January 20, 2002 Heater Checkout

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by	The Staff of Light Plane Maintenance Magazine
	This article originally appeared in the January 2002 edition of LIGHT PLANE MAINTENANCE and is reprinted here by permission.

Winters in northern states like Montana can be brutal which is why the principal means of maintaining controlled flight is with the heater knob. Freezing temperatures can begin in late September and are last seen sometime in April. This is quite a chore for the lowly aircraft heating system, akin to the old Volkswagen beetle's air-cooled engine and non-heater. Air-cooled engines inherently seem to be very poor at keeping the occupants warm either in the air or on the ground. Needless to say, heater performance is a crucial part of flying in this frigid environment and the specter of carbon monoxide is an ever-present possibility. To maximize the inherently marginal heater performance while minimizing the inherent CO dangers of the system, good heating system maintenance is essential.

Ducted Air and Mixing Gasses

Right hand heater shroud. Note inlet and outlet ducts used to direct airflow through the heater.  [Click on any picture for a higher-resolution image]

In almost all single-engine aircraft, heat is provided through a shroud arrangement that ducts ram air across the engine muffler(s) where it is heated. This heated air is then routed to the cabin through flexible ducting with a flow control valve located somewhere in between, usually the firewall. To get the common light aircraft heat-exchange system to work properly, one must be aware of how a typical system is designed, where shortcomings of the system can be reduced and why inherent dangers in the system must be closely monitored.

The heat exchange begins by ducting ram air into a confined space around the engine muffler(s) where the air extracts heat from the hot metal separating the air from the exhaust stream. As more air rushes into the ram air duct, the heated air gets pushed out of the muffler/heat exchanger and is forced through ducting to a blending valve where it is mixed in adjustable proportions with cooler ambient air. The mixed parcel of air then moves through the firewall into heater ducting in the cabin and from there is distributed to heater outlets at various locations in the aircraft. Once in the cabin, the air is distributed via a central heating duct usually running just above the rudder pedals and attached to the back of the firewall.

Air going through this plenum is distributed to other ducts running along either side of the cabin. A separate duct is used to provide windshield defroster heat for one-or-more locations in the glareshield to keep the windscreen clear. We get no shortage of questions from readers on how to improved this marginal airflow to the windshield.

In terms of safety, the most important element of a heater inspection is ensuring exhaust gases don't mix with heater air. The area of utmost concern is the interface between the muffler and the heater shroud. Careful examination of these areas requires inspection of the muffler beneath the shroud, which is usually accomplished by removing several sheet metal screws and/or large hose clamps. Additionally, many installations require the removal of end plates and/or heater hoses to get the stainless steel shroud pulled back sufficiently to perform a thorough inspection.

Regardless of the method used, the intent is to keep the shroud snug against the muffler end plates. As with most things of aircraft quality and design, engineers weren't particularly concerned with the problems for mechanics, so a good selection of screwdrivers, including at least one offset, is required to get the thing(s) out of the way. An electric screwdriver is an asset for this operation, especially when dealing with hose clamps in those awkward situations where one is trying to hold two pieces together with one hand while removing and replacing a screw with the other. Aside from access difficulty, shroud removal is straightforward. After one has removed the required screws and/or clamps, the shroud can be pulled off or, as in several Cessna installations, pulled back sufficiently to gain visual access to the muffler.

Blowing the Exhaust for Leaks

With heater shields removed one can see the welded nubs on the muffler, which help in the heat transfer process. They are also prone to corrosion.

Though aircraft mufflers come in many shapes and sizes, they share the same problems associated with any thin metal repeatedly heated to high temperatures. Usually constructed with a combination of welded joints and sheet metal seams, these natural weak spots of dissimilar metal densities tend to concentrate stress loads. It's particularly important to pay close attention to these areas. With welds and sheet metal joints, normal manufacturing processes make it more difficult to identify cracks. Welds tend to have a wavy appearance and cracks can hide in the small valleys and dips of the joint. Some sheet metal bends have a substantial amount of the joint hidden from view entirely, making inspection that much more difficult.

