cabin heater scat tube size

How to keep the Cessna 170 flying and airworthy.

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GAHorn
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Re: cabin heater scat tube size

Post by GAHorn »

Without addressing the legality of this modification… I’d like to comment on what may not be particularly “obvious” …

If additional heat into the cabin is desired…. simply increasing the tube or “scat” size at the entrance to the cabin may not make any improvement at all. In fact, it might DECREASE effectiveness.

How? In order for the air to be heated it must pass over a heat-source. In this case…it is the exhaust muffler surfaces. The amount of heat absorbed to be conducted into the cabin will depend upon HOW MUCH TIME is spent by the moving air at the source of heat. If the air is “speeded up” in its’ passage-of-exposure to the heat source… that air will have LESS time to absorb the heat which will be carried to the cabin. In other words, the cabin air input may be at a lesser temperature than prior to the modification.

If the exit of the heat-source is increased in size…such as increasing the 2” scat to 3” scat… then the back-pressure of the air inside the heat source (the inside of the muffler “shroud”…. will likely be reduced and will speed up the air flow into the cabin…. resulting in more air perhaps…but at a lower temperature than previous.

What I am attempting to illustrate is that an increase of the scat between the muffler and the firewall may actually REDUCE the calories introduced to the cabin. It may actually prove MORE efficient to increase the size of the scat INTO the muffler-shroud.

Hope this helps complicate matters.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
hilltop170
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Re: cabin heater scat tube size

Post by hilltop170 »

No matter what is decided in this matter, be sure there is a positive shut-off of the air flow from the muffler to the cabin. The original heater valve had this function and I cannot tell from the pictures if that feature was removed in the modification of the valve. It is necessary in case of fire and smoke in the engine compartment.
Richard Pulley
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1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
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c170b53
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Re: cabin heater scat tube size

Post by c170b53 »

Looks like the firewall has seen some action. Best to try to go back to stock.
Jim McIntosh..
1953 C170B S/N 25656
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voorheesh
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Re: cabin heater scat tube size

Post by voorheesh »

Quick story on an accident I investigated years ago in Modesto, CA involving a different aircraft type, but one that emphasizes the need for proper aviation exhaust system inspection and maintenance.

A private pilot was making his first flight in a Tri Pacer after an owner assisted annual. He brought a young co worker along for her first flight in a small airplane. Shortly into the flight, the pilot reported a “burning” odor and made an emergency landing at KMOD, about 1/2 hour from his home base of KLVK. It was a weekend and no qualified technicians were available, but a helpful line boy offered to drive the pilot to Walmart to buy a replacement clamp after he discovered that the heater scat tube had come off the muffler.

After repair was made, they took off to return to KLVK. This time the pilot reported hearing a bang and then a stronger burning odor resulting in a second emergency landing. Inspection revealed the scat tube had come off again and the pilot decided he hadn’t secured it tightly enough. Once corrected, another takeoff was made, but during climb out, the pilot was now experiencing smoke coming into the cabin from the firewall area and from beneath the floor. He declared his third emergency for the day and went straight for the runway. On final, flames were entering the cabin from the floor. At touchdown the cabin was engulfed and the passenger was screaming in terror. The pilot ordered her to jump out which she did and became caught in the landing gear and essentially run over as the plane rolled to a stop. She was taken to a hospital in serious condition, but fortunately recovered. The pilot was not injured. The Tri Pacer was substantially damaged. True story.

The Tri Pacer has an AD on the muffler system that calls for an extensive visual inspection and a pressure test. The IA who supervised the annual insisted this was accomplished, despite the forensic inspection of the system after the accident which indicated otherwise. As in many cases, the evidence was insufficient for the FAA to follow up with anything other than a really serious lesson to pay attention to aviation exhaust systems.

This accident involved an experienced pilot who certainly meant well. He had taken a sincere interest in his airplane’s maintenance but was not aware of a basic vulnerability, particularly present in older aircraft with rudimentary systems. He was under pressure from his innocent passenger and her desire to just get her back home. He also was a victim to “Plan continuation bias” which will lead a pilot to ignore warnings that conflict with an originally decided course of action. So I resist the temptation of labeling him an idiot and recognize how we can all learn from others and also be thankful that in this case everyone survived.
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Bruce Fenstermacher
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Re: cabin heater scat tube size

Post by Bruce Fenstermacher »

What you haven't shown is the size of the scat going from the baffle to the inlet of the heat shroud around the muffler. And you haven't said what side this is on, right or left muffler. I'd bet it is the left. And you haven't shown the muffler but I'll bet it is for a '53 or later cause otherwise the funnel wouldn't work either as an inlet or an exit. The '53 and later has a funnel similar to the one you show but that funnel is the input at the front of the left hand muffler.

Looking at your valve at the firewall, it is not clear if this valve can still cut off the air entering the cabin. If it can't, this is a big safety issue regardless of the scat size.

If the hole cut in the valve to open up the valve inlet is indicative of the workmanship of the rest of the system I can see why your IA might not like it.

