In a high-tension magneto ignition system, the primary winding circuit current is interrupted by the action of the:
ACS code: AM.III.F
Correct answer: breaker points opening
Rationale: In a high-tension magneto, primary current flows while the breaker (contact) points are closed, building a magnetic field. When the points open, the primary circuit is interrupted, the field collapses rapidly, and the changing flux induces the high voltage in the secondary winding that fires the plug. The distributor only routes the high voltage to the correct cylinder; the secondary does not interrupt primary current.
What is the function of the capacitor (condenser) connected across the breaker points in a magneto?
ACS code: AM.III.F
Correct answer: Reduce arcing at the points and speed collapse of the primary field
Rationale: The capacitor is connected in parallel with the breaker points. As the points open, the capacitor momentarily absorbs the primary current, which minimizes arcing and pitting of the points and causes the primary magnetic field to collapse more rapidly. A faster field collapse produces a higher induced secondary voltage. It does not increase primary current or directly step up secondary voltage by transformer action.
When an aircraft magneto switch is placed in the OFF position, the primary circuit is:
ACS code: AM.III.F
Correct answer: grounded so the magneto cannot produce a spark
Rationale: The ignition switch is wired through the P-lead to ground the magneto primary circuit. With the switch OFF, the primary is grounded, so the field cannot collapse to induce high voltage and the magneto cannot fire. This is why a broken or disconnected P-lead leaves a magneto 'hot,' and a propeller must always be treated as if the ignition is on. The switch does not open the primary or connect it to the battery.
The purpose of an impulse coupling on a magneto is to:
ACS code: AM.III.F
Correct answer: provide a stronger, retarded spark for starting
Rationale: An impulse coupling is a spring-and-flyweight device that momentarily holds the magneto rotating magnet, then releases it so it snaps through the neutral position rapidly. This produces a hotter spark at low cranking speed and retards the timing for starting, reducing kickback. Once the engine starts and rpm rises, the flyweights disengage and the magneto runs at normal timing. It does not advance timing or distribute spark.
During a magneto ignition system check, an excessive rpm drop when operating on a single magneto most likely indicates:
ACS code: AM.III.F
Correct answer: a fouled spark plug or faulty ignition lead
Rationale: A normal mag check shows a small rpm drop on each single magneto because only one plug per cylinder is firing. An excessive drop on one magneto points to a problem in that ignition circuit, such as a fouled or defective spark plug, a faulty ignition lead, or a magneto-related fault, which causes a misfire. Mixture and fuel-nozzle issues affect both magnetos and overall power rather than producing a drop on only one magneto.
In a turbine engine, the high voltage required to fire the igniter plugs is produced by the:
ACS code: AM.III.F
Correct answer: exciter unit of the ignition system
Rationale: A turbine (capacitor-discharge) ignition system uses an exciter unit that stores electrical energy in a capacitor and then discharges it at high voltage through the ignition lead to the igniter plug, producing a high-energy spark for starting. The fuel control meters fuel, and the constant-speed drive governs generator speed; neither generates igniter voltage.
Which device limits and controls the output of an aircraft engine-driven alternator to protect the electrical system?
ACS code: AM.III.F
Correct answer: Voltage regulator
Rationale: The voltage regulator controls alternator output by varying the field current, holding system voltage within limits as electrical load and engine speed change. This prevents overcharging the battery and protects connected equipment. A circuit breaker is an overcurrent protective device, and a bus tie relay connects buses; neither regulates alternator output voltage.
The brushes in a typical DC starter-generator or generator make electrical contact with the:
ACS code: AM.III.F
Correct answer: commutator
Rationale: In a DC machine, carbon brushes ride on the rotating commutator to carry current to and from the armature windings; the commutator also provides the switching action that produces or uses DC. The stator core and field frame house the field windings and magnetic structure and are not where the brushes ride. Worn or sticking brushes and a dirty commutator are common causes of poor generator output.
What is the main advantage of a turbine engine starter-generator compared with separate starter and generator units?