Fortunately, heat and combustion by-products in engine exhaust systems leave telltale signs, which help identify leakage at joints. When exhaust gasses force their way through a weakness in the metal, they tend to leave a feather-like discoloration from the crack radiating outward in the direction of the airflow across the muffler. On most engines, you can see examples of these small leaks where an exhaust stack joins a cylinder head and/or on the clamps joining two pieces of exhaust pipe. While these small leaks may not be a concern at those locations, they are a strict no-no when they are in-or-near the heater shrouds.

One sure-fire method of detecting leaks in these areas is by attaching a vacuum cleaner hose to the exhaust output on the vacuum and joining it to the exhaust pipe with some duct tape. While pumping air up the exhaust, take a leak detection fluid (made by mixing a dose of dishwashing detergent with water) and brush the solution onto the various joints of the muffler and nearby exhaust system. The appearance of any bubbles will tell of a hidden crack.

Since the heat output of a muffler is directly related to its surface area, many mufflers incorporate a series of studs or ribs welded onto the muffler's surface. While these studs enhance the performance of the muffler, the welds add another area where cracking can occur. It's important with this kind of muffler to check the areas around these studs for cracks and make certain that none of the studs are working loose.

If the worst happens and you find cracks, you may be able to salvage your muffler(s) by having them repaired or rebuilt. One repair station claims something like 97% of all the mufflers they receive are repairable. The prices for these repairs can vary widely as can prices for new mufflers. Indeed, a rebuilt muffler at one place may be more expensive than a new one at another. For example, a new muffler for a 1978 Cessna 172 found prices ranging from $295 to $370 with only a little checking; one rebuilder wanted as much as $330 for a rebuilt unit. A little research on the phone and calls to local shops may go a long way in saving some money. Shopping around will pay dividends, but don't make the mistake of dropping the muffler off with Joe, the local auto welder. Some of the metals involved in aircraft exhaust systems are very demanding, welding-wise. Even though it will cost more, stick with recommendations of experienced aircraft mechanics who continually have good luck with a particular specialty shop.

You may find even better prices at salvage yards, though you will want to give the muffler a very thorough going-over prior to purchase. There are many shops, which specialize in exhaust repair work, and the Net is a good source for starters, but we still like word-of-experienced-mouth as the first choice. For some starters, try Knisley Welding, Inc., 3450 Swetzer Rd, Loomis, CA 95650, phone: 916-652-5891 or 1-800-522-6990, Aircraft Exhaust System, Inc., P.O. Box 159, Jumping Branch, WV 25969, phone: 304-466-1724 or 1-800-227-5951 or Dawley Aviation Corp., 140 Industrial Dr, Burlington, WI 53105, phone: 414-763-3113 or 1-800-338-5420.

Scat Tubing and Sceet Ducting

This exhaust port on a TCM IO-360 turbo was badly neglected, resulting in erosion and stud breakage. All cylinders were like this, and dumped blazing carbon monoxide into the engine compartment, secondarily diminishing the heater as well. Expensive to fix.

To move air from one area to another, flexible ducting is used throughout most heater systems. Typically, you'll find Scat or Sceet tubing (generic names for a fiberglass and silicone tubing used in most engine compartments) between the ram air port and muffler shroud and the shroud and blending valve. This tubing is also used at other ram air ports (frequently located in the engine cooling baffles) to provide unheated (ambient) air to the blending valve. Inside the cockpit, this ducting is located between the central duct on the firewall and various heater outlets in the defroster.

Since heater performance is dependent on getting all the heated air to the cabin, leaks in any of these ducts will degrade performance, sometimes drastically, since the output is marginal to start with. As any system ages it will start to leak, and the accumulation of several small leaks is as bad as one big one. In addition, severe deterioration of the ducting may cause failure of the hose at an attachment point, leaving the duct to thrash around in the engine compartment airflow and, of course, completely fail the heating system.

