Free AMT Aircraft Electrical Systems Questions and Answers 1 — Questions and Answers
Question 1: In a typical large transport aircraft AC electrical system, what is the primary purpose of a Constant Speed Drive (CSD) unit?
- To convert variable engine RPM into a constant generator speed. (Correct answer)
- To rectify AC voltage to DC voltage for the battery bus.
- To increase the output voltage of the generator during high-demand periods.
- To act as a circuit breaker in the event of a generator fault.
Correct answer: To convert variable engine RPM into a constant generator speed.
The AC generators on most large aircraft must turn at a constant speed to produce alternating current at a stable frequency, typically 400 Hz. [1, 2, 3] Since the engine's accessory gearbox speed varies with engine thrust settings, a Constant Speed Drive (CSD) is used as a hydro-mechanical transmission to ensure the generator rotates at a constant RPM, regardless of the engine's speed. [1, 4]
Question 2: A technician is troubleshooting a circuit that repeatedly trips its protective device shortly after being reset. The aircraft is in a high-vibration environment. Which of the following is the most likely cause?
- A faulty circuit breaker.
- An intermittent short to ground in the wiring. (Correct answer)
- The battery is overcharged.
- The load on the circuit is too low.
Correct answer: An intermittent short to ground in the wiring.
An intermittent short to ground is a common cause for a circuit breaker to trip repeatedly. In a high-vibration environment, wiring insulation can chafe against the aircraft structure, creating a temporary path to ground that draws excessive current and trips the breaker. Once the vibration stops or changes, the contact may be broken, allowing the breaker to be reset, only for it to trip again when the fault reoccurs.
Question 3: When comparing a typical aircraft DC alternator to a DC generator, what is a significant advantage of the alternator?
- It can produce electrical power even with a completely dead battery.
- It is less susceptible to damage from reversed polarity.
- It produces sufficient charging current at lower engine RPMs. (Correct answer)
- It does not require a voltage regulator for operation.
Correct answer: It produces sufficient charging current at lower engine RPMs.
A key advantage of alternators is their ability to produce significant electrical output at low engine speeds, including idle. [12, 16, 27] This is crucial for modern aircraft with high electrical loads. Generators, in contrast, typically require higher RPM to produce adequate charging current, and can even drain the battery at idle. [24, 32]
Question 4: According to 14 CFR Part 25, if the ability to replace a fuse is essential to safety in flight, what is the minimum number of spare fuses that must be available?
- One for each essential circuit.
- At least 25% of the number of fuses of each rating.
- At least 50% of the number of fuses of each rating. (Correct answer)
- A number equal to the total number of fuses installed.
Correct answer: At least 50% of the number of fuses of each rating.
14 CFR § 25.1357(d) states: 'Where fuses are used, there must be spare fuses for use in flight equal to at least 50% of the number of fuses of each rating required for complete circuit protection.' [11, 14] This regulation ensures that flight crews have sufficient spares for critical systems.
Question 5: While reviewing an aircraft wiring diagram, a technician notes a component symbol consisting of a coil and a set of contacts. When the coil is energized, it mechanically closes the contacts in a separate circuit. What does this symbol represent?
- A diode.
- A resistor.
- A capacitor.
- A relay. (Correct answer)
Correct answer: A relay.
This describes the function and schematic representation of a relay. A relay is an electromechanical switch that uses a small current in a control circuit (the coil) to control a larger current in a separate power circuit (the contacts). [9] This allows for remote switching and the control of high-power circuits with a low-power signal.
Question 6: Which type of aircraft battery is known for being lightweight and having a high energy density, but is also susceptible to thermal runaway if not properly managed by a sophisticated charging and monitoring system?
- Vented lead-acid.
- Nickel-cadmium (Ni-Cd).
- Lithium-ion (Li-ion). (Correct answer)
- Sealed lead-acid (VRLA).
Correct answer: Lithium-ion (Li-ion).
Lithium-ion batteries offer significant advantages in weight and energy density compared to older technologies like lead-acid and Ni-Cd. [22] However, they are chemically volatile and require complex battery management systems to prevent overcharging, overheating, and thermal runaway, a condition that can lead to fire. [22]
Free AMT Aircraft Electrical Systems Questions and Answers 1
In a typical large transport aircraft AC electrical system, what is the primary purpose of a Constant Speed Drive (CSD) unit?