What is the difference between a circuit breaker and a fuse?

Both open the circuit when current exceeds their rating, protecting the wiring from overheating. A fuse does it by melting a link and has to be replaced once it operates, while a circuit breaker opens mechanically and can be reset. Aircraft breakers are normally trip-free, meaning they open on a fault even if the handle is held in, so a fault cannot be overridden by force.

What does an inverter do on an aircraft?

An inverter converts DC power into AC so equipment that needs alternating current can run from a DC bus. Its complement is the transformer rectifier unit, which converts AC to DC, typically for battery charging and DC loads on an aircraft whose primary generation is AC. Candidates lose points by swapping the two, so fix the direction in your head: inverter DC to AC, transformer rectifier AC to DC.

Why keep nickel-cadmium and lead-acid batteries apart?

Their electrolytes are chemical opposites: sulfuric acid in a lead-acid cell, an alkaline potassium hydroxide solution in a nickel-cadmium cell. A trace of one carried across on a tool, hydrometer, syringe or bench top will neutralize and contaminate the other and ruin the battery. Separate servicing areas, separate tools and separate protective equipment are the standard shop practice.

Which LED lead is the cathode?

The shorter lead, on the side where the body of the LED is flattened, is the cathode, which is the negative side. The longer lead is the anode. An LED conducts in one direction only, so installing it backward leaves the circuit dark rather than merely dim, and it still needs a current-limiting resistor to keep the device inside its rated forward current.

Why is shielded wire used on some aircraft circuits?

A conductive shield around the conductor contains the electromagnetic field a current-carrying wire produces and keeps outside fields off the inner conductor. That stops noisy circuits, such as ignition and strobe leads, from interfering with radios and sensitive instrument circuits. The shield only does its job when it is properly grounded, so shield terminations are part of the installation rather than an optional extra.

What is the main functional difference between a circuit breaker and a fuse?

A circuit breaker can be reset after a fault, while a fuse must be replaced

A circuit breaker protects only AC circuits, while a fuse protects only DC circuits

A circuit breaker increases voltage on a fault, while a fuse decreases it

What is the primary function of a voltage regulator in an aircraft generator system?

To maintain a constant generator output voltage as load and speed vary

To rectify the generator AC output into usable DC

To open the circuit when output current becomes excessive

In a single-wire (ground-return) electrical system common to metal aircraft, what serves as the return path back to the power source?

The metal aircraft structure used as a common ground

The shielding braid around each individual conductor

A second dedicated return wire run to every load

When two electrical loads are connected in parallel across a 24-volt bus, what is true of the voltage applied to each load?

Each load receives the full bus voltage of 24 volts

The 24 volts is divided equally, giving each load 12 volts

Each load receives a voltage proportional to its resistance share

What is the main reason for using shielded wire on certain aircraft electrical circuits?

To reduce electromagnetic interference radiated to or from the conductor

To increase the current-carrying capacity of the conductor

To provide the primary mechanical support for the wire bundle

Why must nickel-cadmium and lead-acid batteries never share the same servicing area or tools?

Their electrolytes are chemically opposite and cross-contamination ruins the cells

Their charging voltages are identical and would create a short circuit

Their terminals use the same polarity and would discharge together

On a transport-category aircraft, what is the main purpose of an inverter in the electrical system?

To convert DC power into AC power for AC-operated equipment

To convert AC power into DC power for battery charging

To raise DC voltage to a higher DC value for distribution

What precaution is required when removing electrostatic-discharge-sensitive (ESDS) equipment from an aircraft?

Wear an anti-static wrist strap connected to an approved ground point.

Wrap the component in ordinary plastic film.

Wear insulated rubber gloves while handling the unit.

A flight deck display line-replaceable unit (LRU) is being replaced. What does its ESDS label tell the technician?

Do not touch the connector pins or leave them exposed.

Remove all electrical power before disconnecting only.

Apply insulated rubber gloves before any handling.

When installing an electronic flight instrument system (EFIS) display, the technician must

avoid touching the connector terminals.

use only fully insulated hand tooling.

remove all AC power from the airplane.

An anti-static wrist strap is worn when working with ESDS devices in order

to continuously dissipate and prevent static build-up on the operator.

to protect the operator from receiving an electric shock.

to provide a one-time ground for residual static on the operator.

A humid atmosphere has what effect on static electricity build-up?

It reduces static charge build-up.

It has no effect on static charge.

It doubles the static charge build-up.

An electrostatic discharge (ESD) occurs when

materials at different electrical potentials are brought into close proximity.

materials of the same potential are placed in direct contact with each other.

materials of similar composition are physically separated from one another.

Before removing a static-sensitive assembly from an aircraft, you must first ensure minimum current flow between

the technician and the assembly being handled.

the airframe and the external ground point.

the technician and the airframe structure.