If you haven't you should look in the IPC at the Hanlon Wilson setup to see what you should have in a '52 and how much yours has been modified.

Beside a repaired firewall heat valve and what ever repair is needed to the front baffles if they've been changed, to go back to stock, you will need at least one muffler.

BTW how is your carb heat hooked up?
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c170b53
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Re: cabin heater scat tube size

Post by c170b53 »

the workmanship of the rest of the system I can see why your IA might not like it.
Bruce you wrote what likely many have thought. I tried to ease into it with the comment on the firewall repair, it does not look pleasing to the eye nor do the many holes shown in that small firewall section. This is all repairable so the owner should not panic but rather work with the wrenches to get it airworthy.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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GAHorn
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Re: cabin heater scat tube size

Post by GAHorn »

Bruce Fenstermacher wrote:What you haven't shown is the size of the scat going from the baffle to the inlet of the heat shroud around the muffler. And you haven't said what side this is on, right or left muffler. I'd bet it is the left. And you haven't shown the muffler but I'll bet it is for a '53 or later cause otherwise the funnel wouldn't work either as an inlet or an exit. The '53 and later has a funnel similar to the one you show but that funnel is the input at the front of the left hand muffler.

Looking at your valve at the firewall, it is not clear if this valve can still cut off the air entering the cabin. If it can't, this is a big safety issue regardless of the scat size.

If the hole cut in the valve to open up the valve inlet is indicative of the workmanship of the rest of the system I can see why your IA might not like it.

If you haven't you should look in the IPC at the Hanlon Wilson setup to see what you should have in a '52 and how much yours has been modified.

Beside a repaired firewall heat valve and what ever repair is needed to the front baffles if they've been changed, to go back to stock, you will need at least one muffler.

BTW how is your carb heat hooked up?
I believe Bruce was referring to the “right’ hand muffler and associated parts, which on ‘53 and later B-models used a 3” hose to introduce inlet air to the RH muffler “funnel” ..which has the appearance of an old canning-jar funnel used by our grandparents to fill Mason Jars….VERY POORLY depicted in the B-model IPC, Fig 55, Item 36, PN 0713019-32…and which is virtually UN-obtainable except from salvage yards.
9542662F-8114-47C3-97A9-8FD232CEC138.png
The exhaust adaptor to that unique RH muffler heater shroud is also used on ‘53 and later B-models to accommodate the 3” hose to the firewall heater valve, Fig 51, Item 46, PN 0550157-23..also very difficult to find on part shelves.
18ECBE1C-22DE-49E5-A63A-E525B38D016F.png
1moretw wrote:…... So, the real question is, how does one make the 3 inch system legal or can it be given that this a 1952 B model.
In my opinion, any A&P can make a logbook entry that the left hand air duct was modified in a “minor alteration” (listing the parts used and noting their provenance) to increase air flow to the cabin. (I believe this to be correct for cabin heating/ventilation only. If one were to alter the carburetor heater inlet system a Form 337 would likely be required with Field Approval or DER support.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Bruce Fenstermacher
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Re: cabin heater scat tube size

Post by Bruce Fenstermacher »

In reality, besides visual appearance and perhaps strict legality, there are 3 things critical to the muffler/heat system. And it makes little difference in it is a 2" or 3" scat tube system. First, with a failed muffler or even a muffler joint, CO2 is injected into the airstream. Doesn't matter what size system or if you have the exact correct muffler. And by that I mean the correct Hanlon Wilson muffler or your year of aircraft VS one slightly larger for a 172 VS on for a '53 with no end flanges VS a '52 flanged when you need the the '53. Second, very critical because of the first and any other source of toxin entering the cockpit, the valve at the firewall must be able to close. And third, something we haven't really discussed, that the system be able to supply enough hot air to the carb to prevent / clear carb icing.

In my A&P mind so long as the work performed and the material used ie a Hanlon Wilson muffler for a '52 or '53 or even a later appropriate 172 and the work meets standard practice, the muffler is airworthy and the valve works, the size of the scat on the cabin heat alone makes no difference and is a minor alteration.

However the sticking point on '52 and earlier 170s is that the carb heat was directly tied to the cabin heat in the original configuration. And carb heat can effect the performance and operation of the engine and that could more easily be considered a major alteration.

In a ’52 or prior with pancake mufflers, both sides feed air to a t which feed air from both sides either to the carb or the cabin and there is a one why flap in the t to stop blowback to the cabin from the carb as I recall. The approved Hanlon Wilson installation in ’52 and prior, SK70A51-1, has the cabin heat feed from the right directly and the carb heat feed from the left directly.

A link to SK70A51-1: viewtopic.php?f=36&t=6919

On ’52 and prior, I’ve seen carb heat being feed from the left and cabin heat feed from the right and feel this is a minor alteration and both the left and right systems are mirror images.