Unsecured scat tubing, while heat resistant to 600 degrees F, will often find its way against red-hot exhaust components, which increases the chance of in-flight fire or, at the very least, can provide some anxious moments in the cockpit when the smell of smoking duct work hits the nostrils. To prevent this problem, take a good look at the condition of the tubing, especially at endpoints where they are clamped. Remove the hose at the muffler shroud and visually inspect the interior of the hose. Hoses that appear satisfactory on the outside sometimes show extreme heat damage on the interior surfaces. Tears, delamination, unraveling and worn-through spots are all cause for rejection. Be sure to use an inspection mirror for out-of-the-way places to check for chaffing. And don't skip any because it's had to get at. The hose is relatively cheap and available from many aviation supply stores like Aviall and from local shops at prices ranging from $3 to $7 per foot (most single tubing runs are less than three feet long), depending on hose diameter.

In many cases, it's necessary to remove ducting to better access the heater shroud and nearby components. There are a couple of things to know about this procedure. First, when re-installing a hose, tighter isn't necessarily better, especially if you're using an electric screwdriver. Running the clamp down until the screwdriver won't turn any more will cause buckling of the sheet metal attachment and cutting of the tubing ends. While this is easy to repair with a pair of duck-billed pliers, you may not notice your mistake until the next annual inspection or when the hose comes off in mid-winter. Only tighten the hose clamp until the hose is firmly attached—no more is necessary and it certainly won't enhance the reliability of the attachment by "cranking down on it a couple of extra turns." When installing a new hose, be sure you cut back the reinforcement wire enough to ensure you aren't clamping over it. Also, make sure the tail you've got left in the hose won't poke holes through the ducting after it is installed. Bend the end at a 90-degree angle and push it part way into the hose. Lastly, position the outer cotton reinforcing string under the clamp to avoid unraveling of the coated thread.

After installation, check the entire length of the hose for places it can rub against engine compartment components. Sometimes, during installation, a twist will develop in the hose, forcing it into unwanted areas. Loosening the clamp, straightening the hose, and re-clamping the ends can cure this. Avstrap (a brand name for some tie-wraps) works well in keeping the hose away from components. But remember, most tie-wraps have a quite low temperature tolerance, unless special Teflon-type or other high temperature material is used, which is not always easy to find. Use good judgment in how you place these wraps to minimize high temperature exposure. Avoid the use of safety wire since it can rub holes in ducting and pressure tape because the adhesive and/or tape material can deteriorate quickly in the hellish environment under the cowling.

Flapper Valves and Rubber Seals

This is a typical cab in heat blending valve. Yours may be somewhat different in appearance but similar in function.

As pointed out before, the blending valve uses a combination of heated air and ambient air to adjust the heater output to the desired level. The valve consists of a flapper plate that rotates on a rod extending outside the valve housing where an actuator arm is affixed for attachment of the control cable. Operating in a similar fashion to the carburetor heat flapper in the carb air box, positioning of the blending valve forces more heated air into the cabin or more ambient air. When fully positioned to "hot," the flapper's rubber seal blocks entry of cold air into the cabin so undiluted hot air can be used. It is this seal that becomes deteriorated with time and should be carefully inspected for damage.

To inspect the seal and flapper plate, move the cabin heat control to a mid-range position so the plate can be fully viewed. Using a flashlight and mirror (when necessary), check the seal at the front and back as well as on the sides. On the sides, the seal should be right against the side wall of the valve while the seal at the front and rear of the plate should be reasonably straight with only a little curl from where they sit against the top and bottom of the valve housing. Now, actuate the flapper to the full-hot position and check the flapper seal.

You should have little or no clearance between the seal and the top or bottom. If you do, cold air will be shoved into these cracks, reducing the heated air temperature and limiting heater operation. If you find the rubber seal deteriorated or sealing poorly, replacement is a simple matter of finding another piece of rubber or baffle seal material and attaching it to the plate. On Cessna aircraft, the seal is usually sandwiched and riveted between two pieces of the sheet metal making up the flapper valve. Removal of the flapper is pretty straightforward and requires little time.