ACS code: AM.III.F
Correct answer: One unit serves both functions, saving weight and space
Rationale: A starter-generator is a single machine that acts as a motor to start the engine and, after start, is reconnected to operate as a generator. Combining both functions in one unit reduces weight, complexity, and the number of accessory drive pads, which is valuable on turbine engines. It still needs a power source such as the battery or external power to start, and it is a DC machine, not a main AC source.
On a turbine engine, the electronic engine control (EEC) receives its primary electrical power from
ACS code: AM.III.F
Correct answer: a separate engine-driven permanent-magnet alternator.
Rationale: Per FAA-H-8083-32, the EEC/FADEC is powered by its own dedicated permanent-magnet alternator driven by the engine so that the control remains operational independent of the aircraft electrical buses. As long as the engine is rotating, this alternator supplies the EEC, which is why a main or emergency AC bus is not relied upon for primary power.
In aircraft electrical terminology, feeders are
ACS code: AM.III.F
Correct answer: heavy-duty cables carrying generator output to the busbars.
Rationale: Feeders are the heavy-duty cables that carry the main generator output to the distribution busbars. They are not conduits or fluid hoses; their large cross-section reflects the high currents they must carry.
An electronic engine control (EEC/FADEC) typically exchanges digital data with aircraft systems using
ACS code: AM.III.F
Correct answer: ARINC 429 formatted data
Rationale: The EEC commonly exchanges digital information using ARINC 429, the standard one-way data-bus format for this generation of engine controls. ARINC 629 is a different, less common bus used on specific airframes, so the typical format is ARINC 429 (FAA-H-8083-32, Engine Electrical Systems).
What supplies the primary electrical power for a turbine engine's electronic engine control (EEC) once the engine is running?
ACS code: AM.III.F
Correct answer: A dedicated permanent magnet alternator driven by the engine
Rationale: An electronic engine control receives its primary power from a dedicated permanent magnet alternator (PMA) driven by the engine gearbox. This makes the control self-powering once the engine is turning and independent of the aircraft electrical buses, so the EEC keeps operating even if the normal 115V AC supply is lost.
What is the primary purpose of a bonding jumper in an aircraft engine electrical system?
ACS code: AM.III.F
Correct answer: To provide a low-resistance path between metallic parts to prevent static buildup and ensure a common ground
Rationale: Per FAA-H-8083-32, bonding jumpers provide a low-resistance electrical path between metallic components to prevent the buildup of static charge and to ensure a continuous ground path, which reduces radio interference and the hazard of arcing.
What is the function of the commutator in a DC generator?
ACS code: AM.III.F
Correct answer: It mechanically rectifies the AC generated in the armature into DC for the external circuit
Rationale: Per FAA-H-8083-32, the commutator acts as a mechanical rectifier, reversing the armature connections to the external circuit at the proper instant so that the alternating EMF generated in the armature is delivered to the external circuit as direct current.
Why is a DC alternator (alternator with a rectifier) generally preferred over a DC generator in modern aircraft?
ACS code: AM.III.F
Correct answer: It provides more constant output over a wide RPM range, including at low engine speeds
Rationale: Per FAA-H-8083-32, alternators produce a more constant output over a wide RPM range and can provide rated output at lower engine speeds (such as idle), and they are generally lighter and require less maintenance than DC generators because field current is carried through slip rings rather than a heavily loaded commutator.
In a paralleled generator system, what is the function of the equalizing (paralleling) circuit?
ACS code: AM.III.F
Correct answer: It causes each generator to carry an equal share of the total electrical load
Rationale: Per FAA-H-8083-32, when two or more generators are operated in parallel, the equalizing circuit (equalizer) compares the voltages and causes the regulators to adjust field strength so that each generator carries an equal share of the total electrical load.
What condition is indicated by a commutator that shows a smooth, polished, copper-colored or chocolate-brown surface?
ACS code: AM.III.F
Correct answer: A normal, desirable condition indicating proper brush seating and good commutation
Rationale: Per FAA-H-8083-32, a glossy chocolate-brown or polished film on a commutator is a normal and desirable condition that indicates proper brush seating and good commutation; it should not be removed during cleaning.
