November 1, 2011
Technical Counselor Note #7
Will Fox
Should I Use a Gascolator on My Homebuilt?
Back when men were men, airplanes were airplanes, and avgas was full of dirt and
water, real airplanes had gascolators. Nowadays we are seeing a lot of homebuilts
without this marvelous device. Why the change? Let’s see if we can answer that
question by taking a look at what a gascolator is and how it is used in an aircraft fuel
system. A little history on the subject might be useful. At the beginning of the industrial
age, when you bought fuel for your car or tractor from a drug or hardware store, it often
contained impurities, that included sediment and water. These impurities needed to be
filtered out before the fuel could be used or you would end up with a plugged up
carburetor and an engine that wouldn't run. Early cars were sold with a chamois so you
could properly filter your fuel. If you happened to own an airplane, you also used a
chamois to filter your fuel (a practice that is still quite common in many third world
countries and Alaska). The problem was that the fuel could still be contaminated even
after you filtered it, because of a hole in the chamois, leaky gas caps, or even
condensation in the fuel tanks. Needless to say, early pilots faced many an engine failure
due to contaminants in their fuel. About this time someone came up with the idea of
collecting the fuel in a small sump prior to it entering the engine and letting gravity help
to separate the water from the fuel. The addition of a fine wire screen filter on top of the
sump to separate out the dirt and bugs before the fuel left the sump and headed to the
engine, improved the situation immensely. Making the sump out of glass so the fuel and
associated contaminants could be readily seen prior to flight and adding a drain on the
bottom of the sump to dump all the bad stuff out resulted in the device known today as
the fuel strainer or gascolator.
Modern aircraft gascolators typically don't have a glass sump bowl for reasons associated
with crash survivability and fire safety, but all the other elements are present and include
an inlet and outlet for the fuel, a removable sump or sediment bowl to collect
contaminants, a drain located on the bottom of the sump to drain the water and any other
contaminants, a medium size fuel screen (approximately 100 micron) to separate solid
contaminants from the fuel, and a small port to provide fuel for a mechanical primer if
required. Figure 1 shows a variety of common aircraft gascolators available for use in
today's aircraft.
Figure #1: Different types of aircraft gascolators that are readily available for homebuilt
aircraft.
Lets discuss the installation of a gascolator in modern aircraft. Figure 2 shows a
schematic of a simple aircraft fuel system courtesy of Tony Bingellis.
Figure # 2: A simple aircraft fuel system, illustrated by Tony Bingelis from his book,
“The Sportplane Builder”.
As you can see the gascolator is located between the fuel selector and the carburetor at
the lowest point in the fuel system. This location is important because, water being
heavier than fuel, ends up finding its way to the lowest point in the fuel system, in this
case the gascolator, where it settles out and thus is not introduced to the carburetor. If this
were not the case, the water would end up in the carburetor float bowl where it could
cause corrosion or even a misfiring engine.
Figure 3 shows a fuel system layout for the Cirrus SR20 with a fuel injected engine. You
can see the gascolator is located after the fuel selector valve and between the electric and
mechanical fuel pump. Once again it is located at the lowest point in the fuel system to
best collect water. Since the mechanical fuel pump is mounted to the engine at a higher
location, the gascolator must be located upstream from it to properly do its job. While
gascolators are commonly associated with carbureted engines, they are also used on most
certificated aircraft using fuel injected engines.
Figure #3: SR20 Aircraft fuel system schematic courtesy of Cirrus Aircraft, Pilot
Operating Handbook.
In order for the gascolator to be mounted at the lowest point in the fuel system and close
to the engine at the same time, it is usually located on the lower part of the firewall.
Often times the gascolator on a fuel injected engine is located upstream of the firewall to
minimize fuel vaporization problems. We will cover this in more detail later. Figure 4
shows a typical firewall installation in a homebuilt aircraft.
Figure #4: A blue gascolator is shown mounted to the bottom of the firewall at the lowest
point in the fuel system. Also note the sump valve located on the bottom of the
gascolator.
The drain for the gascolator is on the bottom of the sump and is usually located such that
it is parallel with the bottom of the cowl or protrudes slightly below it so that a fuel
sample can be easily taken during the preflight of the aircraft. A gascolator can be
mounted either before or after the electric fuel pump in a fuel injected engine, but if it is
mounted after the electric fuel pump it must be rated for the nominal operating fuel
pressure that is usually between 20-30 psi. Mounting the gascolator after the electric fuel
pump can reduce the likelihood of vapor lock but it also exposes the electric fuel pump to
contaminants that would normally be separated out by the gascolator.
So why doesn't everyone want to use a gascolator in their homebuilt? I think it
has to do with several different things. First, most homebuilder don't live in third world
countries or Alaska, so they are used to getting clean fuel just about wherever they go in
good ol’ America. Second, most of them hangar their aircraft so they don't have to worry
much about getting water in their fuel due to leaky gas caps. Third, the increase in high
speed homebuilts with tightly cowled engines has resulted in higher temperatures in front
of the firewall. This combined with, low wing aircraft and fuel injected aircraft engines,
leads to one of the problems associated with gascolators and that is vaporization of the
fuel prior to reaching the engine, leading to a rough running engine. You see, the
gascolator has a lot of surface area exposed to the high temperatures in the engine
compartment and so the fuel in it can get hot. Combine this with a mechanical or electric
fuel pump sucking on it, and no appreciable pressure head provided by a low wing
aircraft, and you can see how vaporization can occur. This is most likely to occur on
climb out at low speed, or at cruise at high altitude where cooling demands are the
highest and so are engine compartment temperatures. It also commonly happens during
prolonged ground operations on hot days. Yet, almost every certified piston powered
aircraft, fuel injected or not, has a gascolator and they seem to work fine, so like they say
on TV, "How Do They Do It"?