Once you have the flapper valve out, drill out the rivet heads (use a #40 drill for -3 rivets and a #30 drill for -4 rivets) and separate the two plates from the rubber in between. Using the old seal as a pattern cut a new piece from rubber material available through any of several sources, including the original manufacturer and local FBOs. Assemble the sandwich back together as it was prior to disassembly, clamping the new package together in a vice or by using locking pliers. Drill through the existing holes with a smaller-than-original-size bit (this locates the holes in the rubber without enlarging the rivet holes in the aluminum sheet). Pull apart the sandwich and re-drill the holes you've located before with a slightly-larger-than-original drill bit (the flexibility of the rubber will make it a chore to get the rivets through if they are the exact size of the rivets). Reassemble the entire sandwich back together again using clecos. Push through the rivets and squeeze or buck them back together to finish the job. Time required for the repair-about 20 minutes on an older 172.

Operational Considerations

The heat muff on this Piper is clamped at both ends. Any holes in the exhaust will leak directly to the heater system.

Mufflers, heater shrouds, and various metal components suffer the same problems other metals do when exposed to cyclical applications of heat and cold; they can only stand so much before they deteriorate and crack. There are some things you can do to lengthen the service life of these components as well as increase your comfort level in the cockpit...here are a few ideas:
Avoid rapid power changes. Alternate applications of extreme heat and relatively cold cool-off periods tend to create cyclical stresses in exhaust system components. How rapid this cycle is generated will determine the degree of deterioration you can expect.

The principal reason cracking occurs in the first place is due to thicker pieces of metal expanding at slower rates than thinner pieces. The joint area between these varying thicknesses is where most cracks develop due to stresses incurred at these points. If one rapidly reduces the throttle from a 1,400 degree F exhaust gas temp to, say, a 600 degree F idle setting, the thin sheet metal drops that 800 degrees F quickly and shrinks around thicker welded joints and flanges. This creates interference between two metals having different thermal expansion rates and stresses the shared joint area. If one reduces the throttle in smaller increments and allows a period of stabilization time, there is a smaller thermal distance between the welds, exhaust pipes and sheet metal pieces, thus, less stress. Gradual reduction and, where practical, slow increases in throttle will go a long way in keeping your mufflers in good health (as well as your wallet).

Keep up the power. Since the heater is extracting heat from the exhaust gases, it only makes sense that the hotter the exhaust, the more heat there is to transfer to the air. This is particularly noticeable to those who throttle back ten miles from their destination and find themselves freezing the remainder of the trip (their cylinders don't appreciate this much, either). Instead, descents at near-cruise power from a long distance out will be more comfortable in terms of heat output and the engine will appreciate the lack of shock cooling.

Pre-flight the ram-air vent. While performing your preflight, winter or summer, take a second to notice your ram air opening(s) to the muffler. While this seems obvious in the winter, it is also important in the summer. The ram air for heat in the winter is also cooling air for the heater shrouds and hoses in the summer. Since airflow is constant through the shrouds, any reduction in flow will mean additional heat deterioration and can lead to very high localized temperatures. Finally, controlling the temperature turnover rate in the cabin will go a long way to increase the efficiency of your heater. Most light aircraft, over the years, lose some of the insulation installed to protect the cabin from the elements. Age, varmints, inattentive owners and sloppy mechanics all take their toll on insulation.

A thorough inspection will find missing sections of insulation in almost any aircraft, leaving a sheet of thin aluminum separating the cabin from the cold. On my airplane, which I bought in Texas, all of the insulation had been torn out sometime previously. While the insulation may not have been missed in Texas, it certainly was the first winter I flew in North Dakota. Good insulation though, won't mean a thing if the doors are leaking. Inspect your doors and windows for proper fit and closure. Check the seals for missing sections, rotted areas and obvious leaks. Take a willing helper along on your next flight and have him feel around all doors and windows for leaks (don't forget the baggage door). Also check for any other protrusions through the cabin area such as steering. Flexible boot covers age and leak air.

A little water on the hand from a damp rag will detect even the smallest of air leaks into the cabin. Do this while it's still warm because any pinhole leak in the summer will be an arctic blast in very cold weather. Keeping warm is a combination of a tight cabin and a tight heater. Let either slip and you'll end up with cold feet.