18 Aircraft Electrical Systems Practice Questions & Answers

1. 1. When a new circuit is installed in an aircraft, what determines the current rating of the circuit breaker selected for it?

   • A.The inrush current the component draws at the moment of switching.
   • B.The current rating of the wire that the breaker protects.✓ Answer
   • C.The continuous current drawn by the component being supplied.
   • D.The maximum current the aircraft main bus is able to supply.

   A circuit breaker protects the wire, not the equipment at the end of the circuit. It must open before the current rating of the attached wire is exceeded, so a breaker sized to the load rather than to the conductor can allow the wire to overheat and burn while the breaker never trips.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (circuit protection devices)Report a problem with this question

2. 2. All resettable circuit breakers installed in aircraft must be "trip free." What does this requirement mean?

   • A.They can be reset in flight without first determining the cause of the fault.
   • B.They open the circuit on an overload regardless of the control position.✓ Answer
   • C.They may be used as switches without any reduction in service life.
   • D.They reset themselves automatically once the overload has cleared.

   Trip free means the internal mechanism opens the contacts on a fault even if the operating handle is being held in the closed position, so a crewmember cannot force current through a faulted circuit. Automatic-reset breakers are not recommended for aircraft precisely because they re-apply power to a fault that has not been investigated.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (circuit protection devices)Report a problem with this question

3. 3. Two independent requirements govern the selection of conductor size for an aircraft electrical circuit. What are they?

   • A.The number of strands in the conductor, and the current carried without overheating.
   • B.The insulation temperature rating, and the wire length between support points.
   • C.The allowable voltage drop over the run, and the circuit breaker rating chosen.
   • D.The allowable voltage drop over the run, and the current carried without overheating.✓ Answer

   The conductor must be large enough that the voltage drop from bus to load stays within the allowance for the system voltage, and large enough to carry the required current continuously without exceeding the temperature limit of its insulation. Whichever requirement calls for the larger conductor governs, and both must be checked because a long run can fail the voltage-drop test even when the current is small.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (wire size selection)Report a problem with this question

4. 4. Aircraft wire bundles that are not enclosed in troughs, ducts, or conduit must be supported at intervals of not more than —

   • A.12 inches.
   • B.36 inches.
   • C.15 inches.
   • D.24 inches.✓ Answer

   Support at intervals no greater than 24 inches keeps the bundle from sagging into moving parts and prevents the weight of the wire from working the terminations loose under vibration. The 15-inch figure is the maximum spacing for wire identification markings, not for clamps, which is why it is a common wrong choice.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (wire support and routing)Report a problem with this question

5. 5. What is the minimum bend radius permitted for an aircraft wire bundle that is not supported at the bend?

   • A.6 times the outside diameter of the largest wire in the bundle.
   • B.10 times the overall outside diameter of the finished bundle.
   • C.10 times the outside diameter of the largest wire in the bundle.✓ Answer
   • D.3 times the outside diameter of the largest wire in the bundle.

   A bend tighter than 10 times the diameter of the largest wire stretches and thins the insulation on the outside of the bend and crushes the conductor strands on the inside, both of which lead to chafing and eventual shorting. The 3-times figure applies only where the wire is suitably supported at the bend, and the 6-times figure applies to radio-frequency coaxial cable.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (bend radii)Report a problem with this question

6. 6. What is the accepted means of protecting a wire bundle where it passes through a hole in a bulkhead or rib?

   • A.Wrap the bundle with vinyl tape where it passes through the hole.
   • B.Install a grommet in the hole so the bundle cannot contact the edge.✓ Answer
   • C.Clamp the bundle tightly to the bulkhead on each side of the hole.
   • D.Fit a spiral plastic wrap over the bundle at the point of passage.

