AMT Aircraft Electrical Systems 3 — Questions and Answers
Question 1: What is the function of a voltage regulator in an aircraft generator system?
- To limit maximum generator RPM
- To maintain constant generator output voltage under varying loads (Correct answer)
- To convert AC output to DC for the battery
- To disconnect the generator when voltage is too low
Correct answer: To maintain constant generator output voltage under varying loads
The voltage regulator controls generator field current to maintain a steady output voltage regardless of electrical load or engine speed changes.
Question 2: When troubleshooting an open circuit, a voltmeter connected across the break will read:
- Zero volts
- Full system voltage (Correct answer)
- Half system voltage
- A negative voltage
Correct answer: Full system voltage
In an open circuit, the voltmeter reads full supply voltage across the break because no current flows and the entire EMF appears across the open point.
Question 3: Aircraft wiring AWG (American Wire Gauge) sizing is selected primarily based on:
- The length of the wire run only
- The operating voltage of the circuit
- The current load and allowable voltage drop over the wire length (Correct answer)
- The temperature rating of the insulation
Correct answer: The current load and allowable voltage drop over the wire length
Wire gauge must handle the circuit's current without overheating and keep voltage drop within acceptable limits for the length of the run.
Question 4: A reverse current cutout relay (or reverse current relay) in a DC generator system opens when:
- Generator voltage exceeds battery voltage
- Generator output falls below battery voltage causing reverse current flow (Correct answer)
- The battery is fully charged
- The bus voltage drops below a preset threshold
Correct answer: Generator output falls below battery voltage causing reverse current flow
When generator voltage drops below battery voltage, current would flow backward from battery to generator; the relay opens to prevent this reverse flow and possible generator damage.
Question 5: Which of the following best describes a 'phantom load' in an aircraft electrical system?
- An intermittent short circuit
- A continuous small current drain from devices left energized when the master switch is off (Correct answer)
- Inductive kickback from relay coils
- A resistance load created by faulty bonding straps
Correct answer: A continuous small current drain from devices left energized when the master switch is off
Phantom (or parasitic) loads are small but continuous drains from clocks, memory circuits, or improperly wired equipment that can discharge the battery over time.
Question 6: The purpose of a current limiter (fuse link) between the battery and main bus is to:
- Limit charging current to the battery
- Protect the heavy feeder cable from a catastrophic fault current (Correct answer)
- Regulate the bus voltage
- Prevent reverse polarity connection
Correct answer: Protect the heavy feeder cable from a catastrophic fault current
A current limiter is a high-amperage slow-blow protective device that guards the main feeder against fault currents that smaller circuit breakers cannot handle.
Question 7: In an AC aircraft electrical system, frequency is expressed in hertz (Hz). What is the standard frequency used in most transport-category aircraft?
- 60 Hz
- 50 Hz
- 400 Hz (Correct answer)
- 800 Hz
Correct answer: 400 Hz
Aircraft AC systems operate at 400 Hz, which allows smaller, lighter transformers and motors compared to the 60 Hz used in commercial ground power.
AMT Aircraft Electrical Systems 3
What is the function of a voltage regulator in an aircraft generator system?