Minimal functional requirement of
fuel systems is to supply fuel to the engine in sufficient quantity and quality
and at a certain pressure. The system contains of two sub-systems -- the fuselage
fuel system and the engine fuel system. This web page deals with the former sub-system.
The construction of the fuel tank
system largely depends on the task of getting the fuel from the tank to the tank's
exit, and from the tank's exit to the engine. The location of the tanks is usually
determined by the platform's structure, available space and impact on CG (centre
of gravity) location. In some aircraft, external tanks, belly tanks or tip tanks
may be fitted. When additional fuel is needed, for example, for an extended range,
ferry tanks are employed.
Plumbing is the name given to the fuel supply
lines. They are depicted as solid in the illustration but in real life obviously
this is not true.
Lines are generally made from metallic tubing,
not pipes.
Routing must be placed on the opposite side
of the aircraft from the oxygen supply and below any electrical wiring.
The flow of fuel inside the fuel lines may
produce static electricity. The lines are therefore electrically connected to
prevent any electrical charge accumulation.
Fuel tank
This is the fuel storage component. It must
be large enough to carry fuel for the whole mission.
Fuel tanks must have means of dumping fuel
in cases of emergency.
They must be vented so that used fuel may
be replaced by air or inert gases. If there is insufficient flow of air into
the tank, it may collapse (due to loading caused by high performance or steep
descend from altitude to sea level). For integral wing tanks, the pressure difference
may cause structural damage and component failure.
Fuel tanks may have a sump which collects
water and solid particles in fuel.
Level indicator
As the name suggests, this component provides
the flight crew with the current measurement of fuel content.
Fuel measurement are difficult to measure
as they depend on fuel tank shape and aircraft attitude.
Pumps
The pumps may be mounted externally or immersed
in the fuel.
It is important that the pumps are free of
fuel vapour intrusion as they supply fuel to the engine. To avoid this, air
separators are installed at the pump inlets.
Pressure switch
The switch gives no indicator of the pressure
value but rather reacts to the presence of pressure which is above the pre-set
lower limit of the switch.
Relying solely on pump power indication is
insufficient because even though power switch may be on, the pump may not be
working due to seized rotor or blocked inlet. The pressure switch accounts for
this.
It gives the flight crew an indication of
which pumps are operating since not all pumps are switched on during operation
(during take-off and aerobatics, both pumps are on).
Non Return Valves (NRV's)
Since during operation only one pump may be
working, the NRV ensures that fuel pumped from the operating pump is not returned
to the fuel tank via inoperative pump.
Pump by-pass valve
This valve allows the fuel from other tanks
to enter the indicated tank.
If incorporated with a cross feed valve, it
becomes a three way valve.
Tank isolation valve
This valve isolates the tank from the fuel
system. This is usually done in cases of emergency or if a fuel leak develops
in the tank, thus draining the remaining aircraft fuel.
Under normal operating conditions, this valve
is open.
Cross feed valve
Requirements state that the engine may only
be supplied with fuel from one tank at a time. The cross feed valve allows for
this. It supplies fuel from other tanks.
This valve is usually closed under normal
operating conditions.
Low pressure valve
This valve is fitted after all tanks have
had access to the fuel line.
It allows maintenance on any part of the fuel
circuit in the fuselage that is not connected to the tank fuel distribution
circuit.
Power drain valve
This valve allows quick discharge of fuel
from the system for maintenance or emergency purposes.
Power drain valve must be closed before engine
start up.
High pressure feed system
Backing pump
This is not an essential component, however,
in case the engine requires a fuel pressure higher than that produced by tank
pumps, this pump is able to do so.
It is required that the engine must still
operate without the use of this pump.
Fuel Cool Oil Cool (FCOC)
The volatility (ability of evaporate) of fuel
may be increased by heating it. This is particularly important if fuel is supplied
from wing tanks where it has been exposed to low temperature soak at altitude.
The hot oil from engine heats up the cool
fuel and in the process cools itself off, thus reducing the number of oil cooling
components in the platform.
Fuel heater
If additional fuel heating is required, the
fuel heater is able to do this by the use of high pressure compressor gas.
Fuel filter
As the name suggests, this component filters
out unwanted particles contained in the fuel.
The filter may be configured to trap water,
in which case, it may become blocked up due to ice formation.
To avoid clogging when contaminants and/or
ice are trapped, a bypass valve is installed to get around a blocked filter.
Regular inspection is essential unless the
filter is equipped with an electrical signaling device which indicates blockage.
Generally, change over from fuselage to engine
occurs between these two components.
Flow meter
Quantity of fuel supplied to the engine is
an important piece of information for management of flight fuel use.
If installed as it is depicted in the above
illustration, it doesn't account for the fuel bypassing the engine and returning
to tanks.
To give a correct fuel flow to engine indication,
it can be installed after the fuel control unit (FCU) where any fuel flowing
must go to the engine.
Pressure transmitter
This transmitter measures the pressure of
the fuel entering the engine.
If there is a problem during the mission,
the flight crew can eliminate the fuel pressure as a cause of this problem by
reading the value off the pressure transmitter.
Pressure switch
This switch monitors the pressure of fuel
entering the engine. If a low pressure is sensed, the switch automatically alerts
the crew and activates master caution warning.
High Pressure pump (HP pump)
This gear pump supplies the engine with fuel
and is driven through an accessory gearbox mounted to the engine.
It is capable of producing pressures of up
to 5.5 MPa.
