Most modern aircraft are equipped with 2 or more
fuel tanks (or cells). In high wing aircraft, the cells are housed in the wings.
Since they are higher than the engine, the fuel flows down to the engine by the
force of gravity.
On low wing aircraft fuel pumps are required. To initially get fuel to the
engine for starting, an electrical “boost pump” is turned on to pump fuel to the
engine. After the engine is started, a mechanical fuel pump driven by the engine
feeds fuel to the engine. The electric boost pump can now be turned off.
Each fuel tank is equipped with a drain valve located at the lowest point in
the tank. This drain allows the pilot during preflight walk-around to check for
and drain off any water which may have accumulated in the fuel tank. There is
usually another drain located at the lowest part of the fuel piping system. This
valve must also be drained during pre-flight to eliminate any water which may
have accumulated in the fuel lines. Associated with this drain is a fuel
strainer which filters out foreign matter which may be in the fuel system.
A vent line allows air to enter the tank as fuel is used. During hot weather,
fuel may expand and overflow through the vent when tanks are full.
A fuel selector valve located inside the cockpit allows the pilot to select
which tank(s) are to be in use during flight. Most small aircraft operate with
the selector set on Both, such that both the left and right fuel
tanks are simultaneous feeding fuel to the engine. The pilot may set the
selector on Left or Right tank as a means of equalizing the
loading of the aircraft. Usually, the selector should be set to
both for take-off and landing. Pilots of low wing aircraft should
exercise caution in their fuel management if tank selection is other than
both. Running a tank dry can cause the engine to
quit and vapour lock to occur in the fuel lines. It may be impossible to restart
the engine under these conditions.
There is a fuel gauge in the cockpit for each fuel
tank. The lower 1/4 of the fuel gauge indication is marked with a red
line as a caution to the pilot of a low fuel condition. The pilot should never
rely on the fuel gauge as the sole measure of fuel remaining. The gauges on
aircraft are subject to a variety of indicator errors. The pilot should
therefore double check the fuel remaining based on the power setting of the
engine in flight and time in flight.
Inside the cockpit a fuel mixture control and a fuel primer pump are located
on the instrument panel. The mixture control is used to adjust the air/fuel
mixture for the altitude being flown. It allows the pilot to adjust the fuel/air
ratio entering the engine. As altitude is gained, the intake air becomes less
dense. Less fuel must be fed through the carburettor to permit the fuel/air
mixture to remain correct proportion. If leaning is not accomplished by the
pilot, a rich mixture (too much fuel) results. This is not only
wasteful of fuel, but can result in fouled spark plugs due to
carbon and soot buildup on the spark plugs. A rough running engine results. An
additional gauge called an Exhaust Gas Temperature Gauge can be installed in the
aircraft as an aid in achieving the proper “leaning” of the engine.
The fuel primer is a plunger that can be used in cold weather
to inject fuel directly into the carburettor as an assist in starting the engine
in cold conditions.
Three different grades of fuel are used in reciprocating engine aircraft.
These grades are designated by octane rating and are colour coded so the pilot
can insure the proper grade of fuel is being pumped into the tanks.
These grades are:
OCTANE RATING..........FUEL COLOUR
..... 80/87........ ..... ..... .........Red
.......100LL (low lead).
...... 100/130.... ..... ...............Green
When refuelling, if the appropriate grade of fuel is not available, USE THE
NEXT HIGHER GRADE. Using a lower grade can cause overheating and damage to the
engine. Sparks during refuelling can be an extreme fire hazard. The following
precautions should be taken when refuelling is in progress.
1. Attach a ground wire between the fuel pump or truck to a metal part of the
aircraft. This will neutralize any static charge which may exist between the
pump and the aircraft.
2. The fuel nozzle should be grounded to the side of the fuel filler hole
3. The fuel truck should be grounded to both the aircraft and the ground.
Do not use automotive fuel unless the engine has been specially modified for
automotive fuel use.