18 Fuel & Fire Protection Systems Practice Questions & Answers

1. 1. A high-wing single-engine airplane uses a gravity-feed fuel system with a tank in each wing and a selector valve that includes a BOTH position. What design feature makes the BOTH position workable on this airplane?

   • A.A check valve at each tank outlet prevents fuel from crossing between the two tanks.
   • B.An engine-driven pump draws from each tank in turn through a sequencing valve in the line.
   • C.Each tank has its own boost pump, and both pumps are regulated to the same outlet pressure.
   • D.The vapor spaces of both tanks are interconnected by the vent system, so both feed evenly.✓ Answer

   Gravity feed works on the head of fuel above the engine, so both tanks can only feed at the same rate if the pressure above the fuel is the same in each; interconnecting the vapor spaces through the vent system accomplishes that. Pump-fed low-wing singles omit a BOTH position because a pump would draw air from whichever tank emptied first.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — gravity-feed fuel systemsReport a problem with this question

2. 2. How does a fuel strainer differ from a fuel filter in its ability to deal with water carried in the fuel?

   • A.A strainer is coarse mesh and does not inhibit the flow of water; a finer filter element can trap it.✓ Answer
   • B.A strainer blocks water at its mesh; a filter passes the water on toward the fuel metering device.
   • C.Both use the same mesh, so they stop water equally and differ only in sediment bowl size.
   • D.A strainer traps fine sediment and water; a filter removes only the coarse debris in the fuel.

   A strainer is relatively coarse wire mesh intended to catch large debris, and water passes straight through it. A filter uses a much finer element — micronic elements on turbine aircraft capture particles in the 10 to 25 micron range — so it can trap fine sediment and help hold back water, which is why a strainer alone cannot be relied on to keep water out of the system.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel strainers, filters, and drainsReport a problem with this question

3. 3. Where is the sump of an aircraft fuel tank located, and how is the tank outlet positioned relative to it?

   • A.At the high point of the tank, with the tank outlet located below the sump.
   • B.At the low point of the tank, with the tank outlet located above the sump.✓ Answer
   • C.At mid-height in the tank, with the tank outlet located level with the sump.
   • D.At the low point of the tank, with the tank outlet located inside the sump.

   Water and heavy sediment are denser than fuel and settle to the lowest area of the tank, so the sump with its drain valve is placed at that low point where the contaminants collect and can be sampled off. The tank outlet is deliberately positioned above the sump so that settled water and debris are not drawn into the engine feed line.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel tank sumps and drainsReport a problem with this question

4. 4. Which statement correctly describes the displacement characteristics of aircraft fuel pumps?

   • A.The vane-type engine-driven pump is variable displacement; the centrifugal boost pump is constant displacement.
   • B.The vane-type engine-driven pump is constant displacement; the centrifugal boost pump is variable displacement.✓ Answer
   • C.Both the vane-type engine-driven pump and the centrifugal boost pump are constant displacement pumps.
   • D.Both the vane-type engine-driven pump and the centrifugal boost pump are variable displacement pumps.

   A vane pump has an eccentric rotor with sliding vanes that displace a fixed volume every revolution, so it is a constant (positive) displacement pump and must be fitted with an adjustable relief valve that returns excess fuel to the pump inlet. A centrifugal boost pump throws fuel outward with an impeller and its output varies with demand and speed, making it variable displacement; submerged in the tank, it supplies positive pressure that helps prevent vapor lock.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel pumpsReport a problem with this question

5. 5. A fuel leak is discovered on the lower wing skin of an airplane. Which factor most directly determines whether the leak must be repaired before further flight?

   • A.Whether the airplane is serviced with aviation gasoline or with a grade of turbine fuel.
   • B.Whether the tank involved is an integral tank or an installed bladder-type fuel cell.
   • C.Whether the leak lies inboard or outboard of the wing tank sump drain valve.
   • D.Whether the leaking fuel or its vapor can collect in an enclosed area of the aircraft.✓ Answer

   Leaks are classified by the surface area the fuel covers during a specified observation period after the area is wiped dry, running from a stain through a seep and heavy seep to a running leak, but classification alone does not release the aircraft. The controlling airworthiness question is the fire hazard: any leak that lets fuel or vapor accumulate in an enclosed area must be repaired before flight, while a purely external leak may be deferred under the manufacturer's instructions and monitored.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel leak classification and repair; 14 CFR 43.13Report a problem with this question

6. 6. A mechanic must enter an integral (wet wing) fuel tank to replace sealant. Which set of precautions is required?