   A grommet presents a smooth, resilient surface that keeps the sheet-metal edge from cutting the insulation as the airframe flexes and the bundle moves. Tape and spiral wrap wear through against a metal edge and hide the damage underneath, and clamping alone does not remove the sharp edge from contact with the wire.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (wire routing through structure)Report a problem with this question

7. 7. Wiring routed near mechanical control cables must be installed so that, under light hand pressure, the wire cannot come closer to a moving control cable than —

   • A.1 inch.
   • B.1/4 inch.
   • C.2 inches.
   • D.1/2 inch.✓ Answer

   A moving control cable acts like a slow file, so contact will saw through wire insulation and can eventually part the control cable itself. Half an inch of clearance under light hand pressure allows for normal structural flexing and slack in the bundle; where that clearance cannot be maintained, additional clamping must be added to hold the wire away.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (clearance from control cables)Report a problem with this question

8. 8. A 28-volt DC lamp circuit draws a continuous current of 3 amperes, and the derating factor for a lamp load at 28 volts DC is 8. What nominal switch rating is required?

   • A.3 amperes.
   • B.11 amperes.
   • C.0.4 ampere.
   • D.24 amperes.✓ Answer

   To find the nominal rating a switch must carry, multiply the continuous load current by the derating factor: 3 amperes times 8 equals 24 amperes. The factor is large for lamps because a cold filament has very low resistance and draws an inrush many times the running current; dividing instead of multiplying gives 0.4 ampere and would select a badly undersized switch.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (switch derating factors)Report a problem with this question

9. 9. How should a stranded aircraft wire be prepared before a pre-insulated solderless terminal lug is crimped onto it?

   • A.Strip the insulation, crimp the barrel, then flow solder into the joint.
   • B.Strip the insulation, tin the strands with solder, then crimp the barrel.
   • C.Strip the insulation, fold the strands back, then crimp over the fold.
   • D.Strip the insulation and crimp the bare, untinned strands in the barrel.✓ Answer

   A crimp works by cold-flowing the conductor strands and the barrel into gas-tight metal-to-metal contact, and solder prevents that from happening. Tinned strands flow and relax under load so the joint loosens, and solder added to a finished crimp wicks up the wire and creates a rigid section that cracks under vibration.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (solderless terminals and splices)Report a problem with this question

10. 10. When several splices must be made in the wires of a single bundle, how should the splices be positioned?

    • A.Placed at the point of greatest bundle movement so slack absorbs strain.
    • B.Staggered along the bundle so its diameter is not increased at one point.✓ Answer
    • C.Grouped at one location so a single protective sleeve covers them all.
    • D.Located immediately next to the terminal block where the bundle ends.

    Splices are larger in diameter than the wire, so grouping them swells the bundle at one station, prevents the clamps from gripping evenly, and makes the bundle stiff where it should flex. Staggering keeps the bundle a uniform diameter, and splices must also be kept away from termination devices and avoided altogether in areas of extreme vibration.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (splicing of electrical wire)Report a problem with this question

11. 11. A bonding jumper must be installed between a stainless steel component and the surrounding structure. Which jumper material is correct?

    • A.Cadmium-plated aluminum, which is specified for corrosion-prone joints.
    • B.Copper, which is specified for stainless steel, brass, and bronze parts.✓ Answer
    • C.Aluminum alloy, which is specified for all steel and copper-bearing parts.
    • D.Steel, which is specified whenever the part being bonded is itself ferrous.

    Jumper material is chosen to limit galvanic action at the joint, and copper is specified where the bonded part is stainless steel, cadmium-plated steel, copper, brass, or bronze, while aluminum alloy jumpers are used on aluminum structure. The mating surfaces must also be cleaned of paint or anodic finish so the completed connection carries current with very low resistance.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (bonding and grounding)Report a problem with this question

12. 12. Why can the state of charge of an installed nickel-cadmium battery not be determined with a hydrometer?

    • A.Its electrolyte specific gravity changes very little from charged to discharged.✓ Answer
    • B.Its specific gravity changes too rapidly for an accurate reading to be taken.
    • C.Its electrolyte would be contaminated by contact with the hydrometer glass.
    • D.Its electrolyte is a paste that cannot be drawn into a hydrometer barrel.

    In a lead-acid cell the sulfuric acid is consumed during discharge, so specific gravity tracks the state of charge, but the potassium hydroxide electrolyte of a nickel-cadmium cell only transfers ions and its specific gravity stays essentially constant. The state of charge of a nickel-cadmium battery is therefore established by a capacity or discharge test rather than by a hydrometer reading.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), aircraft batteriesReport a problem with this question

13. 13. Thermal runaway in an installed nickel-cadmium battery is most likely to occur when —

    • A.its electrolyte level is checked immediately at the end of a charging cycle.
    • B.it is charged from a constant-current source at a rate below its rated capacity.
    • C.it is charged from a constant-voltage source while a cell is shorted or overheating.✓ Answer
    • D.it is discharged rapidly at low ambient temperature through a very heavy load.