How is a fiber optic data line protected against electromagnetic interference (EMI) at its electrical interface?

By installing an opto-isolator at the electrical interface.

By clamping a ferrite core over the fiber line.

By adding a braided metallic shield around the fiber.

How do aircraft manufacturers build in protection against high-intensity radiated fields (HIRF)?

By designing specific avionics areas that are shielded against HIRF.

By installing additional bonding leads on external antenna mounts.

By routing all electrical wiring through separate conduit channels.

How is a line replaceable unit (LRU) itself protected against high-intensity radiated fields (HIRF)?

By built-in filters and overvoltage protection circuits.

By external grounding and surge protection only.

By shielded wiring and ferrite core clamps only.

To prevent a ground loop from forming on a shielded cable, the cable shield should be grounded at

both ends of the shield securely.

neither end, leaving the shield floating.

At what point is an aircraft's immunity to high-intensity radiated fields (HIRF) demonstrated by test and analysis?

During development and at initial type certification.

During scheduled maintenance checks at each service interval.

By periodically listening for interference on all systems.

When making a bonding-resistance check on a surface protected by an anodic (anodized) film, the technician must

remove the anodic film locally to ensure a good metal contact.

press the bonding probe firmly through the anodic layer.

seal the anodic film with conductive paste before testing.

When clamping a wire bundle that contains coaxial cable, the technician must

avoid distorting the coaxial cable so its dielectric spacing is maintained.

bond the coaxial cable's outer braid to the clamp being used.

allow slight jacket distortion if the inner conductor is unaffected.

When installing an antenna (RF) cable in an aircraft, it should be

routed separately from the main wire bundle.

run just outside the bundle for easy access.

placed at the center of the wire bundle.

Electromagnetic compatibility (EMC) in an aircraft's electrical and avionics installation is achieved by

shielding, screening, grounding, bonding, and interference filters.

coating all exposed cable runs with conductive paint and clamps.

wrapping all wire bundles in ferrite sleeves and conductive tape.

For a braided (screened) cable, the amount of braid coverage

determines how well radiated energy is kept inside the emitting circuit or away from a susceptible circuit.

completely isolates the conductor from all adjacent wiring in the bundle.

provides the low-resistance return-current path to the airframe bonding network.

Before crimping a terminal onto an aluminum wire conductor, what preparation should be performed?

Apply an oxide-inhibiting compound of petroleum jelly and zinc oxide

Degrease the stripped conductor with solvent only

Simply crimp the terminal with no further preparation

What is the purpose of a current-limiter fuse in an aircraft electrical system?

It allows a high overload current to flow for a few seconds

It opens instantly at any current above its rating

It reacts only when the surrounding circuit gets warm

In a typical aircraft electrical distribution system, the loads are connected:

in parallel with one another across the busbar

in series with one another across the busbar

in series with the busbar and with each other

The power factor of an AC generator indicates the proportion of apparent power that is

real power that does useful work

reactive power that does useful work

apparent power lost as circuit resistance

If both the output voltage and frequency of an engine-driven AC generator drop to zero in flight, this most likely indicates that the

generator drive shaft has sheared

phase-sequence detection circuit has tripped

bus-tie interlock has become inoperative

If the voltage and frequency of an AC generator both drop to zero in flight, it indicates that the

constant speed drive input shaft has sheared

bus tie interlock has become inoperative

phase sequence detection circuit has operated

Assuming all systems are operating normally, as aircraft electrical load increases, generator output voltage will

remain constant while amperage output increases

increase while amperage output increases

decrease while amperage output increases

One of the main purposes of a constant speed drive (CSD) in an AC system is to

enable generators to be paralleled

prevent the engine from being overloaded

maintain a constant load on the generator

In an undervoltage condition in an AC generator system, the most likely immediate consequence is

activation of the time delay circuit

energizing of the bus tie relay

de-activation of the field regulator circuit

Differential protection in an AC generator system protects against

line-to-line, three-phase, and line-to-ground faults

low resistance short circuits only

reverse current flowing from the battery

The power factor of an AC generator identifies the proportion of

apparent power from the generator that does useful work

reactive power from the generator that does useful work

real power from the generator that does useful work

The frequency of a hydraulically powered emergency electrical generator is controlled by varying the

angle of the hydraulic motor's swash plate

opening of a fixed restrictor valve

setting of an integrated drive generator

A purely inductive reactive load on an AC generator causes

no change in torque but additional heat to be dissipated

an increase in torque with no additional heat

an increase in both torque and heat to be dissipated

Increasing the real (resistive) load on an AC alternator primarily

increases both output voltage and frequency

A paralleling relay in a DC system is energized and held connected by a

voltage coil and a current coil

With no variation in electrical loading, the controlling element of a carbon pile voltage regulator is

stationary and holding a steady output

continuously fluctuating about the set point

modulating its output by pulse width

Aircraft Electrical Systems — FAA A&P Test Questions (ACS AM.II.K)

Aircraft Electrical Systems is one of the larger Airframe areas and it rewards fundamentals. The FAA tests Ohm's law thinking, series versus parallel behavior on a bus, circuit protection, generator and alternator control, wiring and shielding practice, battery servicing, and the DC to AC conversion used on transport aircraft. Questions are worded in aircraft terms, a 24-volt bus or a single-wire ground return, but the reasoning underneath is basic circuit theory. Work the theory until it is automatic and the aircraft-specific items reduce to definitions. This page maps the area and drills the published questions.

