What is the main functional difference between a circuit breaker and a fuse?
ACS code: AM.II.K
Correct answer: A circuit breaker can be reset after a fault, while a fuse must be replaced
Rationale: 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.
What is the primary function of a voltage regulator in an aircraft generator system?
ACS code: AM.II.K
Correct answer: To maintain a constant generator output voltage as load and speed vary
Rationale: 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.
In a single-wire (ground-return) electrical system common to metal aircraft, what serves as the return path back to the power source?
ACS code: AM.II.K
Correct answer: The metal aircraft structure used as a common ground
Rationale: 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.
When two electrical loads are connected in parallel across a 24-volt bus, what is true of the voltage applied to each load?
ACS code: AM.II.K
Correct answer: Each load receives the full bus voltage of 24 volts
Rationale: 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.
What is the main reason for using shielded wire on certain aircraft electrical circuits?
ACS code: AM.II.K
Correct answer: To reduce electromagnetic interference radiated to or from the conductor
Rationale: 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.
Why must nickel-cadmium and lead-acid batteries never share the same servicing area or tools?
ACS code: AM.II.K
Correct answer: Their electrolytes are chemically opposite and cross-contamination ruins the cells
Rationale: 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.
On a transport-category aircraft, what is the main purpose of an inverter in the electrical system?
ACS code: AM.II.K
Correct answer: To convert DC power into AC power for AC-operated equipment
Rationale: 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.
On a standard LED, the shorter lead located next to the flat side of the body identifies the
ACS code: AM.II.K
Correct answer: cathode (negative terminal).
Rationale: 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.
What precaution is required when removing electrostatic-discharge-sensitive (ESDS) equipment from an aircraft?
ACS code: AM.II.K
Correct answer: Wear an anti-static wrist strap connected to an approved ground point.
Rationale: 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.
A flight deck display line-replaceable unit (LRU) is being replaced. What does its ESDS label tell the technician?
ACS code: AM.II.K
Correct answer: Do not touch the connector pins or leave them exposed.
Rationale: 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.
When installing an electronic flight instrument system (EFIS) display, the technician must
ACS code: AM.II.K
Correct answer: avoid touching the connector terminals.
Rationale: 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.
At a relative humidity of 10% to 20%, walking across a carpet can generate a body electrostatic voltage of approximately
ACS code: AM.II.K
Correct answer: 35,000 volts.
Rationale: 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.
An anti-static wrist strap is worn when working with ESDS devices in order
ACS code: AM.II.K
Correct answer: to continuously dissipate and prevent static build-up on the operator.
Rationale: 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.
A humid atmosphere has what effect on static electricity build-up?
ACS code: AM.II.K
Correct answer: It reduces static charge build-up.
Rationale: 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.
An electrostatic discharge (ESD) occurs when
ACS code: AM.II.K
Correct answer: materials at different electrical potentials are brought into close proximity.
Rationale: 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.
Before removing a static-sensitive assembly from an aircraft, you must first ensure minimum current flow between
ACS code: AM.II.K
Correct answer: the technician and the assembly being handled.
Rationale: 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.
How is a fiber optic data line protected against electromagnetic interference (EMI) at its electrical interface?
ACS code: AM.II.K
Correct answer: By installing an opto-isolator at the electrical interface.
Rationale: 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.
How do aircraft manufacturers build in protection against high-intensity radiated fields (HIRF)?
ACS code: AM.II.K
Correct answer: By designing specific avionics areas that are shielded against HIRF.
Rationale: 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.
How is a line replaceable unit (LRU) itself protected against high-intensity radiated fields (HIRF)?
ACS code: AM.II.K
Correct answer: By built-in filters and overvoltage protection circuits.
Rationale: 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.
To prevent a ground loop from forming on a shielded cable, the cable shield should be grounded at
ACS code: AM.II.K
Correct answer: only one end of the shield.
Rationale: 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.
At what point is an aircraft's immunity to high-intensity radiated fields (HIRF) demonstrated by test and analysis?
ACS code: AM.II.K
Correct answer: During development and at initial type certification.
Rationale: 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.
When making a bonding-resistance check on a surface protected by an anodic (anodized) film, the technician must
ACS code: AM.II.K
Correct answer: remove the anodic film locally to ensure a good metal contact.
Rationale: 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.
When clamping a wire bundle that contains coaxial cable, the technician must
ACS code: AM.II.K
Correct answer: avoid distorting the coaxial cable so its dielectric spacing is maintained.
Rationale: 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.
When installing an antenna (RF) cable in an aircraft, it should be
ACS code: AM.II.K
Correct answer: routed separately from the main wire bundle.
Rationale: 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.
