In cold weather operations, the heater system is critical to both your comfort and health. Here's what to check out.
January 20, 2002
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
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
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
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
Scat Tubing and Sceet Ducting
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
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
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.
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
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.