep 2003 05:09:29 GMT, "Darryl Gibbs"
> wrote:

>Hi Ben.
>
>I have noted a number of replies saying yes and no to your question.
>Unfortunately I cannot comment with authority on the 172's system, but on
>CT-4s small pinprick size holes in both the heated and cool air scatting
>were normally found to be the cause of higher than allowable levels of
>carbon monoxide in the cockpit on post maintenance ground runs.
>
>I tend to be wary with these matters. You can't just pull over to the side
>of the road for some fresh air if you start feeling tired. :-)
>
>Darryl Gibbs.
>
>
>"Ben Jackson" > wrote in message
et...
>> 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/
>

[Newps]

Darryl Gibbs wrote:

> I tend to be wary with these matters. You can't just pull over to the side
> of the road for some fresh air if you start feeling tired. :-)

In a Cessna you can. It's called opening the window. I could probably
route the exhaust pipe into the cabin and not get sick. Four vents and
the opening window oughta take care of it.

[Dan Luke]

"Newps" wrote:
>Four vents

Five

[Jim Weir]

I'm just curious, coming in rather late to this discussion...

How many answers came from those who do this sort of stuff for a living...with
the right initials behind their name?

How many came from those who have no direct knowledge other than hearsay or
supposition?

Jim



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Russell Kent]

Jim Weir wrote:

> I'm just curious, coming in rather late to this discussion...
>
> How many answers came from those who do this sort of stuff for a living...with
> the right initials behind their name?
>
> How many came from those who have no direct knowledge other than hearsay or
> supposition?

I'm afraid the initials after my name aren't the ones you're talking about, so I
guess that puts me in the "hearsay or supposition" camp. Was I wrong?

Russell "almost always willing to learn" Kent

[Stu Gotts]

initials here, along with common sense.

On Fri, 19 Sep 2003 13:03:54 -0700, Jim Weir > wrote:

>
>I'm just curious, coming in rather late to this discussion...
>
>How many answers came from those who do this sort of stuff for a living...with
>the right initials behind their name?
>
>How many came from those who have no direct knowledge other than hearsay or
>supposition?
>
>Jim
>
>
>
>Jim Weir (A&P/IA, CFI, & other good alphabet soup)
>VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
>http://www.rst-engr.com

[Steve Dold]

On Fri, 19 Sep 2003, Jim Weir wrote:

>
> I'm just curious, coming in rather late to this discussion...
>
> How many answers came from those who do this sort of stuff for a living...with
> the right initials behind their name?
>
> How many came from those who have no direct knowledge other than hearsay or
> supposition?


The guy that works on my plane is an IA with about 20 years
experience and I'm a complete idiot with regard to this stuff. He has no better idea
than I do whether or not the air flows into or out of the hole in the
hose. He's never mesured the pressure inside a cowling in flight. I say
just replace the hose because it's cheap and you don't really know.


--
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