Electromagnetic compatibility (EMC) in an aircraft's electrical and avionics installation is achieved by
ACS code: AM.II.K
Correct answer: shielding, screening, grounding, bonding, and interference filters.
Rationale: 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.
For a braided (screened) cable, the amount of braid coverage
ACS code: AM.II.K
Correct answer: determines how well radiated energy is kept inside the emitting circuit or away from a susceptible circuit.
Rationale: 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.
The acronym HIRF stands for
ACS code: AM.II.K
Correct answer: High-Intensity Radiated Field.
Rationale: 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.
An HF antenna is protected against a lightning strike by a
ACS code: AM.II.K
Correct answer: spark gap.
Rationale: 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.
Before crimping a terminal onto an aluminum wire conductor, what preparation should be performed?
ACS code: AM.II.K
Correct answer: Apply an oxide-inhibiting compound of petroleum jelly and zinc oxide
Rationale: 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.
What is the purpose of a current-limiter fuse in an aircraft electrical system?
ACS code: AM.II.K
Correct answer: It allows a high overload current to flow for a few seconds
Rationale: 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.
Non-essential loads such as galleys and cabin lighting are normally supplied from the:
ACS code: AM.II.K
Correct answer: transfer (non-essential) bus
Rationale: Non-essential services such as galleys and cabin lighting are fed from a transfer (non-essential) bus, which can be disconnected during load shedding or abnormal supply conditions without affecting essential systems. Ground service and ground handling buses are energized only for specific maintenance and turnaround functions, not for normal non-essential loads.
In a typical aircraft electrical distribution system, the loads are connected:
ACS code: AM.II.K
Correct answer: in parallel with one another across the busbar
Rationale: 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.
The power factor of an AC generator indicates the proportion of apparent power that is
ACS code: AM.II.K
Correct answer: real power that does useful work
Rationale: 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.
If both the output voltage and frequency of an engine-driven AC generator drop to zero in flight, this most likely indicates that the
ACS code: AM.II.K
Correct answer: generator drive shaft has sheared
Rationale: 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.
If the voltage and frequency of an AC generator both drop to zero in flight, it indicates that the
ACS code: AM.II.K
Correct answer: constant speed drive input shaft has sheared
Rationale: 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.
Assuming all systems are operating normally, as aircraft electrical load increases, generator output voltage will
ACS code: AM.II.K
Correct answer: remain constant while amperage output increases
Rationale: 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.
One of the main purposes of a constant speed drive (CSD) in an AC system is to
ACS code: AM.II.K
Correct answer: enable generators to be paralleled
Rationale: 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.
In an undervoltage condition in an AC generator system, the most likely immediate consequence is
ACS code: AM.II.K
Correct answer: activation of the time delay circuit
Rationale: 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.
When resetting the constant speed drive (CSD) on the ground, the engine should be
ACS code: AM.II.K
Correct answer: stationary and not rotating
Rationale: 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.
Differential protection in an AC generator system protects against
ACS code: AM.II.K
Correct answer: line-to-line, three-phase, and line-to-ground faults
Rationale: 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.
The power factor of an AC generator identifies the proportion of
ACS code: AM.II.K
Correct answer: apparent power from the generator that does useful work
Rationale: 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.
The frequency of a hydraulically powered emergency electrical generator is controlled by varying the
ACS code: AM.II.K
Correct answer: angle of the hydraulic motor's swash plate
Rationale: 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.
A purely inductive reactive load on an AC generator causes
ACS code: AM.II.K
Correct answer: no change in torque but additional heat to be dissipated
Rationale: 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.
Increasing the real (resistive) load on an AC alternator primarily
ACS code: AM.II.K
Correct answer: decreases the output frequency
Rationale: 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.
A paralleling relay in a DC system is energized and held connected by a
ACS code: AM.II.K
Correct answer: voltage coil and a current coil
Rationale: 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.
In a transistorized voltage regulator, the output voltage is controlled by
ACS code: AM.II.K
Correct answer: Zener diodes and transistors
Rationale: 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.
With no variation in electrical loading, the controlling element of a carbon pile voltage regulator is
ACS code: AM.II.K
Correct answer: stationary and holding a steady output
Rationale: 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.
In a carbon pile voltage regulator, the resistive element is connected
ACS code: AM.II.K
Correct answer: in series with the field, changing resistance with contact surface area
Rationale: 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.
Under the ATA-100 numbering system, aircraft electrical power and battery servicing procedures are found in
ACS code: AM.II.K
Correct answer: ATA Chapter 24
Rationale: 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.
If an aircraft has no dedicated battery charger, the battery is charged by the
ACS code: AM.II.K
Correct answer: constant-voltage method from the regulated DC bus
Rationale: 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.