Carb heat was feed from the left with 2” scat ’53 and later and cabin heat was feed from the right muffler with 3” scat from the front baffle to the funnel at the front of the muffler and 3” scat from a V exit of the back of the muffler and into the a different style heat valve on the left side of the firewall where the battery was prior and now relocated to the left side of the fire wall. Check out the IPC to see how the '53 and later was done.
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ghostflyer
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Re: cabin heater scat tube size

Post by ghostflyer »

It’s interesting to hear how different authorities in different countries have the same ideas regardless of climatic conditions . I have never used my heater [maybe once because my wife complained her feet were cold ] .so when i was fitting my new shiny stainless firewall i didn’t want to fit the heater and cut a hole in my firewall. Well the situation got bigger than Ben Hur . Leaving a heater out was a Major modification and then it was to have another weight and balance check ,plus drawings all approved by the authorities PLUS a letter from Cessna stating it wasnt detriment to the safety of the aircraft blah blah. I was then told checking it’s operation should be part of my preflight check .”really” in summer the OAT temperature cruises from 80degF to 100 degF. during taxi out my cockpit is already over 120 degF . so the 3in scat went in ,heater went in but never used .
while it could be argued that the heater is part of the type certificate but saying that there are items on the type certificate that are not fitted .
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Bruce Fenstermacher
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Re: cabin heater scat tube size

Post by Bruce Fenstermacher »

ghostflyer wrote:while it could be argued that the heater is part of the type certificate but saying that there are items on the type certificate that are not fitted .
The only items required by TCDS are as follows:

170 - In addition to the pertinent required basic equipment specified in CAR 3, the following items of equipment must be installed:
Landplane: Items 1(a), 103, 104, 201(a), 202(a), 204(a), 402(a).
Skiplane: Items 1(a), 103, 104, 204(a), 208(a), 402(a), and (e) or (d).
Note: For night flying, cabin dome light and instrument lights or equivalent, to provide illumination of all placards and instruments are required in addition to equipment required by CAR 3.

170A - In addition to the pertinent required basic equipment specified in CAR 3, the following items of equipment must be installed:
Landplane: Items 1(a), 103, 201(a), 202(a), 204(a), 402(a).
Skiplane: Items 1(a), 103, 204(a), 208(a), 402(a) and (e) or (d).
Seaplane: Items 1(a), 103, 209(a), 402(a).
Note: For night flying cabin dome light and instrument lights or equivalent, to provide illumination of all placards and instruments are required in addition to equipment required by CAR 3.

170B - In addition to the pertinent required basic equipment specified in CAR 3, the following items of equipment must be installed:
Landplane: Items 1(a), 110, 201(a), 202(a), 204(a), 402(f) and 607.
Skiplane: Items 1(a), 110, 201(a), 202(a), 204(a), 402(f) and 607.
Seaplane: Items 1(a), 103, 209(b), 402(h).
Note: For night flying, cabin dome light and instrument lights or equivalent, to provide illumination of all placards and instruments are required in addition to equipment required by CAR 3.

I've not the time at the moment to double check CAR 3. But I doubt there is a cabin heat requirement. So the TCDS doesn't, Likely not CAR 3 and I know of no current FARs, that require cabin heat.
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GAHorn
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Re: cabin heater scat tube size

Post by GAHorn »

We’re diverting from the Topic of the Thread, but in the Spirit of the Diversion…. I believe Ghostflyer meant to refer to the “Production Certificate” rather than the Type Cert. **

And that Bruce meant to refer to toxic Carbon Monoxide (CO) rather than Carbon Dioxide (CO2) which makes up about 1% of every breath we take from the ordinary atmosphere every day. :wink:

** (I ran into this perplexity when I visited with FSDO-FTW in an attempt to clarify whether or not it is a minor or a major alteration to disable the parking-brake. The TCDS does not require a parking brake…which led me to argue that it is a minor alteration, however FSDO-FTW wanted me to submit my question in Writing so they could forward it to FAA Engineering/Airworthiness in OKC because FTW thought that altering anything specified in the Production Certificate may constitute a Major alteration.

Since this smelled like a can of worms I decided to let the sleeping giant snooze and ignore FTW, especially since my own parking brake was disabled 20+ years ago when in 1971 an FAA Airworthiness Inspector in Jacksonville (who used to TEACH the FAA classes on Airworthiness) had approved an initial Airworthiness Certificate for my airframe which, as an imported airframe, was receiving its’ FIRST FAA Airworthiness Certificate…. and he not only designated the airplane Airworthy with the incorrect engine (O-300-C) and prop (EM7655) but also declared it to be a 1971 B-model…. which had no parking brake mechanisms installed.)
The point being that SOME inspectors think the Production Certificate is as important as the Type Cert….and SOME Faa Inspectors don’t know what they Teach. :roll:

We’re all human. :)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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ghostflyer
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Re: cabin heater scat tube size

Post by ghostflyer »

Gee Gahorn ,you gave me that warm fuzzy feeling when you put me in the category of “human” . i should have put it in the production certificate and my mind was else where .
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