   • A.Purge the tank, then close it up to keep dust out, and check the atmosphere once immediately before entry.
   • B.Drain the tank, blow it dry with shop air, and judge the atmosphere inside using a portable inspection light.
   • C.Purge the tank and verify with a combustible gas indicator, ventilate continuously, and station an observer outside.✓ Answer
   • D.Drain the tank, let it air out overnight, and enter alone wearing a dust mask while the work proceeds.

   Draining a tank does not make it safe, because the remaining vapor in a nearly empty tank is more dangerous than liquid fuel; the vapor must be purged and the atmosphere proven safe with a combustible gas indicator. Ventilating air must keep flowing the whole time the tank is open, and an observer must remain outside to pull the mechanic out, since fuel vapor is toxic as well as explosive.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — integral fuel tank entry and purgingReport a problem with this question

7. 7. Which statement correctly describes an electronic capacitance-type fuel quantity indicating system?

   • A.It uses a float-driven wiper inside the tank and indicates the quantity of fuel by weight.
   • B.It has no moving parts inside the tank and indicates the quantity of fuel by weight.✓ Answer
   • C.It uses a float-driven wiper inside the tank and indicates the quantity of fuel by volume.
   • D.It has no moving parts inside the tank but indicates the quantity of fuel by volume only.

   The tank units are concentric plates held a fixed distance apart and open top and bottom, so the fuel and air between them act as the dielectric; as the fuel level rises, capacitance rises, and the probes wired in parallel are compared against a reference capacitor. Nothing moves inside the tank, and the indication is given by weight, which is what matters for performance planning; because the dielectric constant of fuel changes with temperature, a compensator unit low in the tank corrects the reading.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — capacitance-type fuel quantity indicating systemsReport a problem with this question

8. 8. Why can dissolved and entrained water not be eliminated simply by draining the fuel tank sumps and the filter bowls?

   • A.That water is lighter than fuel and floats above the tank outlet, where no drain valve is fitted.
   • B.That water is trapped inside the tank vent lines, which are built with no provision for draining.
   • C.That water is held in suspension within the fuel and does not settle out as free water at the drain.✓ Answer
   • D.That water reacts with the fuel to form a stable compound that no drain valve is able to remove.

   Only free water is denser than fuel and settles to the low point where a sump drain can remove it; dissolved and entrained water stays distributed through the fuel, entrained water showing as a cloudy appearance. As the fuel cools during a climb this water comes out of solution as free water or ice crystals that clog filters, which is precisely why anti-icing additive and fuel heaters are used instead of relying on sump draining.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel contamination and waterReport a problem with this question

9. 9. During over-the-wing refueling, when is the bonding wire between the fueling equipment and the aircraft connected and when is it removed?

   • A.Connected after the filler cap is opened, and removed just before the nozzle is withdrawn.
   • B.Connected before the filler cap is opened, and removed only after fueling has been completed.✓ Answer
   • C.Connected only when turbine fuel is used, and removed at any time after fueling begins.
   • D.Connected after the nozzle is inserted, and removed as soon as the tank indicates full.

   Bonding equalizes the electrical potential between the fueling equipment and the airframe so that no spark can jump at the filler opening, where fuel vapor is present. The bond must therefore already be in place before the cap is opened and must stay connected until fueling is finished and the cap is closed, because a static discharge at any point in the operation can ignite the vapor.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fueling and defueling safetyReport a problem with this question

10. 10. An airplane approved for grade 100LL aviation gasoline is found to have been serviced with a fuel that is colorless. What is the correct conclusion and action?