Written by the AMTprep editorial team · Published July 27, 2026 · Last reviewed October 21, 2026

Written against the FAA primary sources cited at the foot of this page.

What ACS AM.II.K covers

The area opens with circuit fundamentals applied to an airplane. Loads across a bus are in parallel, so each load sees the full bus voltage while branch currents add, and a series string shares one current and divides the voltage. Ohm's law and the power relationship show up in wire-sizing and load-analysis questions, where allowable voltage drop, current, wire length and whether the wire runs in a bundle all matter. Circuit protection follows. A fuse is a one-time device that must be replaced after it opens; a circuit breaker opens on overcurrent and can be reset. Both are sized to protect the wire rather than the equipment on the end of it, and a trip-free breaker cannot be held closed against a fault. Metal aircraft use a single-wire, ground-return system in which the airframe structure carries current back to the source, which makes bonding, clean ground terminations and corrosion control electrical concerns rather than cosmetic ones. Generation and control comes next: a voltage regulator holds output voltage steady across changing engine speed and electrical load by controlling field current, and alternators, generators, reverse-current relays and bus arrangements all appear. On transport aircraft an inverter converts DC to AC for equipment that needs alternating current, while a transformer rectifier unit converts AC to DC. Installation practice covers stranded conductors chosen for vibration, service loops, clamping, routing away from fluid lines, and shielded wire used to contain electromagnetic interference so noisy circuits do not disturb radios and instruments. Battery questions separate lead-acid from nickel-cadmium, including why their electrolytes, tools and servicing areas must be kept apart, and solid-state items such as LED polarity round the area out. FAA-H-8083-31 is the reference behind these topics.

Where this sits on the test

ACS AM.II.K is tested on the FAA Airframe written test, one of 1,739 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-31

Three traps candidates fall into

Series rules get applied to a parallel bus. Two loads across a 24-volt bus each see the full 24 volts and their currents add; the voltage does not split between them. Candidates who divide voltage in parallel usually also compute a total resistance larger than either branch, which cannot happen.
A hydrometer gets reached for on a nickel-cadmium battery. The potassium hydroxide electrolyte barely changes specific gravity as the cell charges and discharges, so a hydrometer reading says nothing about state of charge. That takes a measured capacity discharge check. Specific gravity is a lead-acid measurement only.
Circuit protection is assumed to protect the component. A fuse or breaker is sized to protect the wire from overheating, which is why fitting a higher-rated fuse to stop nuisance blows is a fire risk rather than a fix, and why a tripped breaker points at the circuit and not just the load.

