Heater Checkout

In cold weather operations, the heater system is critical to both your comfort and health. Here's what to check out.

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

Ducted Air and Mixing Gasses

 

Cabin Heater Shroud
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 shroudarrangement 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 flowcontrol valve located somewhere in between, usually the firewall. To get thecommon light aircraft heat-exchange system to work properly, one must be awareof how a typical system is designed, where shortcomings of the system can bereduced and why inherent dangers in the system must be closely monitored.

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

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

In terms of safety, the most important element of a heater inspection isensuring exhaust gases don’t mix with heater air. The area of utmost concern isthe interface between the muffler and the heater shroud. Careful examination ofthese areas requires inspection of the muffler beneath the shroud, which isusually accomplished by removing several sheet metal screws and/or large hoseclamps. Additionally, many installations require the removal of end platesand/or heater hoses to get the stainless steel shroud pulled back sufficientlyto perform a thorough inspection.

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

Blowing the Exhaust for Leaks

Exhaust With Heater Shields Removed
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 sameproblems 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 concentratestress loads. It’s particularly important to pay close attention to these areas.With welds and sheet metal joints, normal manufacturing processes make it moredifficult to identify cracks. Welds tend to have a wavy appearance and crackscan hide in the small valleys and dips of the joint. Some sheet metal bends havea substantial amount of the joint hidden from view entirely, making inspectionthat much more difficult.

Fortunately, heat and combustion by-products in engine exhaust systems leavetelltale signs, which help identify leakage at joints. When exhaust gasses forcetheir way through a weakness in the metal, they tend to leave a feather-likediscoloration from the crack radiating outward in the direction of the airflowacross the muffler. On most engines, you can see examples of these small leakswhere an exhaust stack joins a cylinder head and/or on the clamps joining twopieces of exhaust pipe. While these small leaks may not be a concern at thoselocations, 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 avacuum cleaner hose to the exhaust output on the vacuum and joining it to theexhaust pipe with some duct tape. While pumping air up the exhaust, take a leakdetection fluid (made by mixing a dose of dishwashing detergent with water) andbrush the solution onto the various joints of the muffler and nearby exhaustsystem. 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’ssurface. While these studs enhance the performance of the muffler, the welds addanother area where cracking can occur. It’s important with this kind of mufflerto check the areas around these studs for cracks and make certain that none ofthe studs are working loose.

If the worst happens and you find cracks, you may be able to salvage yourmuffler(s) by having them repaired or rebuilt. One repair station claimssomething like 97% of all the mufflers they receive are repairable. The pricesfor these repairs can vary widely as can prices for new mufflers. Indeed, arebuilt 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 $295to $370 with only a little checking; one rebuilder wanted as much as $330 for arebuilt unit. A little research on the phone and calls to local shops may go along way in saving some money. Shopping around will pay dividends, but don’tmake 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 ofexperienced aircraft mechanics who continually have good luck with a particularspecialty shop.

You may find even better prices at salvage yards, though you will want togive the muffler a very thorough going-over prior to purchase. There are manyshops, which specialize in exhaust repair work, and the Net is a good source forstarters, but we still like word-of-experienced-mouth as the first choice. Forsome 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. Box159, Jumping Branch, WV 25969, phone: 304-466-1724 or 1-800-227-5951 or DawleyAviation Corp., 140 Industrial Dr, Burlington, WI 53105, phone: 414-763-3113 or1-800-338-5420.

Scat Tubing and Sceet Ducting

 Worn Flange
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 throughoutmost heater systems. Typically, you’ll find Scat or Sceet tubing (generic namesfor a fiberglass and silicone tubing used in most engine compartments) betweenthe ram air port and muffler shroud and the shroud and blending valve. Thistubing is also used at other ram air ports (frequently located in the enginecooling baffles) to provide unheated (ambient) air to the blending valve. Insidethe cockpit, this ducting is located between the central duct on the firewalland various heater outlets in the defroster.

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

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

In many cases, it’s necessary to remove ducting to better access the heatershroud and nearby components. There are a couple of things to know about thisprocedure. First, when re-installing a hose, tighter isn’t necessarily better,especially if you’re using an electric screwdriver. Running the clamp down untilthe screwdriver won’t turn any more will cause buckling of the sheet metalattachment and cutting of the tubing ends. While this is easy to repair with apair of duck-billed pliers, you may not notice your mistake until the nextannual inspection or when the hose comes off in mid-winter. Only tighten thehose clamp until the hose is firmly attached—no more is necessary and itcertainly won’t enhance the reliability of the attachment by "cranking downon it a couple of extra turns." When installing a new hose, be sure you cutback 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 theducting after it is installed. Bend the end at a 90-degree angle and push itpart way into the hose. Lastly, position the outer cotton reinforcing stringunder the clamp to avoid unraveling of the coated thread.

