Airframe Mechanic Aircraft Electrical Systems Questions and Answers — Questions and Answers
Question 1: An aircraft's electrical schematic shows a device that uses a small current flowing through a coil to electromagnetically close a set of contacts, allowing a much larger current to flow to a high-power component like a starter motor. Which device performs this function?
- A circuit breaker
- A diode
- A relay (Correct answer)
- A resistor
Correct answer: A relay
A relay is an electromechanical switch that uses a small control current to activate an electromagnet, which in turn closes contacts to control a separate, high-power circuit. This allows for remote switching and reduces the need for heavy-gauge wiring to the cockpit controls.
Question 2: Which of the following is the primary purpose of a Transformer Rectifier Unit (TRU) in an aircraft electrical system?
- To convert 115V AC power to 28V DC power (Correct answer)
- To step up DC voltage for long-distance transmission
- To convert 28V DC power to 115V AC power for sensitive instruments
- To regulate the frequency of the AC generator output
Correct answer: To convert 115V AC power to 28V DC power
A Transformer Rectifier Unit (TRU) is a standard component in aircraft with AC power generation. Its main function is to transform the high-voltage AC (typically 115V, 400Hz) and then rectify it into stable, low-voltage DC (typically 28V) for use by numerous aircraft systems.
Question 3: A technician is performing maintenance on a nickel-cadmium (Ni-Cad) battery and observes a white, powdery residue on the battery case near the cell vents. This is most often caused by:
- Normal outgassing during a deep discharge cycle.
- Electrolyte reacting with lead-acid fumes from a nearby battery.
- The battery freezing in extremely low ambient temperatures.
- Electrolyte spewage due to overcharging or excessive heat. (Correct answer)
Correct answer: Electrolyte spewage due to overcharging or excessive heat.
The white, powdery substance found on Ni-Cad batteries is potassium carbonate. It forms when the potassium hydroxide electrolyte is spewed from the cells, often due to overcharging or high temperatures, and subsequently reacts with carbon dioxide in the air.
Question 4: What is the primary function of a Constant Speed Drive (CSD) connected to an engine-driven AC generator?
- To provide over-voltage protection for the electrical bus.
- To convert the variable engine RPM into a constant generator RPM. (Correct answer)
- To act as a clutch, allowing the generator to be disengaged.
- To cool the generator using a dedicated oil system.
Correct answer: To convert the variable engine RPM into a constant generator RPM.
AC generators must rotate at a precise, constant speed to produce alternating current at a stable frequency (typically 400 Hz). A Constant Speed Drive is a hydro-mechanical transmission that takes the variable RPM from the engine's accessory gearbox and outputs a constant RPM to drive the generator.
Question 5: In a brushed DC generator, what is the combined function of the commutator and brushes?
- To regulate the generator's output voltage.
- To cool the armature windings by providing an air gap.
- To provide a sliding electrical connection and convert the internal AC to external DC. (Correct answer)
- To vary the magnetic field strength of the stator.
Correct answer: To provide a sliding electrical connection and convert the internal AC to external DC.
As the armature rotates, it generates AC internally. The commutator, consisting of segmented copper bars, rotates with the armature. The stationary carbon brushes ride on the commutator, acting as a mechanical rectifier to reverse the current direction every half-turn, resulting in a DC output to the external circuit.
Question 6: To verify the integrity of an electrical bond between an equipment rack and the aircraft structure, a technician should use a multimeter to measure for:
- a very low resistance. (Correct answer)
- a specific AC voltage.
- the absence of continuity.
- the capacitance of the bonding strap.
Correct answer: a very low resistance.
Electrical bonding ensures that all conductive parts of the aircraft are at the same electrical potential. A proper bond provides a path of least resistance for fault currents, lightning strikes, and static discharge. A test of a good bond is a measurement of very low resistance (typically in the milliohm range) between the two points.
Airframe Mechanic Aircraft Electrical Systems Questions and Answers
An aircraft's electrical schematic shows a device that uses a small current flowing through a coil to electromagnetically close a set of contacts, allowing a much larger current to flow to a high-power component like a starter motor.
Which device performs this function?