Fuel system

INFORMATION: Fuel System Fundamentals

Fuel Management Panel

The fuel management panel (fig. 4-1) provides a centralized means of controlling fuel

movement within the fuel system. It has gauges that reflect quantities and system operation.

Its location on most aircraft is in the control cabin/cockpit, though some aircraft have an

additional fuel management panel that is located externally to control fuel flow on the ground

for servicing operations. A typical fuel management panel includes the necessary switches

and indicators to accomplish the following:

• Control and monitor individual engine fuel feed

• Control tank-to-tank fuel transfer within aircraft

• Indicate quantity of fuel stored in fuel tanks

• Alert the pilot of fuel system malfunctions with warning lights

• Control/monitor air or ground refuel operation

• Rapidly dump (release) fuel in emergency situations

Fuel tanks or cells

Provide fuel storage and can vary in design, size, construction, location and shape based on

the aircrafts particular need (fig. 4-2, 4-3). Usually, a fuel tank is designed as an integral

(built-in) part of a wing or the fuselage but can also be bladder and external styles.

Integral fuel cells

Contained in the wing structure; however, in some aircraft, integral fuel cells are also built

into the fuselage (main body) of the aircraft. An integral cell is a part of the aircraft structure

that has been built with such precision that the seams, structural fasteners, and access doors

have been properly sealed with fuel resistant sealant to prevent leaking.

Fuel Bladders

Bladder type fuel tank (fig. 4-4) are installed into compartments called cavities and are

constructed of either rubber or nylon.

Tank Filler Cap (fig. 4-5)

The filler cap allows for over-the-wing (gravity) servicing of both Integral and external tanks.

The Filler cap is also used for visual and dip stick checks of fuel quantity in each tank. Filler

caps typically have a locking tab to control the locking mechanism.

External fuel storage

Increase range or mission by providing additional fuel. For some aircraft, the external fuel

system consists of the fuel (drop) tanks (fig. 4-6), a transfer system, and a jettison (release)

system

Fuel pumps

The aircraft fuel system uses various types of pumps to move fuel through the system

plumbing. Electrically powered fuel pumps are mounted inside the tanks and use fuel for

cooling while operating. The following pumps are used to move fuel from the tanks to the

engine or from tank-to-tank.

Boost pumps

Ensure a positive fuel supply to engine driven pumps and can also be used to defuel tanks.

Transfer pumps

Utilized for the transfer of fuel from tank-to-tank and can also be used to defuel tanks.

Ejector pumps

Operates by fuel pressure from a transfer pump using the Venturi (fig. 4-7) principle to

remove fuel from tanks or cavities that may have fuel fumes that could explode in the

presence of a faulty or non-submerged electrical pump.

Fuel valves

A valve is a device used to control fluid passing through a pipe or duct. They can be operated

manually, mechanically, or electrically. Unless otherwise specified, the control switch for

each valve will be located on the fuel management panel. The following valves are examples

of what can be controlled on the different types of aircraft.

Engine Feed Valve

The engine feed valve controls the flow of fuel from a designated tank to its respective engine

(i.e., from #1 main fuel tank to #1 engine). Closing the valve for an engine prevents fuel from

its respective main tank to feed the engine. This allows the aircrew to use fuel from other

tanks first and conserve the main tanks for a later part of the flight. In some aircraft, fuel in the main tank can gravity drain into the engine if the valve is open.

Cross Feed Valve

The cross feed system permits interconnection between the left and right sides of the aircraft

and engine feed lines by using a shutoff valve referred to as a cross feed valve. This valve

provides personnel the ability to move fuel from tank-to-tank on either side of the aircraft, as

well as allow any engine to be fed by any fuel tank.

Engine Firewall Shutoff Valve

Engine firewall shut-off valves are located on each engines firewall and provides the pilot the

capability to electrical shutoff fuel to each engine in the event of an emergency. The normal

(open) position holds the firewall shutoff valve open to allow fuel to the engine. T-handles or

emergency pushbuttons located on or above the forward instrument panels in the crew

compartment control the corresponding firewall shutoff valve.

Check Valve

Check valves are installed in the fuel system wherever fuel flow in one direction is required.

Direction of flow is indicated by an arrow on the valve. Pressure forces the valve open by

pushing against a spring.

Fuel Level Control Valve

Fuel level control valves are located internally at the top of each

fuel tank. Fuel flows through this valve when servicing your

aircraft’s fuel tanks, stop the flow of fuel into a fuel tank and

prevent it from overfilling. You can selectively open and close

individual tank valves at any point during the refuel operation

using switches on the refuel management panel. Lastly the

operation of this valve can be compared to the tank reservoir

float on your toilet (fig. 4-8). As the fuel level in the tanks reach

their maximum capacity, a float inside each tank senses the level

and automatically closes the valve.

Pre-Check Selector Valve

There are two potential hazards if the fuel level control valve does not close (fails), fuel will

be forced from the tank through vents to the ground, or worse, the tank can be pressurized to

the point it ruptures. To guard against these hazards some aircraft use a pre-check selector

valve to check the fuel control valve for proper operation. Positioning the pre-check selector

valve switch on the refuel control panel to the test position simulates a full tank. This causes

the fuel level control valve to close and stop the flow of fuel into the aircraft’s fuel tanks.

Condensation Drain Valve (Sump Drain)

Condensation drain valves are located at the lowest point of each sump to provide the ability

to remove the water and other contaminants from the tank (water can enter fuel as

condensation). It also provides a means to take a fuel sample from each tank. Condensation

drain valves are spring-loaded valves held closed until they are operated by simply pushing

upward on the bottom of the individual valves.

Fuel Manifolds

Fuel manifolds (tubing and hoses) transports (carry) fuel from place-to-place within the fuel

system and are constructed from steel, aluminum, or rubber and will vary in size depending

on the quantity of fuel that needs to pass through them. Typical manifolds in the fuel system

include the Single Point Refueling (SPR) manifold, which routes fuel from the SPR receptacle

to each tank, the engine feed manifold which routes fuel from a fuel tank to its respective

engine and the cross feed manifold which routes fuel from tank to tank or any tank to any

engine. Pumps pressurize the fuel and valves control the fuel’s movement.

Fig. 4-8, Fuel Level

Control Valve

CUI//Rel To USA, AF, AR, BL, BR, CI, CO, EC, ET, ASFF (H5 and J3), FR, GB, HO, ID, IQ, JO, KU, MX, NK, NI, PE, PI, RO, SR, TU, TK AND UY

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Fuel Filters

Filters are placed in the fuel system to remove foreign material. This material may enter the

fuel system during fuel servicing, maintenance, or from the wear of parts such as pumps and

valves. The small openings make this type of filter susceptible to clogging; therefore, a bypass

valve is necessary because without fuel the engine cannot operate.

Heat Exchangers

As stated before, a heat exchanger is comparable to a car’s radiator. In a fuel system, the heat

exchanger heats the fuel for better atomization and cools the oil or hydraulic fluid for better

lubrication and cooling. Fuel flows into the heat exchanger through a series of tubes. Oil or

hydraulic fluid passing around the tubes warms the fuel (fig. 4-9).

Fuel Subsystems

Fuel vents and pressurization systems aid in fuel transfer and usage by maintaining positive

air pressure in the tanks and prevents excessive pressure buildup. The vent components

prevent excessive pressure due to temperature changes or during refueling/defueling. This is

accomplished by allowing air to enter tanks to displace fuel being removed during engine

operation, transfer, or defuel.

The pressurization system keeps the tanks at a positive pressure which aids the pumps in

keeping fuel flowing and helps control fuel vaporization resulting from negative pressure. A

pressure relief valve is critical to the safe operation of the fuel system by ensuring the

negative and positive pressures within the fuel system are maintained within safe limits

throughout flight as the aircraft ascends and descends.

Fuel Dump System

The aircraft fuel dump system provides a means of rapidly off-loading fuel in-flight in case of

an emergency. It is used to reduce the possibility of fire during emergency landing, or to

reduce the total weight of the aircraft. A dump valve is opened allowing fuel to transfer to a

port, sometimes called a dump mast, where fuel is dumped overboard. This mast may be

located near the wing tips or on the empennage.

Fuel Quantity Indicating System

Regardless of the aircraft, a fuel quantity system must continually show the total quantity of

fuel (in pounds) remaining in the aircraft fuel tanks. The tank sensor/probe is a transmitter

that electrically measures the amount (level) of fuel in the tank. This information is sent to a

to a fuel quantity indicator on the fuel management panel.