50 free sample questions from ACS AM.II.K

AM.II.KTap an answer
What is the main functional difference between a circuit breaker and a fuse?
Show the answer and explanation
Answer: A circuit breaker can be reset after a fault, while a fuse must be replaced
Why: Both devices open a circuit on an overcurrent condition to protect wiring. The key difference is that a circuit breaker uses a mechanical trip mechanism that can be reset and reused, whereas a fuse contains a metal element that melts and must be replaced after it opens. Neither is voltage- or AC/DC-specific in the way the distractors state.· FAA-H-8083-31 — Circuit Protection Devices
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What is the primary function of a voltage regulator in an aircraft generator system?
Show the answer and explanation
Answer: To maintain a constant generator output voltage as load and speed vary
Why: A voltage regulator controls field excitation so that the generator's output voltage stays within a set range despite changing rotational speed and electrical load. Rectification is performed by a rectifier, and protection against excessive current is the job of a current limiter or circuit protection device, not the regulator.· FAA-H-8083-31 — Voltage Regulation
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In a single-wire (ground-return) electrical system common to metal aircraft, what serves as the return path back to the power source?
Show the answer and explanation
Answer: The metal aircraft structure used as a common ground
Why: In a single-wire system, current flows out to the load through the wire and returns through the aircraft's metal structure, which is bonded to the negative terminal of the source. This saves weight by eliminating a separate return wire. Shielding is for interference control, not the normal current return path.· FAA-H-8083-31 — Electrical System Grounding / Bonding
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When two electrical loads are connected in parallel across a 24-volt bus, what is true of the voltage applied to each load?
Show the answer and explanation
Answer: Each load receives the full bus voltage of 24 volts
Why: In a parallel circuit, every branch is connected directly across the same two points, so each branch sees the full source voltage. Voltage division (the other two options) occurs in series circuits, not parallel ones. This is why aircraft loads are wired in parallel from the bus.· FAA-H-8083-31 — Basic Electricity (series and parallel circuits)
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What is the main reason for using shielded wire on certain aircraft electrical circuits?
Show the answer and explanation
Answer: To reduce electromagnetic interference radiated to or from the conductor
Why: Shielding is a conductive braid surrounding the wire that is grounded to contain or exclude electromagnetic fields, thereby reducing interference with sensitive circuits such as radios and instruments. It does not raise ampacity (gauge of the conductor does that) and is not a structural support member.· FAA-H-8083-31 — Wiring (shielding / EMI)
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Why must nickel-cadmium and lead-acid batteries never share the same servicing area or tools?
Show the answer and explanation
Answer: Their electrolytes are chemically opposite and cross-contamination ruins the cells
Why: Lead-acid batteries use an acidic (sulfuric) electrolyte while nickel-cadmium batteries use an alkaline (potassium hydroxide) electrolyte. Each neutralizes and contaminates the other, so separate areas, tools, and supplies must be used to avoid degrading the cells. Polarity and charge-voltage statements in the distractors are not the reason for separation.· FAA-H-8083-31 — Aircraft Batteries (lead-acid vs. ni-cad servicing)
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On a transport-category aircraft, what is the main purpose of an inverter in the electrical system?
Show the answer and explanation
Answer: To convert DC power into AC power for AC-operated equipment
Why: An inverter changes direct current into alternating current to supply instruments and equipment that require AC, often as a backup or for specific AC loads in a primarily DC system. A device that converts AC to DC is a rectifier (or transformer-rectifier), and a DC-to-DC voltage increase describes a converter—not an inverter.· FAA-H-8083-31 — Inverters / Transformer-Rectifiers
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On a standard LED, the shorter lead located next to the flat side of the body identifies the
Show the answer and explanation
Answer: cathode (negative terminal).
Why: On a light-emitting diode, the lead next to the flattened edge of the case is the shorter lead, and it identifies the cathode (negative terminal). The longer lead is the anode. Knowing this polarity marking lets the technician connect the LED in forward bias so it conducts and emits light.· FAA-H-8083-31 — Aircraft Electrical Systems
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What precaution is required when removing electrostatic-discharge-sensitive (ESDS) equipment from an aircraft?
Show the answer and explanation
Answer: Wear an anti-static wrist strap connected to an approved ground point.
Why: When handling ESDS equipment, the technician must be at the same electrical potential as the equipment. This is achieved by wearing an anti-static wrist strap connected through its safety resistor to an approved ground point, which drains static charge safely and prevents a damaging discharge through the component.· FAA-H-8083-31 — Aircraft Electrical Systems
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A flight deck display line-replaceable unit (LRU) is being replaced. What does its ESDS label tell the technician?
Show the answer and explanation
Answer: Do not touch the connector pins or leave them exposed.
Why: The ESDS label warns that the unit contains electrostatic-discharge-sensitive devices, so its connector pins must not be touched or left exposed where a static discharge could reach them. The pins should be kept covered or shorted and handled only with proper anti-static precautions to prevent latent damage.· FAA-H-8083-31 — Aircraft Electrical Systems
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When installing an electronic flight instrument system (EFIS) display, the technician must
Show the answer and explanation
Answer: avoid touching the connector terminals.
Why: An EFIS display is an electrostatic-discharge-sensitive unit, so during installation the technician must avoid touching the connector terminals where a static discharge could reach the internal devices. The pins are kept protected and handled only with the wrist strap and grounding precautions in place.· FAA-H-8083-31 — Aircraft Electrical Systems
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At a relative humidity of 10% to 20%, walking across a carpet can generate a body electrostatic voltage of approximately
Show the answer and explanation
Answer: 35,000 volts.
Why: Static-charge tables show that walking across a carpet at low relative humidity (about 10 to 20 percent) can build a body voltage of roughly 35,000 volts. Dry air allows charge to accumulate readily, which is why such high voltages occur and why anti-static precautions are essential when handling sensitive devices.· FAA-H-8083-31 — Aircraft Electrical Systems
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An anti-static wrist strap is worn when working with ESDS devices in order
Show the answer and explanation
