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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems Airframe Systems
Fuel Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Fuel Gauges
The fuel quantity gauges indicate the amount of fuel
measured by a sensing unit in each fuel tank and is displayed
in gallons or pounds. Aircraft certification rules require
accuracy in fuel gauges only when they read "empty." Any
reading other than "empty" should be verified Do not depend
solely on the accuracy of the fuel quantity gauges. Always
visually check the fuel level in each tank during the preflight
inspection, and then compare it with the corresponding fuel
quantity indication.

If a fuel pump is installed in the fuel system, a fuel pressure
gauge is also included. This gauge indicates the pressure in
the fuel lines. The normal operating pressure can be found
in the AFM/POH or on the gauge by color coding.

Fuel Selectors
The fuel selector valve allows selection of fuel from various
tanks. A common type of selector valve contains four positions:
LEFT, RIGHT, BOTH, and OFF. Selecting the LEFT or RIGHT
position allows fuel to feed only from that tank, while selecting
the BOTH position feeds fuel from both tanks. The LEFT or
RIGHT position may be used to balance the amount of fuel
remaining in each wing tank. [Figure 6-31]

Fuel selector valve.
Figure 6-31. Fuel selector valve.

Fuel placards will show any limitations on fuel tank usage,
such as "level flight only" and/or "both" for landings and

Regardless of the type of fuel selector in use, fuel consumption
should be monitored closely to ensure that a tank does not run
completely out of fuel. Running a fuel tank dry will not only
cause the engine to stop, but running for prolonged periods
on one tank causes an unbalanced fuel load between tanks.
Running a tank completely dry may allow air to enter the
fuel system and cause vapor lock, which makes it difficult to
restart the engine. On fuel-injected engines, the fuel becomes
so hot it vaporizes in the fuel line, not allowing fuel to reach
the cylinders.

Fuel Strainers, Sumps, and Drains
After leaving the fuel tank and before it enters the carburetor,
the fuel passes through a strainer which removes any moisture
and other sediments in the system. Since these contaminants
are heavier than aviation fuel, they settle in a sump at the
bottom of the strainer assembly. A sump is a low point in a fuel
system and/or fuel tank. The fuel system may contain sump,
fuel strainer, and fuel tank drains, which may be collocated.
The fuel strainer should be drained before each flight Fuel
samples should be drained and checked visually for water
and contaminants.

Water in the sump is hazardous because in cold weather the
water can freeze and block fuel lines. In warm weather, it
can flow into the carburetor and stop the engine. If water is
present in the sump, more water in the fuel tanks is probable
and they should be drained until there is no evidence of water.
Never take off until all water and contaminants have been
removed from the engine fuel system.

Because of the variation in fuel systems, become thoroughly
familiar with the systems that apply to the aircraft being flown.
Consult the AFM/POH for specific operating procedures.

Fuel Grades
Aviation gasoline (AVGAS) is identified by an octane or
performance number (grade), which designates the antiknock
value or knock resistance of the fuel mixture in the engine
cylinder. The higher the grade of gasoline, the more pressure
the fuel can withstand without detonating. Lower grades of
fuel are used in lower-compression engines because these
fuels ignite at a lower temperature. Higher grades are used
in higher-compression engines, because they ignite at higher
temperatures, but not prematurely. If the proper grade of fuel
is not available, use the next higher grade as a substitute.

Never use a grade lower than recommended. This can cause
the cylinder head temperature and engine oil temperature to
exceed their normal operating ranges, which may result in

Several grades of AVGAS are available. Care must be
exercised to ensure that the correct aviation grade is being
used for the specific type of engine. The proper fuel grade is
stated in the AFM/POH, on placards in the flight deck, and
next to the filler caps. Auto gas should NEVER be used in
aircraft engines unless the aircraft has been modified with a
Supplemental Type Certificate (STC) issued by the Federal
Aviation Administration (FAA).