Note: We asked Don Maxwell to give his comments on the proper operation
and maintenance of carburetor heat systems on Mooney models M20C
and M20G. Don maintains lots of these airplanes in his shop and
he sees all the problems with carburetor heat systems. We're having
a high number of carb ice incidents this year. We were wondering
if maybe the problems could be mechanically related - are the carburetor
heat systems working properly in the majority of the fleet? We think
the answer is no. As pilots, when we pull out that carb heat control,
we assume the system is working properly to deliver the correct
amount of heat into the carburetor. In many cases it's not and
there is no way to melt ice if we get it. That's' bad. Here is what
Don had to say on the subject of carburetor heat systems in Mooneys.
The bottom line is this - get your carburetor heat system checked
out soon by someone who knows how the system is supposed to work.
Your safety could depend on it.
This article actually came about after a call from Bob Kromer.
Bob mentioned that the Mooney community had experienced a high number
of carburetor ice incidents this season and that the NTSB had called
with some concerns. Bob asked me if it might be possible that the
problems could be mechanically related. In other words, maybe the
pilots were doing the right thing in the cockpit (pulling on carb
heat ) but that the systems weren't working as they were supposed
to. Bob mentioned that in the course of writing an article for a
previous LOG on car ice, he had taken a look at the carb heat system
on a randomly selected C model and found it missing some important
pieces. Could this be a common problem with the Mooney fleet in
At the time he made the call, we had two “C” models
and one “B” model in for annual inspections. I went
out and took a look at the carburetor heat systems on these three
planes. The Mooney’s we had in for annual had the three different
carb heat systems that have evolved over the years.
The carb heat box on the “B” model is fairly typical
of other aircraft types. One of the first things we learn as new
pilots is that the purpose of a carburetor is to mix fuel and air
into a combustible mixture. Unfortunately, while doing this carburetors
also do a pretty good job of making ice under the right conditions
and circumstances. To combat this problem, aircraft manufacturers
have devised ways to introduce heated air into the carburetor to
prevent and remove ice that forms inside. Over the years, several
different variants of carburetor heat system have been installed
on the carbureted Mooneys we fly.
The three Mooneys in my shop had three different variants of the
carb heat system that had evolved over the years. None were the
same. On the '61 model, the carb heat system is fairly typical of
those found on other general aviation airplanes.
Intake air for the carb heat system is taken in at the back of the
“doghouse” baffle around the engine. This air (not hot
yet) is routed through a flexible hose to a shroud surrounding the
muffler. The air is then heated as if flows past the engine's muffler
inside the shroud. Once heated, the now hot air is ducted to both
the carb heat valve located on top of the carb intake and cabin
heat valve located on the firewall. Select carb heat and the hot
air goes into the carburetor. Select cabin heat and the hot air
goes into the cabin. Select both and the hot air goes both places.
On the “B” and early “C” models there
was a problem with this system. If neither cabin heat or carburetor
heat was in use. there was no place for the heated air to flow out.
There was no "bypass" valve allowing for good circulation
around the muffler with out carb heat or cabin heat selected. The
extreme heat of the un circulated air around the muffler could cause
muffler damage. Photo1 shows a distorted muffler and heat shroud
caused by this condition and its something to watch for on your
B or early C model.
The damage to this particular muffler wasn't related as much to
the design as it was to an unapproved repair. The muffler was originally
a Hanlon-Wilson studded type. Unfortunately, one side of the muffler
was replaced in the field by the local Midas franchise. That's a
no-no. The muffler was then distorted by uneven heating. This is
really dangerous. You can't jury rig repairs like this. Cabin and
carb heat comes from air circulated around the muffler. A crack
or break in the muffler due to an unapproved repair can kill occupants
due to carbon monoxide. A large exhaust leak under the shroud can
cause the engine to run very rough when carb heat is applied. All
exhaust systems are repairable, no matter how bad the damage. Companies
such as Aircraft Exhaust Systems and Dawley can repair damaged mufflers
correctly and properly. Go to them - not Midas.