Answer: to continuously dissipate and prevent static build-up on the operator.
Why: The wrist strap connects the operator through a high-value resistor to a common ground point so any static charge bleeds away continuously and never builds to a level that could discharge into an ESDS device. It is not for shock protection, and continuously dissipating charge is more accurate than describing a one-time grounding of residual charge.· FAA-H-8083-31 — Aircraft Electrical Systems
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A humid atmosphere has what effect on static electricity build-up?
Show the answer and explanation
Answer: It reduces static charge build-up.
Why: Moisture in the air makes surfaces slightly conductive, allowing static charge to leak away rather than accumulate, so a humid atmosphere reduces static build-up. This is why ESD problems are worst in dry, low-humidity conditions; humidity neither doubles nor ignores the charge.· FAA-H-8083-31 — Aircraft Electrical Systems
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An electrostatic discharge (ESD) occurs when
Show the answer and explanation
Answer: materials at different electrical potentials are brought into close proximity.
Why: Electrostatic discharge is the sudden transfer of charge when two objects at different electrical potentials are brought together or close enough for the field to bridge the gap. Items at the same potential have no driving voltage to discharge, and separating similar materials is the triboelectric charging mechanism rather than the discharge event.· FAA-H-8083-31 — Aircraft Electrical Systems
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Before removing a static-sensitive assembly from an aircraft, you must first ensure minimum current flow between
Show the answer and explanation
Answer: the technician and the assembly being handled.
Why: Before disturbing a static-sensitive assembly, the technician equalizes potential between themselves and that assembly so no damaging current flows through its components on contact. The reference point is the item being handled, not the general airframe, because it is the assembly's internal devices that must be protected.· FAA-H-8083-31 — Aircraft Electrical Systems
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How is a fiber optic data line protected against electromagnetic interference (EMI) at its electrical interface?
Show the answer and explanation
Answer: By installing an opto-isolator at the electrical interface.
Why: A fiber optic line carries light rather than electric current, so it cannot be shielded against EMI with a metallic braid or ferrite the way a copper conductor is. Interference is controlled at the electrical interface with an opto-isolator, which breaks the electrical path and stops conducted interference from crossing between circuits.· FAA-H-8083-31 — Aircraft Electrical Systems
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How do aircraft manufacturers build in protection against high-intensity radiated fields (HIRF)?
Show the answer and explanation
Answer: By designing specific avionics areas that are shielded against HIRF.
Why: HIRF protection is designed into the airframe by creating dedicated avionics areas that are electromagnetically shielded to attenuate high-intensity radiated fields. Extra antenna bonding leads or wiring in conduit are secondary measures and do not provide the overall shielded-zone protection built into the design.· FAA-H-8083-31 — Aircraft Electrical Systems
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How is a line replaceable unit (LRU) itself protected against high-intensity radiated fields (HIRF)?
Show the answer and explanation
Answer: By built-in filters and overvoltage protection circuits.
Why: An LRU is protected internally by built-in interference filters and overvoltage/transient-suppression circuits that limit the energy reaching its electronics. External grounding, shielded wiring, or ferrite clamps support the installation, but the LRU's own designed-in filtering and overvoltage protection is what protects the unit itself.· FAA-H-8083-31 — Aircraft Electrical Systems
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To prevent a ground loop from forming on a shielded cable, the cable shield should be grounded at
Show the answer and explanation
Answer: only one end of the shield.
Why: Grounding a cable shield at both ends creates a loop in which circulating currents flow because of small potential differences between the two ground points. The shield is therefore grounded at only one end to break that loop. Leaving the shield completely floating would remove its shielding effectiveness.· FAA-H-8083-31 — Aircraft Electrical Systems
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At what point is an aircraft's immunity to high-intensity radiated fields (HIRF) demonstrated by test and analysis?
Show the answer and explanation
Answer: During development and at initial type certification.
Why: HIRF immunity is demonstrated by test and analysis during aircraft development and at initial type certification, when the design's protection is qualified against the required field strengths. Routine in-service listening for interference or checks at each service interval do not constitute the controlled HIRF testing, which is a certification activity.· FAA-H-8083-31 — Aircraft Electrical Systems
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When making a bonding-resistance check on a surface protected by an anodic (anodized) film, the technician must
Show the answer and explanation
Answer: remove the anodic film locally to ensure a good metal contact.
Why: An anodic film is an insulating oxide layer, so a reliable bonding measurement requires the film to be removed locally to expose bare metal for a true low-resistance contact. Pressing a probe through the film gives an unreliable reading, and sealing it with paste would leave the insulating layer in the contact path.· FAA-H-8083-31 — Aircraft Electrical Systems
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When clamping a wire bundle that contains coaxial cable, the technician must
Show the answer and explanation
Answer: avoid distorting the coaxial cable so its dielectric spacing is maintained.
Why: A coaxial cable must not be crushed or distorted, because deforming the spacing between the inner conductor and the outer braid changes the dielectric geometry and therefore the cable's characteristic impedance, degrading RF performance. Even slight distortion of the outer jacket is not acceptable on coaxial runs.· FAA-H-8083-31 — Aircraft Electrical Systems
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When installing an antenna (RF) cable in an aircraft, it should be
Show the answer and explanation
Answer: routed separately from the main wire bundle.
Why: An antenna cable carries or is sensitive to RF energy, so it is routed separately from the main wire bundle to prevent electromagnetic coupling with other conductors. Placing it at the center of the bundle or just outside it for access would still allow unwanted coupling with adjacent wiring.· FAA-H-8083-31 — Aircraft Electrical Systems
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Electromagnetic compatibility (EMC) in an aircraft's electrical and avionics installation is achieved by
Show the answer and explanation
Answer: shielding, screening, grounding, bonding, and interference filters.
Why: EMC is achieved by combining complementary techniques: shielding and screening to contain fields, sound grounding and bonding to control return paths and potentials, and interference filters to suppress unwanted energy. Relying on any single measure, such as ferrite sleeving or conductive coating alone, would not achieve full compatibility.· FAA-H-8083-31 — Aircraft Electrical Systems
