Flight Training - November/December 2024 - 42

FUEL SYSTEMS
Of all the systems on the airplane that your actions
can affect most, the fuel system is at the top of the list.
You'd be shocked by how many off-airport fuel starvation
accidents there are each year in which a pilot
puts down with one full tank and one empty tank
under the impression that all fuel had been expended.
There are nearly as many different fuel systems as
there are different airplanes. Broadly speaking, there
are gravity-feed systems and there are fuel-pump
supply systems-but the devil's in the details. Vents,
sumps, lines, cutoffs, and tank selectors for multiple
tanks all vary. The key point is to understand how a
fuel molecule gets from the bottom of the fuel truck to
the top of one of the cylinders in your engine.
A failure of the fuel delivery system results in a
sputtering engine that quickly goes silent. Assuming
you have fuel, your knowledge of potential roadblocks
can assure you'll clear them should they arise.
Check and cycle the fuel tank selector or selectors.
This ensures that if somehow one of your fuel shutoff
controls has closed, it's now open. Do you have
maintenance shutoffs for the fuel lines
on the side walls or under the panel?
If so, check those by opening and
closing them. And if contamination
is blocking the line, there's
a chance that opening and
closing each may dislodge it, or,
depending on the fuel system,
may allow you to bypass the
blockage and deliver fuel from a
different tank.
INDUCTION SYSTEMS
The induction system is how air is
introduced into the combustion cycle. It
provides a path for the transport of air, mixes that
air with fuel, and delivers the mixture to the cylinders.
Pretty much all induction systems draw air from
the front of the cowl, typically through an air filter. As
the filter might become clogged by debris or blocked
by ice, many airplanes have an alternate air supply,
usually drawing from the inside of the cowl, that are
automatic or pilot-deployed.
The induction system also includes either a
carburetor or fuel injection system, either of which
can create different problems on both the ground
and in the air. On the ground, the starting procedures
vary. In the air, carburetors are subject to carb ice at
a wide variety of temperature, dew point, humidity,
and power combinations-so most are equipped with
a source of hot air from the engine compartment that
can melt off the ice called carb heat. This control can
also be used to prevent carburetor icing in conditions
where it is likely-such as throttling back to land on a
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FLIGHT TRAINING NOVEMBER/DECEMBER 2024
humid day, even when it is quite warm. The design of
the carburetor can lower the temperature of the fuelair
mixture by 70 degrees Fahrenheit.
Carb ice builds up in the venturi, the skinny part of
the carburetor where the throttle butterfly valve lives.
The initial buildup is generally slow, so the first warning
sign is a small reduction in rpm like what you'd get
from a small reduction in throttle. In fact, that's what
you are effectively getting. Just like pulling back on
the throttle causes the butterfly valve to reduce the
flow through the venturi, so too, does ice build-up.
As the ice builds, rpm continue to drop off. If the
ice gets bad, the engine will start running rough and
eventually can be choked off completely and stop in
flight. To avoid this, get the carb heat on as soon as
carb ice is suspected-basically any unexpected rpm
drop-even before checking the other possible rpm
theft suspects. This should be your first action for two
reasons. First, there is no harm in having carb heat
on for a time, even if it didn't turn out to be needed.
Second, if there is ice, clearing it takes time. Picture
holding a hair dryer to an ice cube.
Leave the carb heat on while you troubleshoot.
Check the throttle response. A loose throttle friction
lock can cause an rpm loss, as can a vibration-induced
mixture change. So too, can a magneto failure, which
you can rule out with an in-flight mag check. More
rarely, contaminated fuel can trigger an rpm drop, so
switch tanks. There are many possible causes of an
rpm drop, but carb ice is both the most likely and the
issue that takes the longest to fix. In some airplanes,
engaging carb heat causes yet more of an rpm drop,
which leads some pilots to reverse their action, letting
the ice start to build again.
Like the fuel system, having a mental picture of
how air travels through the induction system is essential
to diagnosing combustion issues.
POWERPLANTS
Knowing the brand, horsepower, and the rest of the
commonly taught rote knowledge on engines doesn't
help in problem-solving engine issues. Instead, know
that engines only need three things to run: fuel, air,
and spark. If your engine isn't running well, or stops
altogether, those are the things you need to check.
The final delivery mechanisms of the fuel system
are the throttle and mixture controls. These are crucial
elements of the system when it comes to troubleshooting.
Power spool back? It could be as simple as a loose
friction lock on the throttle. Engine running rough?
Maybe the mixture is set too lean for your altitude. The
two levers are possible points of failure in the smooth
delivery of fuel from the tanks to the engine.
Now let's look at spark. At first you might think
you have little power over this aspect of the powerplant,
but you do have one control: the ignition switch.
Flight Training - November/December 2024

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