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

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 1,748 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

1. 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.
2. 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.
3. 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.

7 free sample questions from ACS AM.II.K

1. AM.II.KTap an answer

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

   • AA circuit breaker can be reset after a fault, while a fuse must be replaced
   • BA circuit breaker protects only AC circuits, while a fuse protects only DC circuits
   • CA circuit breaker increases voltage on a fault, while a fuse decreases it
2. AM.II.KTap an answer

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

   • ATo rectify the generator AC output into usable DC
   • BTo maintain a constant generator output voltage as load and speed vary
   • CTo open the circuit when output current becomes excessive
3. AM.II.KTap an answer

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

   • AThe metal aircraft structure used as a common ground
   • BThe shielding braid around each individual conductor
   • CA second dedicated return wire run to every load
4. AM.II.KTap an answer

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

   • AThe 24 volts is divided equally, giving each load 12 volts
   • BEach load receives the full bus voltage of 24 volts
   • CEach load receives a voltage proportional to its resistance share
5. AM.II.KTap an answer

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

   • ATo increase the current-carrying capacity of the conductor
   • BTo provide the primary mechanical support for the wire bundle
   • CTo reduce electromagnetic interference radiated to or from the conductor
6. AM.II.KTap an answer

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

   • ATheir electrolytes are chemically opposite and cross-contamination ruins the cells
   • BTheir charging voltages are identical and would create a short circuit
   • CTheir terminals use the same polarity and would discharge together
7. AM.II.KTap an answer

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

   • ATo convert DC power into AC power for AC-operated equipment
   • BTo convert AC power into DC power for battery charging
   • CTo raise DC voltage to a higher DC value for distribution

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.

All 40 ACS areas

Primary sources

• FAA Mechanic Airman Certification Standards (FAA-S-ACS-1)
• 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers