Aircraft Fuel Systems

Question 1

What are two types of fuel cells used in modern aircraft?

Answer 1

Integral fuel cells (cells that are sealed-off portion of the structure), and bladder-type cells.

Question 2

What types of operations are fuel systems designed and certified for?

Answer 2

Any maneuver or operating condition for which the aircraft is certified.

Question 3

Where can you find the correct part number for the fuel quantity sensor to be installed in an aircraft fuel tank?

Answer 3

In the illustrated Parts Catalog for the aircraft.

Question 4

Where are fuel system strainers located?

Answer 4

One strainer is located in the outlet of the tank, and the main strainer is located in the fuel line between the outlet of the fuel tank and the inlet to the fuel metering device.

Question 5

Why do the fuel strainers used on some turbine-engine aircraft have a warning device that signals the flight crew when the strainer is beginning to clog up?

Answer 5

Strainers clog because ice forms on the filter element. The flight crew can route the fuel through a fuel heater to melt the ice.

Question 6

How does water appear in the fuel drained from the tank sumps?

Answer 6

Water appears as a clear globule in the bottom of the container used to collect the fuel drained from the sumps.

Question 7

Why must an aircraft fuel valve have a detent in its operating mechanism?

Answer 7

The detent gives the pilot a positive indication by feel when the selector valve is in the full ON and full OFF position.

Question 8

What must be done before a fuel selector valve can be removed from an aircraft?

Answer 8

The tank to which the valve is connected must be drained or otherwise isolated.

Question 9

What must be done after a fuel valve is replaced?

Answer 9

The fuel system must be checked for leaks.

Question 10

Why are fuel tanks divided into compartments or have baffles installed in them?

Answer 10

The compartments or baffles keep the fuel from surging back and forth as the aircraft changes its attitude in flight.

Question 11

How is a fuel leak indicated on a reciprocating-engine-powered aircraft?

Answer 11

The dye in the gasoline stains the area around the leak.

Question 12

What is the function of the sump in a fuel tank?

Answer 12

The sump, or lowest point, in a fuel tank is located below the fuel outlet. The sump collects water and contaminants and will have a drain to remove them.

Question 13

What are the three different types of fuel tanks?

Answer 13

Built-up tanks, integral tanks, and bladder tanks.

Question 14

What problem can occur in a bladder tank with a blocked vent?

Answer 14

As internal tank pressure decreases, the pressure differential between the atmosphere and the tank can cause the tank restraints to fail and the tank can collapse and block the fuel outlet.

Question 15

How do you prepare an aluminum fuel tank for welding?

Answer 15

Remove all fuel and thoroughly wash the tank, inside and out, with hot water and detergent. The tank should be purged with live stream for at least 30 minutes to remove all residual fuel and vapors.

Question 16

Ater welding an aluminum fuel tank, to what pressure should the tank be tested?

Answer 16

3.5 psi.

Question 17

To what fuel flow is an aircraft fuel system designed for?

Answer 17

Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit (APU) functioning under each likely operating condition.

Question 18

What markings must appear near the filler opening of the fuel tanks on a reciprocating-engine-powered aircraft, and on a turbine-powered aircraft?

Answer 18

On a reciprocating engine powered-aircraft: the work “Avgas” and the minimum grade of fuel. On a turbine engine powered aircraft: the words “Jet Fuel.” the permissible fuel designations, the maximum permissible fueling supply pressure, and the maximum permissible defueling pressure.

Question 19

Why is it important to drain all of the fuel sumps before the first flight of the day?

Answer 19

Water can condense in the fuel tanks and it must be drained out before the aircraft is safe for flight.

Question 20

Is the procedure of draining the sumps with the aircraft in the ground attitude an assurance that all of the water is removed from the tanks?

Answer 20

No; some aircraft require special procedures to remove all of the water. Refer to the airplane flight manual or pilot’s operating handbook.

Question 21

Why is a fuel jettisoning or dump system required for some aircraft?

Answer 21

A fuel dump system is installed on most large aircraft and allows the flight crew to jettison, or dump, fuel to lower the gross weight of the aircraft to its allowable landing weight.

Question 22

What color is grade 100-LL fuel?

Answer 22

Blue.

Question 23

How can you detect jet fuel in the sample taken form the tank sump drains?

Answer 23

Jet fuel is basically clear and it has an odor similar to kerosine.

Question 24

What must be done after a fuel system strainer has been cleaned or replaced?

Answer 24

The system must be tested for leaks by pressurizing the system with the boost pump, if one is used.

Question 25

What must be done if the sump drains sample shows traces of jet fuel?

Answer 25

The fuel system must be drained and flushed out with the proper grade of aviation gasoline.

Question 26

What is used as the sensor in the fuel tank of an electronic-type fuel quantity indicating system?

Answer 26

Tubular capacitor that extend across the fuel tank from top to bottom.

Question 27

What is the principle upon which the electronic-type fuel quantity indicating system operaties?

Answer 27

tubular capacitors extending across the fuel tanks change their capacitance as the fuel level changes. The dielectric constant (k) of the fuel is approximately twice that of air.

Question 28

What is used as a sensor in the fuel tank for the older resistance-type fuel quantity indicating system?

Answer 28

A variable resistor with an arm that is moved by a float riding on top of the fuel in the tank.

Question 29

What is the purpose of a drip gauge in the fuel tank of a large aircraft?

Answer 29

The drip gauge allows a mechanic to check the fuel level in a tank from the bottom of the tank.

Question 30

When must the fuel quantity indicating system indicate “zero”?

Answer 30

During level flight when the fuel in the tank is equal to the unusable fuel supply.

Question 31

What risks are associated with fuel system maintenance?

Answer 31

Improper fuel system maintenance can lead to failure of the fuel system or inflight fires. Always follow the manufacturer’s procedures and after completion, have a second mechanic check fitting for security and proper routing of fuel lines.

Question 32

What risks are associated with fuel contamination?

Answer 32

Fuel contamination can lead to engine failure. Verify that fuel is of the proper type and check fuel sumps for water and other contaminations.

Question 33

What risks are associated with fuel spills?

Answer 33

Vapors coming from spilled fuel can ignite, causing a fire. Fuel spills should be addressed by immediate action that corresponds to the type of fuel and the amount of fuel spilled. Significant fuel spills should be handled by the airport fire department.

Question 34

What safety procedures should be followed when entering a large fuel tank?

Answer 34

All fuel must be emptied from the tank and strict safety procedures must be followed. Fuel vapors must be purged from the tank and a respiratory equipment must be used by the aviation mechanic. A full-time spotter must be positioned just outside of the tank to assist if needed.

Question 35

What precautions should be used when defueling an aircraft?

Answer 35

Follow aircraft manufacturer’s procedures and use the proper equipment. On large aircraft, the same fueling port can be used for defueling.