There is a fuel quantity indicator for each tank that receives the signal from the tank

sensor/probe unit, decodes the signal and shows the amount of fuel in the fuel tank. Heavy

aircraft with multiple fuel tanks may contain different fuel levels in each tank. The fuel total

indicator (totalizer) provides a quick reference point for personnel to determine the total

amount of fuel in all tanks. When a fuel tank reaches empty, a low-level switch in the fuel

level transmitter is activated and turns on an indicator light in the cockpit to warn the aircrew

when the fuel level/ pressure is below a required minimum amount and is almost exhausted.

Refuel/defuel Methods

Refuel Methods

Most aircraft are refueled using a pressurized refueling system known as SPR receptacle. This

system provides for rapid fueling of all tanks by connecting a hose to a quick

connect/disconnect SPR receptacle (fig. 4-10). Fuel from this single point is distributed to the

various wing, fuselage, and external tanks. The SPR receptacle is standard on all aircraft that

use the pressure-fueling method. However, the control panel and operations differ from one

aircraft to another depending upon the complexity of the fuel system.

Gravity (over-the-wing) method provides for individual tank filling through a port covered

with a filler cap (fig. 4-11). Metal-to-metal contact between the nozzle and the aircraft fueling

port must be maintained during the entire fueling operation to prevent static buildup and a

possible explosion. Gravity refueling through tank filler caps are normally used as a backup if

the SPR is inoperable

The In-flight Refueling (IFR) (fig. 4-12) receptacle provides in-flight refueling capabilities. It

allows the aircraft to continue flying without having to stop for fuel

Defuel Methods (fig. 4-13)

Defueling may be necessary for many reasons like, fuel tank/cell repairs, fuel system

components failure, and changing fuel loads. Aircraft that use pressure-fueling are normally

defueled from the SPR, allowing for rapid defueling of all tanks by connecting a hose to the

quick disconnect SPR receptacle.

Gravity defueling is the process of draining individual tanks by opening the drain valves on

the bottom of that aircraft fuel tank. Some aircraft can be defueled by gravity methods using

the condensation drain valves.

Lastly the siphon method is used to remove fuel from external tanks (drop tanks) by inserting

a defueling hose from the defuel truck into the tank’s filler port and having the fuel truck

provide hose suction to remove the fuel out of that tank

Fuel System Safety

Refueling and defueling aircraft is extremely dangerous if not done correctly. You must

follow the TO precisely. During refuels/defuels no other maintenance may be performed and

only authorized personnel are allowed to be in the refuel/defuel area. It is imperative that all

safety precautions are followed.

• Always ensure the aircraft is properly grounded

• Make sure the aircraft is parked no closer than 50 feet from any building and there is

no AGE operating within 50 feet of the pressurized fuel source

• No smoking within 50 feet of the refuel/defuel operation.

• No radio transmissions within 10 feet of the aircraft

• Aircraft engines and ground radar will not be operated within 300 feet.

Aircraft refuel team members will always establish a three-way static grounding and electrical

bonding situation before fuel is added to or taken from the aircraft (fig. 4-14). The aircraft and

all equipment used will be electrically bonded to each other and attached to an approved

ground point located on the parking ramp.

Fuel Leak Classifications

Containment of fuel leaks is critical, if left unchecked they can lead to fires and explosions.

Fuel cell leaks are classified depending on the size of the leak evidence that accumulates

within 6-minutes of the initial evidence being removed. The classification of fuel leaks also

helps in determining the repair requirements for the system.

There are four classifications of fuel leaks shown in Table 4-1, Leak Classifications.

Fig. 4-14, F-16 Triangular Ground

CUI//Rel To USA, AF, AR, BL, BR, CI, CO, EC, ET, ASFF (H5 and J3), FR, GB, HO, ID, IQ, JO, KU, MX, NK, NI, PE, PI, RO, SR, TU, TK AND UY

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Table 4-1, Leak Classifications (fig. 4-15)

Leak Classification Criteria

A—Slow Seep 0-1/4 of an inch

B—Seep Greater than 1/4 to 3/4 of an inch

C—Heavy Seep Greater than 3/4 to 8 inches and/or less than 4 drops per minute

D—Running Leak Greater than 8 inches or 4 drops per minute