    Under constant-voltage charging a rise in cell temperature lowers internal resistance, which increases charging current, which raises the temperature further, so the cycle feeds itself until the battery is destroyed. Constant-current charging breaks the loop because the current cannot climb, and removing the constant-voltage source before the battery exceeds 160 degrees F stops a runaway that has started.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), nickel-cadmium battery chargingReport a problem with this question

14. 14. How does a voltage regulator control the output of an aircraft generator or alternator?

    • A.By varying the current flowing through the field coil.✓ Answer
    • B.By varying the speed at which the generator is driven.
    • C.By varying the current flowing through the armature windings.
    • D.By varying the resistance connected in series with the load.

    Output voltage depends on field strength and speed, and since the generator is driven by the engine at whatever speed the engine happens to turn, the only variable the regulator can control is field current. A carbon-pile regulator does this by squeezing or relaxing a stack of carbon discs to change the field circuit resistance, while a transistorized regulator switches field current electronically.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), voltage regulationReport a problem with this question

15. 15. In a rotary inverter, what controls the frequency of the alternating current output?

    • A.The DC input voltage applied to the inverter.
    • B.The current flowing in the AC generator field winding.
    • C.The rotational speed of the rotating unit.✓ Answer
    • D.The number of turns in the output transformer.

    A rotary inverter is a DC motor driving an AC generator, and the frequency produced by any rotating generator is fixed by how many pole pairs pass a coil each second, so it rises and falls with shaft speed. Output voltage, by contrast, is set by the generator field current, and confusing the two is the usual error.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), invertersReport a problem with this question

16. 16. What does a constant speed drive accomplish in a large aircraft's AC generating system?

    • A.It holds the generator output voltage constant, replacing the voltage regulator.
    • B.It holds the generator drive speed constant so output frequency stays constant.✓ Answer
    • C.It reduces the drive speed to the generator so a smaller unit can be used.
    • D.It disconnects the generator from the bus when output frequency becomes excessive.

    The frequency of an AC generator is set by its number of poles and its rpm, so a generator geared directly to an engine that changes speed would produce a wandering frequency that AC motors and instruments cannot tolerate. The constant speed drive absorbs the engine speed variation hydraulically and turns the generator at the fixed speed needed for constant-frequency output; an integrated drive generator packages that drive and the generator in one housing.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), AC generation and constant speed drivesReport a problem with this question

17. 17. During an inspection of an airplane's position light system, which arrangement of light colors is required?

    • A.Red on the left wing, green on the right wing, white at the rear.✓ Answer
    • B.Green on the left wing, red on the right wing, white at the rear.
    • C.White on the left wing, green on the right wing, red at the rear.
    • D.Red on the left wing, white on the right wing, green at the rear.

    The forward position lights are aviation red on the left wing tip and aviation green on the right, with an aviation white light showing aft, so that another pilot can judge the relative direction of travel from the colors visible. Each forward light must show through a dihedral angle of 110 degrees outboard of the longitudinal axis, and the rear light covers 70 degrees on each side.

    Source: 14 CFR 25.1385 (position light system installation)Report a problem with this question

18. 18. A landing light burns dimly and the wiring and terminals in that circuit are warm to the touch after a few minutes of operation. What fault does this indicate, and how is it confirmed?

    • A.A short to ground; confirm it by measuring insulation resistance to structure.
    • B.A reversed connection; confirm it by checking polarity at the lamp terminals.
    • C.A high-resistance connection; confirm it by measuring voltage drop under load.✓ Answer
    • D.An open circuit; confirm it by checking continuity through the circuit.

    Unwanted resistance in a connection drops part of the source voltage before it reaches the lamp, which is why the lamp is dim, and that lost voltage appears as heat at the bad joint. Because the fault only shows up when current flows, it is found by measuring the voltage drop across each connection while the circuit is energized under normal load; an open would give no operation at all and a short would trip the circuit protection.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 11 (electrical system troubleshooting)Report 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 →