There are really two solutions in use. One is to separate the gascolator from the high
temperature in the engine compartment and the other is to provide some dedicated
cooling air to it. Sometimes they do both, such as on the Cherokee 180 where the
gascolator and electric fuel pump are enclosed in a sheet metal compartment inside the
engine compartment with a two inch cold air duct feeding cool outside air to them. In
Mooney and Bonanza aircraft, the gascolator is part of the fuel selector valve and is
mounted behind the firewall and below the cabin floorboards along with the electric fuel
pump where it is close to the fuel pump and at the lowest point in the fuel system and it
doesn't see the high engine compartment temperatures. Figure 6 shows an example of the
system used in a Bonanza.
Figure #6: Illustration of a Bonanza Fuel System showing the integral fuel selector valve
and gascolator courtesy of Norman Colvin. The gascolator/fuel selector assembly is
located between the fuel tanks behind the firewall and has the stem and fuel selector
handle protruding above it.
The addition of a gascolator to a fuel system also adds complexity and another place to
develop a fuel leak as well as another item to be inspected at the annual condition
inspection. So the hundred dollar question is, do the advantages of using a gascolator
outweigh the additional complexity of installing one? The answer is it depends. If you
always get clean avgas, and don't expose your aircraft to an environment where water can
be easily introduced to the fuel system either through leaks or condensation, you probably
don’t need one. However, if you occasionally fuel your aircraft from sources that are
somewhat questionable, or leave it outside in a bad rainstorm, or with tanks less than full
for long periods of time, you might appreciate the advantages of a gascolator.
Let me tell you about some other advantages that a gascolator has that you might be
interested in. It can provide a temporary source of fuel when engine demands
temporarily exceed flow from the fuel tank. This can occur sometimes in turbulence or
during a slip or skid, when the fuel tank is low and the fuel line from the tank becomes
unported. Another advantage is that the fuel filter in the gascolator is sized such that it is
finer than the finger strainer in the fuel tank but larger than the very fine filter in the fuel
injection servo or carburetor. This results in three levels of fuel filtration that can be less
prone to plugging in some instances. Multiple size filters in series are better at
preventing plugging than a single size filter in this situation. For example, the finger
strainer in the fuel tank stops the big stuff like insects and grass, but let's the fuel, rust,
and dirt through. The gascolator strainer stops the rust, but let's the dirt and fuel through.
The carburetor or servo strainer stops the dirt but let's the fuel through. Whereas a single
fine filter would become plugged with the grass and insects and let no fuel through.
Finally, the location of the gascolator lets you sample your fuel right before it goes into
the engine and see what might have gotten by the fuel tank finger strainer. But make sure
you take a good look at the sample that comes out of the gascolator. It might look like
the sample you see Figure 5. Yup, that’s a mason jar full of mostly dirty water with some
Avgas on top. It came out of the right wing tank of a Beech Bonanza along with three
others just like it. The pilot discovered the contaminated fuel after he experienced an
engine stoppage while flying at night. After fueling earlier in the evening he had
“checked” the fuel sample from the gascolator by draining it on the ramp and looking for
bubbles. This is known as the AWFPH (Accident Waiting For a Place to Happen)
approach to fuel sampling. Studying fuel samples on the tarmac really doesn’t work very
well in the daytime, let alone at night. So do yourself a favor and collect your fuel
sample in a container where you can easily inspect it for contamination.
Figure #5 The brown stuff at the bottom of this Mason jar is dirty water. The pilot
collected this sample -- and three others like it -- from the right tank of a Beech Bonanza,
after the engine quit in flight. The pilot checked the fuel before his nighttime takeoff.
After landing, he said he'll never again check the fuel by draining the fuel on the ramp
and looking for bubbles. "Even when you use a flashlight, you're not going to see dirty
water on black asphalt at night."
I have kidded a bit about the availability of clean fuel in Alaska. Let me tell you a story.
Shortly after I got my private pilot license, a good friend and I flew to Alaska in a
Cessna 150. At one point we had to land on a grass strip in the middle of nowhere and get
fuel. The fuel came out of some very old 55 gallon drums that had seen a lot of abuse.
The "lineman" attached a hand pump to the drum and had my buddy pump the fuel out
of the drum while he fueled the aircraft using a very large funnel lined with a less than
pristine, chamois. After he fueled the aircraft, he showed me what was left in the funnel
sitting on top of the chamois. There was about a pint of dirty water and several
tablespoons of rust along with pieces of leaves and bug parts. I’m not exaggerating here.
I looked at the “lineman” in complete, utter dismay. He said that I should sample my fuel
and see if anything had gotten through, Duh! Amazingly, very little had, but I still found
some water and rust in the gascolator that took about a half a dozen refuelings to get
completely out of the tanks. I was sure glad that I had that gascolator while I was flying
over all those mountains in Alaska. It's also why I have one in my homebuilt today. You
never know when you might want to fly to Alaska.