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For a braided (screened) cable, the amount of braid coverage
Show the answer and explanation
Answer: determines how well radiated energy is kept inside the emitting circuit or away from a susceptible circuit.
Why: The braid acts as a barrier that confines radiated energy within an emitting cable and keeps external radiated energy out of a susceptible circuit, so the coverage of the braid determines how effective that screening is. It does not provide complete isolation, nor is its primary role to serve as the bonding return-current path.· FAA-H-8083-31 — Aircraft Electrical Systems
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The acronym HIRF stands for
Show the answer and explanation
Answer: High-Intensity Radiated Field.
Why: HIRF stands for High-Intensity Radiated Field, the strong external electromagnetic fields produced by transmitters such as radar and broadcast stations that aircraft systems must be protected against. The other expansions are plausible-sounding but are not the correct definition of the acronym.· FAA-H-8083-31 — Aircraft Electrical Systems
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An HF antenna is protected against a lightning strike by a
Show the answer and explanation
Answer: spark gap.
Why: An HF antenna is protected against lightning by a spark gap, which breaks down and conducts the high-voltage strike safely to airframe ground while leaving the normal HF signal unaffected. A fuse link or RF chokes cannot handle or divert the very high transient energy of a direct lightning strike.· FAA-H-8083-31 — Aircraft Electrical Systems
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Before crimping a terminal onto an aluminum wire conductor, what preparation should be performed?
Show the answer and explanation
Answer: Apply an oxide-inhibiting compound of petroleum jelly and zinc oxide
Why: Aluminum forms an insulating oxide film almost instantly once exposed. Before crimping, the stripped strands are coated with an oxide-inhibiting jointing compound (a mixture of petroleum jelly and zinc oxide) that breaks down and excludes the oxide, so a low-resistance, gas-tight crimp is achieved and reformation of the film is prevented.· FAA-H-8083-31 — Aircraft Electrical Systems
AM.II.KTap an answer
What is the purpose of a current-limiter fuse in an aircraft electrical system?
Show the answer and explanation
Answer: It allows a high overload current to flow for a few seconds
Why: A current-limiter is a slow-acting protective link designed to carry a heavy overload for a short interval (on the order of several seconds) without opening. This lets brief surges, such as motor starting or system inrush, pass while still protecting against a sustained fault, so it does not interrupt the current instantly like a fast-acting fuse.· FAA-H-8083-31 — Aircraft Electrical Systems
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In a typical aircraft electrical distribution system, the loads are connected:
Show the answer and explanation
Answer: in parallel with one another across the busbar
Why: Each load is supplied from the busbar through its own feed, but the various loads are connected in parallel with one another across the busbar. This keeps a common bus voltage available to every load while each draws only the current it requires.· FAA-H-8083-31 — Aircraft Electrical Systems
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The power factor of an AC generator indicates the proportion of apparent power that is
Show the answer and explanation
Answer: real power that does useful work
Why: Power factor is the ratio of true (real) power, which actually performs work, to the apparent power supplied by the generator. Reactive power circulates in the circuit without doing useful work, so a low power factor means a larger share of apparent power is reactive. The figure therefore represents the real power that does work.· FAA-H-8083-31 — Aircraft Electrical Systems
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If both the output voltage and frequency of an engine-driven AC generator drop to zero in flight, this most likely indicates that the
Show the answer and explanation
Answer: generator drive shaft has sheared
Why: When both output voltage and frequency fall to zero, the generator has lost its mechanical drive, indicating that the drive input shaft has sheared and the generator is no longer being turned. A protection trip or a bus fault would affect voltage and load distribution but would not collapse the output frequency to zero in this manner.· FAA-H-8083-31 — Aircraft Electrical Systems
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If the voltage and frequency of an AC generator both drop to zero in flight, it indicates that the
Show the answer and explanation
Answer: constant speed drive input shaft has sheared
Why: Both generator voltage and frequency depend on the generator being driven at speed. When voltage and frequency collapse to zero together in flight, the generator has lost its mechanical drive, which points to the constant speed drive (CSD) input shaft having sheared. A protection or interlock fault would not cause both outputs to fall to zero simultaneously.· FAA-H-8083-31 — Aircraft Electrical Systems
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Assuming all systems are operating normally, as aircraft electrical load increases, generator output voltage will
Show the answer and explanation
Answer: remain constant while amperage output increases
Why: The voltage regulator holds generator output voltage at its set value, so as electrical load increases the voltage remains essentially constant. To supply the added demand at constant voltage the current (amperage) drawn from the generator increases, with the regulator adjusting field excitation to maintain a steady output voltage.· FAA-H-8083-31 — Aircraft Electrical Systems
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One of the main purposes of a constant speed drive (CSD) in an AC system is to
Show the answer and explanation
Answer: enable generators to be paralleled
Why: A constant speed drive holds the generator at a fixed rotational speed regardless of engine speed, producing a constant output frequency. This common, stable frequency across all generators is what allows them to be synchronized and paralleled onto a shared bus; differing frequencies would otherwise prevent paralleling.· FAA-H-8083-31 — Aircraft Electrical Systems
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In an undervoltage condition in an AC generator system, the most likely immediate consequence is
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Answer: activation of the time delay circuit
Why: Undervoltage protection does not trip instantly, because momentary dips during transients are normal. A sensed undervoltage starts a time delay circuit, and only if the low voltage persists beyond the set period does the protection act and disconnect the generator, preventing nuisance trips from brief fluctuations.· FAA-H-8083-31 — Aircraft Electrical Systems
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When resetting the constant speed drive (CSD) on the ground, the engine should be
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Answer: stationary and not rotating
