Why is a propeller dangerous even with the ignition switch OFF?

Because the switch stops the magneto by grounding its primary circuit through the P-lead rather than by cutting power to it. A magneto makes its own current whenever it turns, so if the P-lead is broken, chafed, or disconnected, the switch cannot ground it and the magneto still fires. Treat every propeller as live and confirm the grounding circuit before turning one by hand.

What is the function of the capacitor in a magneto?

It is connected across the breaker points in the primary circuit. When the points open, the capacitor absorbs the surge of current that would otherwise jump the gap as an arc, which prevents burning and pitting of the point faces. Suppressing that arc also makes the primary magnetic field collapse much faster, and a faster collapse induces a higher voltage in the secondary winding.

What does an impulse coupling do?

It solves the low-speed starting problem. During cranking, a spring in the coupling winds up while the magnet is held, then releases it so the rotating magnet snaps through its neutral position much faster than the engine is turning. That produces a spark hot enough to fire the mixture, and the delay also retards the spark timing so the engine does not kick back.

What does an excessive rpm drop on one magneto indicate?

A fault somewhere in that magneto's ignition circuit, since the engine is running on that side alone during the check. Common causes are fouled or worn spark plugs, a defective ignition lead or distributor, incorrect internal timing, or a magneto that is out of time to the engine. Troubleshooting works from the cheapest and most likely cause outward before removing the magneto.

How is turbine engine ignition different from a magneto system?

A turbine only needs ignition for starting and for relight, not continuously, so it uses an electrically powered ignition exciter that steps input power up to the high voltage the igniter plugs need. Igniter plugs have a much wider gap and produce a far more energetic spark than a reciprocating engine spark plug, and they are replaced on their own schedule.

Engine Electrical Systems — FAA A&P Test Questions (ACS AM.III.F)

Engine Electrical Systems is a small subject area with an outsized safety payload. AM.III.F questions concentrate on magneto ignition, turbine ignition, and the engine-driven generating equipment that feeds the aircraft bus. You will be asked what opens the primary circuit, what the condenser is there for, what the switch does when it is turned off, and what an abnormal magneto drop is telling you. Get the magneto primary circuit straight and most of this area answers itself, including the questions that keep mechanics from being injured by a live propeller.

What ACS AM.III.F covers

This area covers ignition and engine-driven electrical generation on reciprocating and turbine powerplants, following the treatment in the Powerplant Handbook, FAA-H-8083-32. Magneto theory carries most of the questions. In a high-tension magneto, rotating magnetism induces current in the primary winding, the breaker points interrupt that primary current at the correct instant, and the sudden collapse of the primary magnetic field induces the high voltage in the secondary winding that fires the plug. The capacitor, or condenser, is wired across the points in the primary circuit; it absorbs the surge that would otherwise arc across the opening points, which both protects the point faces and makes the field collapse fast enough to produce a hot spark. The ignition switch works by grounding, not by opening: the P-lead shorts the primary to ground in the OFF position, so a broken or disconnected P-lead leaves that magneto live and the propeller dangerous regardless of switch position. Starting hardware appears through the impulse coupling, a spring device that holds the rotating magnet and then releases it to spin rapidly, delivering both a hot spark and a retarded spark at cranking speed. Troubleshooting questions read the magneto check: an excessive rpm drop on one magneto points to a fault on that side, such as fouled plugs, a failed lead, or incorrect timing, while no drop at all points to a grounding problem in the switch circuit. Turbine ignition is treated separately, with the exciter unit producing the high voltage that fires the igniter plugs. Generation questions close the area with voltage regulators controlling alternator output and brushes riding on the commutator of a DC generator or starter-generator.

Where this sits on the test

ACS AM.III.F is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant 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-8083-32

Three traps candidates fall into

Candidates believe the ignition switch opens the magneto circuit. It grounds the primary instead, so a broken, chafed, or disconnected P-lead leaves the magneto hot with the switch OFF. That is why the propeller of a piston aircraft is treated as live whenever the mag check or P-lead continuity has not been verified.
The condenser is described as storing charge for the spark plug. It sits in the primary circuit across the breaker points, where its job is to prevent arcing at the points and force the primary field to collapse quickly. The high voltage that reaches the plug is induced in the secondary winding, not stored.
A magneto check with no rpm drop gets called a good result. Zero drop usually means that magneto is not being grounded when it should be, so the switch or P-lead is faulty, and the aircraft is unairworthy for the same reason an excessive drop is. Both extremes are findings.

7 free sample questions from ACS AM.III.F

AM.III.FTap an answer
In a high-tension magneto ignition system, the primary winding circuit current is interrupted by the action of the:
AM.III.FTap an answer
What is the function of the capacitor (condenser) connected across the breaker points in a magneto?
AM.III.FTap an answer
When an aircraft magneto switch is placed in the OFF position, the primary circuit is:
AM.III.FTap an answer
The purpose of an impulse coupling on a magneto is to:
AM.III.FTap an answer
During a magneto ignition system check, an excessive rpm drop when operating on a single magneto most likely indicates:
AM.III.FTap an answer
In a turbine engine, the high voltage required to fire the igniter plugs is produced by the:
AM.III.FTap an answer
Which device limits and controls the output of an aircraft engine-driven alternator to protect the electrical system?

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.

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Frequently asked questions

Why is a propeller dangerous even with the ignition switch OFF?: Because the switch stops the magneto by grounding its primary circuit through the P-lead rather than by cutting power to it. A magneto makes its own current whenever it turns, so if the P-lead is broken, chafed, or disconnected, the switch cannot ground it and the magneto still fires. Treat every propeller as live and confirm the grounding circuit before turning one by hand.
What is the function of the capacitor in a magneto?: It is connected across the breaker points in the primary circuit. When the points open, the capacitor absorbs the surge of current that would otherwise jump the gap as an arc, which prevents burning and pitting of the point faces. Suppressing that arc also makes the primary magnetic field collapse much faster, and a faster collapse induces a higher voltage in the secondary winding.
What does an impulse coupling do?: It solves the low-speed starting problem. During cranking, a spring in the coupling winds up while the magnet is held, then releases it so the rotating magnet snaps through its neutral position much faster than the engine is turning. That produces a spark hot enough to fire the mixture, and the delay also retards the spark timing so the engine does not kick back.
What does an excessive rpm drop on one magneto indicate?: A fault somewhere in that magneto's ignition circuit, since the engine is running on that side alone during the check. Common causes are fouled or worn spark plugs, a defective ignition lead or distributor, incorrect internal timing, or a magneto that is out of time to the engine. Troubleshooting works from the cheapest and most likely cause outward before removing the magneto.
How is turbine engine ignition different from a magneto system?: A turbine only needs ignition for starting and for relight, not continuously, so it uses an electrically powered ignition exciter that steps input power up to the high voltage the igniter plugs need. Igniter plugs have a much wider gap and produce a far more energetic spark than a reciprocating engine spark plug, and they are replaced on their own schedule.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

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