In a starter-generator system used on many turbine engines, how is the single unit used during engine start versus normal operation?
ACS code: AM.III.F
Correct answer: It acts as a motor to crank the engine during start, then operates as a generator once running
Rationale: Per FAA-H-8083-32, a starter-generator functions as a motor (starter) to crank the engine during start, then, once the engine is running, is switched to operate as a generator to supply electrical power, saving the weight and complexity of two separate units.
What is the purpose of flashing the field of a DC generator?
ACS code: AM.III.F
Correct answer: To restore residual magnetism in the field so the generator can build up voltage
Rationale: Per FAA-H-8083-32, flashing the field momentarily applies battery voltage to the generator field to restore or re-establish residual magnetism in the field poles, which is necessary for the generator to begin building up output voltage if residual magnetism has been lost.
Which type of generator field circuit is most commonly used in aircraft DC generators because it allows the voltage regulator to control output?
ACS code: AM.III.F
Correct answer: Shunt-wound, with the field connected in parallel with the armature
Rationale: Per FAA-H-8083-32, aircraft DC generators are commonly shunt-wound (the field is connected in parallel with the armature), which permits a voltage regulator to control output by varying the relatively small field current.
What is the primary function of slip rings in an aircraft alternator?
ACS code: AM.III.F
Correct answer: To carry field excitation current to the rotating field winding
Rationale: Per FAA-H-8083-32, slip rings, contacted by brushes, carry the relatively small DC field excitation current to the rotating field winding of an alternator; the AC output is taken from the stationary stator windings, so the slip rings do not carry the main output current.
In a three-phase aircraft alternator with a built-in rectifier, what type of device converts the AC to DC?
ACS code: AM.III.F
Correct answer: A bank of semiconductor diodes (rectifier assembly)
Rationale: Per FAA-H-8083-32, a three-phase alternator's AC output is converted to DC by a bank of semiconductor diodes (a rectifier assembly), which conduct current in only one direction to produce a relatively smooth DC output.
What is the recommended method for cleaning a slightly dirty or oily commutator on an aircraft generator?
ACS code: AM.III.F
Correct answer: Use fine (No. 000) sandpaper, never emery cloth
Rationale: Per FAA-H-8083-32, a dirty commutator can be cleaned by holding a strip of No. 000 (very fine) sandpaper against it while it rotates; emery cloth or other conductive abrasives must never be used because the metallic particles can short the segments.
What type of switch is typically used to control the application of power to an engine starter, and why?
ACS code: AM.III.F
Correct answer: A solenoid/contactor, so a small control current can switch the heavy starter current
Rationale: Per FAA-H-8083-32, because starter motors draw very heavy current, a solenoid (or starter relay/contactor) is used so that a small control current from the cockpit switch can close heavy-duty contacts that carry the large starter current, keeping the high current off the cockpit switch and its wiring.
When measuring brush spring tension on a generator, an excessively weak spring is likely to cause what problem?
ACS code: AM.III.F
Correct answer: Brush bouncing and poor contact, causing arcing and commutator pitting
Rationale: Per FAA-H-8083-32, insufficient brush spring tension allows the brush to bounce and make poor contact with the commutator, causing arcing, sparking, and pitting; excessive tension causes rapid brush and commutator wear. Proper tension must be maintained per specifications.
Why is stranded wire, rather than solid wire, generally used for aircraft engine electrical wiring?
ACS code: AM.III.F
Correct answer: It is more flexible and more resistant to fatigue failure from vibration
Rationale: Per FAA-H-8083-32, stranded wire is used in aircraft because it is far more flexible and far more resistant to fatigue failure from the vibration present in the engine and airframe environment than solid wire of the same gauge.
When selecting the proper size (gauge) of electrical wire for an engine circuit, which two primary factors must be considered?
ACS code: AM.III.F
Correct answer: The current-carrying capacity and the allowable voltage drop over the wire length
Rationale: Per FAA-H-8083-32, wire size selection is based on the amount of current the wire must carry without overheating and the allowable voltage drop over the length of the conductor; both current-carrying capacity and voltage drop must be within limits.