After installation, check the entire length of the hose for places it can rubagainst engine compartment components. Sometimes, during installation, a twistwill 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 brandname for some tie-wraps) works well in keeping the hose away from components.But remember, most tie-wraps have a quite low temperature tolerance, unlessspecial Teflon-type or other high temperature material is used, which is notalways easy to find. Use good judgment in how you place these wraps to minimizehigh temperature exposure. Avoid the use of safety wire since it can rub holesin ducting and pressure tape because the adhesive and/or tape material candeteriorate quickly in the hellish environment under the cowling.

Flapper Valves and Rubber Seals

Cabin Heat Blending Valve
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 airand ambient air to adjust the heater output to the desired level. The valveconsists of a flapper plate that rotates on a rod extending outside the valvehousing 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 airbox, positioning of the blending valve forces more heated air into the cabin ormore ambient air. When fully positioned to "hot," the flapper’s rubberseal 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 carefullyinspected for damage.

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

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

Once you have the flapper valve out, drill out the rivet heads (use a #40drill for -3 rivets and a #30 drill for -4 rivets) and separate the two platesfrom the rubber in between. Using the old seal as a pattern cut a new piece fromrubber material available through any of several sources, including the originalmanufacturer and local FBOs. Assemble the sandwich back together as it wasprior to disassembly, clamping the new package together in a vice or by usinglocking pliers. Drill through the existing holes with asmaller-than-original-size bit (this locates the holes in the rubber withoutenlarging the rivet holes in the aluminum sheet). Pull apart the sandwich andre-drill the holes you’ve located before with a slightly-larger-than-originaldrill bit (the flexibility of the rubber will make it a chore to get the rivetsthrough if they are the exact size of the rivets). Reassemble the entiresandwich back together again using clecos. Push through the rivets and squeezeor buck them back together to finish the job. Time required for the repair-about20 minutes on an older 172.

Operational Considerations

 

Piper Heat Muff
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 sameproblems 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 somethings you can do to lengthen the service life of these components as well asincrease your comfort level in the cockpit…here are a few ideas:
Avoid rapid power changes. Alternate applications of extreme heat and relativelycold cool-off periods tend to create cyclical stresses in exhaust systemcomponents. How rapid this cycle is generated will determine the degree ofdeterioration you can expect.

The principal reason cracking occurs in the first place is due to thickerpieces of metal expanding at slower rates than thinner pieces. The joint areabetween these varying thicknesses is where most cracks develop due to stressesincurred at these points. If one rapidly reduces the throttle from a 1,400degree F exhaust gas temp to, say, a 600 degree F idle setting, the thin sheetmetal drops that 800 degrees F quickly and shrinks around thicker welded jointsand flanges. This creates interference between two metals having differentthermal expansion rates and stresses the shared joint area. If one reduces thethrottle in smaller increments and allows a period of stabilization time, thereis a smaller thermal distance between the welds, exhaust pipes and sheet metalpieces, thus, less stress. Gradual reduction and, where practical, slowincreases 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 exhaustgases, it only makes sense that the hotter the exhaust, the more heat there isto transfer to the air. This is particularly noticeable to those who throttleback ten miles from their destination and find themselves freezing the remainderof the trip (their cylinders don’t appreciate this much, either). Instead,descents at near-cruise power from a long distance out will be more comfortablein terms of heat output and the engine will appreciate the lack of shockcooling.

Pre-flight the ram-air vent. While performing your preflight, winter orsummer, take a second to notice your ram air opening(s) to the muffler. Whilethis seems obvious in the winter, it is also important in the summer. The ramair for heat in the winter is also cooling air for the heater shrouds and hosesin the summer. Since airflow is constant through the shrouds, any reduction inflow will mean additional heat deterioration and can lead to very high localizedtemperatures. Finally, controlling the temperature turnover rate in the cabinwill go a long way to increase the efficiency of your heater. Most lightaircraft, over the years, lose some of the insulation installed to protect thecabin from the elements. Age, varmints, inattentive owners and sloppy mechanicsall take their toll on insulation.

A thorough inspection will find missing sections of insulation in almost anyaircraft, 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 outsometime previously. While the insulation may not have been missed in Texas, itcertainly 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 forproper fit and closure. Check the seals for missing sections, rotted areas andobvious leaks. Take a willing helper along on your next flight and have him feelaround all doors and windows for leaks (don’t forget the baggage door). Alsocheck 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 ofair leaks into the cabin. Do this while it’s still warm because any pinhole leakin the summer will be an arctic blast in very cold weather. Keeping warm is acombination of a tight cabin and a tight heater. Let either slip and you’ll endup with cold feet.

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