Why: Resetting a constant speed drive on the ground must be done with the engine stationary so the CSD is not being driven, eliminating risk to personnel and to the unit while the reset is carried out. Attempting a reset with the engine rotating at idle or at synchronous speed could damage the drive or be hazardous.· FAA-H-8083-31 — Aircraft Electrical Systems
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Differential protection in an AC generator system protects against
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Answer: line-to-line, three-phase, and line-to-ground faults
Why: Differential protection compares the current entering and leaving a generator feeder; any imbalance indicates a fault within the protected zone. It therefore detects feeder faults such as line-to-line, three-phase, and line-to-ground faults, tripping the generator before they cause major damage, rather than guarding against simple battery reverse current.· FAA-H-8083-31 — Aircraft Electrical Systems
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The power factor of an AC generator identifies the proportion of
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Answer: apparent power from the generator that does useful work
Why: Power factor is the ratio of real (working) power to apparent power, so it identifies what proportion of the generator's apparent output actually does useful work. A unity power factor means all the apparent power is real, while a low power factor means much of the current circulates as reactive power doing no work.· FAA-H-8083-31 — Aircraft Electrical Systems
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The frequency of a hydraulically powered emergency electrical generator is controlled by varying the
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Answer: angle of the hydraulic motor's swash plate
Why: A hydraulically driven emergency generator is turned by a variable-displacement hydraulic motor whose output speed sets the generator frequency. Speed, and hence frequency, is regulated by adjusting the angle of the motor's swash plate, which changes its displacement per revolution; a fixed restrictor or an integrated drive generator does not provide this controlled frequency regulation.· FAA-H-8083-31 — Aircraft Electrical Systems
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A purely inductive reactive load on an AC generator causes
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Answer: no change in torque but additional heat to be dissipated
Why: A purely inductive reactive load draws current that is 90 degrees out of phase with voltage, so it transfers no average real power and adds no mechanical torque to the generator drive. The reactive current still flows through the windings, however, raising copper losses and the heat that must be dissipated.· FAA-H-8083-31 — Aircraft Electrical Systems
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Increasing the real (resistive) load on an AC alternator primarily
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Answer: decreases the output frequency
Why: Real (resistive) load draws active power from the alternator, which loads the prime mover or constant speed drive and tends to slow it; until the governor restores speed, the output frequency falls. Output voltage is held by the regulator, so the primary immediate effect of added real load is a drop in frequency.· FAA-H-8083-31 — Aircraft Electrical Systems
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A paralleling relay in a DC system is energized and held connected by a
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Answer: voltage coil and a current coil
Why: The paralleling (differential) relay used in a DC system carries both a voltage coil and a current coil. The voltage coil closes the relay only when generator voltage slightly exceeds bus voltage, and the current coil holds it closed while current flows in the correct direction, opening it on reverse current to prevent the battery from feeding back.· FAA-H-8083-31 — Aircraft Electrical Systems
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In a transistorized voltage regulator, the output voltage is controlled by
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Answer: Zener diodes and transistors
Why: A transistorized voltage regulator uses a Zener diode to establish a stable reference voltage against which the output is compared, and transistors to amplify the error and control the field current. The Zener diode sets the regulation point while the transistors do the switching or controlling, so the output is governed by Zener diodes and transistors working together.· FAA-H-8083-31 — Aircraft Electrical Systems
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With no variation in electrical loading, the controlling element of a carbon pile voltage regulator is
Show the answer and explanation
Answer: stationary and holding a steady output
Why: When the electrical loading is steady, the carbon pile regulator settles to a fixed clamping pressure that holds the output at the regulated value, so the controlling element remains stationary. Pile movement only occurs in response to load or speed changes; with no variation there is nothing to correct, so the output stays constant and steady.· FAA-H-8083-31 — Aircraft Electrical Systems
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In a carbon pile voltage regulator, the resistive element is connected
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Answer: in series with the field, changing resistance with contact surface area
Why: A carbon pile regulator places a stack of carbon discs in series with the generator field, and its resistance is varied by changing the clamping pressure on the pile. Greater pressure increases the effective contact area between discs, lowering resistance and raising field current; the resistance change comes from contact area, not from altering the pile's length.· FAA-H-8083-31 — Aircraft Electrical Systems
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Under the ATA-100 numbering system, aircraft electrical power and battery servicing procedures are found in
Show the answer and explanation
Answer: ATA Chapter 24
Why: Aircraft electrical power, including the battery and its servicing, falls under ATA Chapter 24. Battery charging and servicing procedures are located within Chapter 24, not under Chapter 31, which covers indicating and recording systems.· FAA-H-8083-31 — Aircraft Electrical Systems
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If an aircraft has no dedicated battery charger, the battery is charged by the
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Answer: constant-voltage method from the regulated DC bus
Why: Without a dedicated charger, the battery is maintained directly from the aircraft DC bus, which is held at a regulated value by the generator system. This amounts to constant-voltage charging, where the battery draws a high initial current that tapers as it approaches bus voltage, rather than the controlled constant-current method used by a proper charger.· FAA-H-8083-31 — Aircraft Electrical Systems
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The small pin on a DC external (ground) power connector serves as the
Show the answer and explanation
Answer: positive control connection to the external power relay
Why: The DC ground power receptacle has two large pins carrying the main supply current plus a small pin that acts as a control connection energizing the external power relay. This small pin ensures the ground supply is connected through the relay only when the plug is fully and properly inserted, providing a safety interlock.· FAA-H-8083-31 — Aircraft Electrical Systems