To remedy the potential problem of muffler damage due to excessive
heat buildup inside the shroud, later C and G models (from '62 to
'68) incorporated a bypass valve located on top of the carburetor
heat box. This valve allowed for the hot air circulated around the
muffler to be vented overboard when neither cabin heat or carb heat
were selected on. This arrangement consisted of two valves connected
by a linage and operated by the carb heat control in the cabin.
In normal operation, the carb heat valve is closed and the bypass
valve is closed, keeping all the hot air directed into the carburetor.
That's the way the system is supposed to work. Unfortunately, over
years and hours of vibration and heat, the system tends to develop
problems. On the B model in our shop, the engine mounts were sagging,
allowing the carburetor intake scoop to rub on the bottom cowling.
The scoop was worn enough from this interference that the carb heat
valve was not sealing properly when car heat was selected (see photo
#2) Even though the valve was in good condition, without a seal
surface the car heat on this Mooney M20B would not be effective
if needed. Avery bad situation. The early C model in our shop incorporated
the Mooney design of the time that had the bypass valve on top of
the air scoop. This system design require the most maintenance of
all carbureted Mooneys for the carb heat to work properly. Photo
#3 shows the carb heat system on this early C model before being
removed for inspection. Photo #4 shows the wear we found to the
carb heat valve shaft. The shaft was worn out. Another bad situation,
allowing for partial carb heat at best.
Photo #5 shows in detail the bypass valve and bearing sleeve we
talked about earlier. Remember, this valve allows for hot air to
be ducted when neither cabin or car heat is selected, It is not
uncommon to find this valve missing or even plugged off with a cap.
On several occasions, I have found where some mechanic decided to
attach hose to this valve and route the hot air to such places as
the magnetos, the generator or the fuel pump. Can you imagine that
- to the fuel pump! The correct thing to do with the hot air coming
out of the bypass valve is to attach a short piece of tubing to
it and to vent it overboard as shown in photo #6.
Photo #7 shows the car heat components removed from our early C
Model disassembled on the workbench. It is obvious that many of
these components have been repaired in the past by welding and brazing.
Owners are under the impression that these pieces are not available
any more, so they undergo these home repairs .These are normal stocking
parts for any Mooney service center. Check with them for availability
and price before resorting to home repairs. It's your neck if you
need carburetor heat and the system doesn't work properly.
In 1968, Mooney removed the bypass valve from the top of the intake
scoop and moved it to the firewall at the heater control valve (photo
#8). This was the configuration we found on our third airplane.
This configuration the troublesome linkage arrangement seen on the
earlier M20C models. With this arrangement, the bypass valve was
now controlled by the cabin heat valve. During normal operation
the cabin heat valve is closed and the heated air from the muffler
shroud is vented overboard through a hose from the cabin heat box.
When cabin heat is applied, the valve opens and directs all the
heat from the bypass valve into the cabin. on our third airplane
to inspect, everything seemed to be in good working order. But two
out of three randomly selected carbureted Mooneys in our shop had
problems with the carburetor heat systems. Problems bad enough that
partial to no heat was being routed to the carburetor when the pilot
selected carb heat full on. That's not good and is reason to suspect
that the Mooney carbureted fleet in general could be suffering from
the same problems.
Mooney service instruction M20-14 dated 6/6/66 refers to maintenance
and inspection of the carburetor heat system. Make sure your mechanic
has a copy during you next 100 or annual inspection. And I know
this sound like self promotion, but there is nothing like getting
your airplane into a shop with knowledge and experience working
on Mooneys. There are areas on our airplanes that need someone with
type-specific information and experience. The Mooney carburetor
heat system is one of these area's. Take your carbureted Mooney
to an experienced Mooney shop the next time you need a good inspection
and repair don on the carburetor heat system. There would be nothing
worse than getting carb ice pulling out the carb heat control and
having the system not work. Without carb heat, you're going down
with ice in the carburetor. Call me if you need help or support
in the maintenance of your Mooney. I can be reached in Gladewater,
Texas at (903-643-9902) or e-mail me at