    • A.It holds 100LL that lost its dye in storage, and the fuel may be used just as it is.
    • B.It holds grade 80 aviation gasoline, which is an acceptable substitute for grade 100LL.
    • C.It holds 82UL aviation gasoline, which may be blended with the 100LL already on board.
    • D.It holds turbine fuel from a misfueling, and the system must be drained and flushed.✓ Answer

    Aviation gasoline is dyed by grade — 100LL is blue, grade 100 is green, grade 80 is red, and 82UL and 115/145 are purple — while every turbine fuel is colorless or straw colored, so colorless fuel in a gasoline airplane means turbine fuel. If no pump has run and no start was attempted, drain and flush the tanks and lines and refuel correctly; if the engine was started or a start attempted, the entire system must be flushed and the cylinders, oil, screens and filters inspected before return to service.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 14 — fuel grades, color coding, and misfuelingReport a problem with this question

11. 11. Powerplant fire zones are classified according to which characteristic of the compartment?

    • A.The type of flammable fluid that is routed through the compartment.
    • B.The temperature that the structure within the compartment can reach.
    • C.The amount and pattern of the airflow passing through the compartment.✓ Answer
    • D.The class of extinguishing agent installed to protect the compartment.

    Fire zones are defined by airflow because airflow governs how an agent is distributed and how much of it is needed: Class A has heavy airflow past regular obstructions, Class B heavy airflow past aerodynamically clean obstructions, Class C relatively low airflow, Class D little or no airflow such as wing compartments and wheel wells, and Class X heavy airflow through unusual structure with pockets that require roughly double the agent of a Class A zone. This lettering is separate from the fire classes that describe what is burning.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — fire zone classificationReport a problem with this question

12. 12. On what principle does a thermocouple fire warning system operate, and what does that mean for the conditions it can detect?

    • A.It responds to an absolute temperature, so it warns of both a fire and a slow overheat.
    • B.It responds to the rate of temperature rise, so it does not warn of a slow overheat.✓ Answer
    • C.It responds to an absolute temperature, so it does not warn of a rapidly developing fire.
    • D.It responds to the rate of temperature rise, so it also warns of a short circuit in the loop.

    A thermocouple system uses dissimilar metals forming a hot junction exposed to the compartment and a reference junction in dead air between insulation blocks; voltage is produced only when the hot junction heats faster than the reference junction. A slow, general overheat raises both junctions together, produces no voltage and therefore no warning, and the system likewise gives no indication of a short circuit in its wiring.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — thermocouple fire warning systemsReport a problem with this question

13. 13. In a thermistor-type continuous-loop fire detection element, what happens electrically as the temperature of the element rises?

    • A.The element releases a gas pressure that closes a diaphragm switch inside the control unit.
    • B.Resistance between the two conductors rises, opening the circuit that runs to the control unit.
    • C.The element generates a voltage of its own that increases as the tube temperature increases.
    • D.Resistance between the two conductors falls, giving first an overheat and then a fire signal.✓ Answer

    A thermistor element carries two conductors embedded in a thermistor core inside a metal tube, monitored with 28 volt DC; the core's resistance decreases as temperature rises, so the control unit can give an overheat indication first and a fire warning as heating continues, and the rate at which resistance falls distinguishes an electrical short from a real fire. By contrast, the eutectic salt element uses a single center conductor whose surrounding salt drops sharply in resistance at its set point, and the pneumatic element uses gas pressure to close a diaphragm switch.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — continuous-loop detector systemsReport a problem with this question

14. 14. Why are smoke detectors, rather than thermal detectors, used to protect cargo compartments and lavatories?

    • A.Thermal detectors may not be installed in any compartment that is pressurized during flight.
    • B.Materials there smolder and give off heavy smoke before enough heat exists to trip a thermal unit.✓ Answer
    • C.Cargo compartments run too cold in cruise for a heat-sensing element to function reliably.
    • D.Smoke detectors respond only to open flame, which is the usual form taken by a cargo fire.