That is a free sample of the 103 questions tagged to ACS AM.II.K. A free account adds three practice sessions a day and one timed exam-simulator attempt; the full bank, the adaptive engine that serves you more of whatever you keep missing, and unlimited exam simulations come with the Complete plan.

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Related subject areas

The areas that share the most ground with AM.II.K — study them alongside it.

Fundamentals of Electricity and Electronics (AM.I.A)
Engine Electrical Systems (AM.III.F)
Aircraft Instrument Systems (AM.II.H)
Communications, Light Signals, and Runway Lighting Systems (AM.II.I)

Frequently asked questions

What is the difference between a circuit breaker and a fuse?: Both open the circuit when current exceeds their rating, protecting the wiring from overheating. A fuse does it by melting a link and has to be replaced once it operates, while a circuit breaker opens mechanically and can be reset. Aircraft breakers are normally trip-free, meaning they open on a fault even if the handle is held in, so a fault cannot be overridden by force.
What does an inverter do on an aircraft?: An inverter converts DC power into AC so equipment that needs alternating current can run from a DC bus. Its complement is the transformer rectifier unit, which converts AC to DC, typically for battery charging and DC loads on an aircraft whose primary generation is AC. Candidates lose points by swapping the two, so fix the direction in your head: inverter DC to AC, transformer rectifier AC to DC.
Why keep nickel-cadmium and lead-acid batteries apart?: Their electrolytes are chemical opposites: sulfuric acid in a lead-acid cell, an alkaline potassium hydroxide solution in a nickel-cadmium cell. A trace of one carried across on a tool, hydrometer, syringe or bench top will neutralize and contaminate the other and ruin the battery. Separate servicing areas, separate tools and separate protective equipment are the standard shop practice.
Which LED lead is the cathode?: The shorter lead, on the side where the body of the LED is flattened, is the cathode, which is the negative side. The longer lead is the anode. An LED conducts in one direction only, so installing it backward leaves the circuit dark rather than merely dim, and it still needs a current-limiting resistor to keep the device inside its rated forward current.
Why is shielded wire used on some aircraft circuits?: A conductive shield around the conductor contains the electromagnetic field a current-carrying wire produces and keeps outside fields off the inner conductor. That stops noisy circuits, such as ignition and strobe leads, from interfering with radios and sensitive instrument circuits. The shield only does its job when it is properly grounded, so shield terminations are part of the installation rather than an optional extra.

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