    Baggage, cargo and lavatory waste tend to burn slowly and smolder, producing large quantities of smoke long before the compartment gets hot enough to operate a thermal switch or a continuous-loop element, so a heat-based detector would warn far too late. Photoelectric light-refraction and ionization smoke detectors sense that smoke early, which is why transport rules require a visual indication to the crew within one minute of the start of a fire in these compartments.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — smoke detection systemsReport a problem with this question

15. 15. A mechanic finds that a red disc has been ejected from the thermal discharge indicator port of an installed fire extinguisher container. What does this indicate?

    • A.The container discharged overboard because of excessive heat, and it must be replaced.✓ Answer
    • B.The discharge cartridge has reached the end of its service life and must be replaced.
    • C.The container is fully charged and the indicator was reset after the last servicing.
    • D.The flight crew deliberately discharged the container in flight, and it must be replaced.

    The thermal discharge indicator is tied to the container's temperature and pressure sensitive safety relief diaphragm: if heat raises the internal pressure to the point that the diaphragm ruptures, the agent is dumped overboard and a red disc is ejected. A yellow disc ejected from the other indicator port means the flight crew commanded a normal discharge; in either case the container must be replaced before the next flight, but a red disc also calls for finding the source of the overheat.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — fire extinguisher discharge indicatorsReport a problem with this question

16. 16. Which statement about dry chemical extinguishing agents and the classes of fire they are rated for is correct?

    • A.All dry chemical agents carry an A-B-C rating, and all are also approved for Class D fires.
    • B.Only sodium bicarbonate carries an A-B-C rating; other dry chemicals are rated A and B.
    • C.All dry chemical agents are rated for Class B only, and none may be used on energized equipment.
    • D.Only mono-ammonium phosphate carries an A-B-C rating; other dry chemicals are rated B and C.✓ Answer

    Among the dry chemical powders only mono-ammonium phosphate is the all-purpose agent rated for Class A, B and C fires; every other dry chemical is rated for Class B and C only. Halons and halocarbon clean agents are likewise rated only for Class A, B and C and must never be used on a Class D metal fire, which requires a specialized dry powder, and dry chemicals are unsuitable in the cabin or flight deck because they corrode electronics, obscure vision and are difficult to clean up.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — extinguishing agents; NFPA 10 fire classesReport a problem with this question

17. 17. How is the service life of a fire extinguisher container discharge cartridge determined?

    • A.From the hours of operation accumulated by the aircraft since the cartridge was installed.
    • B.From the number of hours the cartridge has been held below a specified temperature.
    • C.From the manufacturer's date stamp on the face of the cartridge, expressed in years.✓ Answer
    • D.From the container gauge pressure reading compared against a temperature curve chart.

    Cartridge life is a shelf life, not a usage life: it is figured from the date stamp the manufacturer places on the face of the cartridge and is expressed in years, and reading it usually means removing the electrical leads and the discharge line so the plug body can be taken off the container. Comparing gauge pressure against a temperature curve is how the container's agent charge is checked, not how cartridge life is found, and a cartridge taken from one discharge valve must not be fitted to a different valve assembly because contact protrusion varies.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — discharge cartridge service lifeReport a problem with this question

18. 18. During inspection, a continuous-loop fire detector sensing element is found with a small kink in the tubing. What is the correct action?

    • A.Do not straighten it; evaluate it against the manufacturer's acceptable dent and kink limits.✓ Answer
    • B.Disregard it, because deformation of the tubing does not affect operation of the element.
    • C.Straighten it with a tube bender, then reinstall the element with an added support clamp.
    • D.Straighten it carefully by hand, then return the element to service if it still has continuity.

    The handbook is explicit that an acceptable dent or kink must never be straightened, because the stress induced by straightening can cause the tubing to fail in service; whether the deformation is acceptable at all is decided by the limits the manufacturer publishes for that element. A kink also matters because it can short the inner conductor to the tube and produce a false fire warning, which is found with an ohmmeter while tapping the suspect area.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 17 — continuous-loop detector maintenance and inspectionReport a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Airframe (FAA-H-8083-31). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the AIRFRAME written test only; the General and Powerplant written tests and the oral and practical tests are separate. Torque values, cable tensions, inflation pressures, servicing quantities, wear limits, and repair criteria always come from the manufacturer's current maintenance data, the structural repair manual, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →