Ignition and Starting Systems — FAA A&P Test Questions (ACS AM.III.H)

What ACS AM.III.H covers

The FAA builds this area around the magneto, because the magneto is the one component on a piston engine that generates its own current. FAA-H-8083-32 walks through the sequence you are tested on: the rotating magnet turns, flux builds in the primary coil, the breaker points open at the E-gap position a few degrees past neutral, the primary field collapses rapidly, and the secondary coil induces the high voltage that jumps the spark plug gap. The capacitor across the points belongs to that sequence — it absorbs the current that would otherwise arc across the opening points, so the collapse is fast and the points survive. Expect questions on coming-in speed, on impulse couplings and shower-of-sparks systems that retard the spark for starting, and on distributor and harness construction. The ignition switch questions are really P-lead questions: the switch does not remove power from a magneto, it grounds the primary circuit, which is why a broken P-lead leaves the magneto live and the propeller dangerous. Runup questions follow the same logic — an excessive drop on one magneto points at that magneto's plugs, leads or timing, while no drop at all suggests the magneto was never grounded. Turbine content sits alongside it: high-energy capacitor-discharge ignition units, igniter plugs with wide gaps that fire intermittently for start and relight, and starting sequences built around air-turbine starters or starter-generators. Maintenance items round the area out — internal timing versus timing to the engine, spark plug reach and heat range, lead-fouled plugs — and the records for that work fall under 14 CFR Part 43.

Where this sits on the test

ACS AM.III.H 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

1. Many candidates describe the capacitor as a device that stores voltage to strengthen the spark. It does not add voltage. It absorbs the current that would arc across the breaker points as they open, forcing the primary field to collapse quickly, and burned or pitted points usually mean that capacitor has failed.
2. The ignition switch is treated as a power switch. A magneto makes its own current, and the switch works by grounding the primary circuit through the P-lead. Candidates who reason from battery logic miss the classic question: an engine that keeps running with the switch OFF has an open P-lead.
3. Turbine ignition gets treated like magneto ignition. Turbine igniters do not fire continuously through the flight — the capacitor-discharge unit is intermittent duty, used for starting, relight and selected icing conditions, and its stored energy can be lethal, so it must bleed down before you touch the igniter lead.

7 free sample questions from ACS AM.III.H

1. AM.III.HTap an answer

   In a reciprocating engine magneto ignition system, what is the primary function of the breaker points?

   • ATo interrupt current flow in the primary coil and induce high voltage
   • BTo distribute high voltage to each spark plug in firing order
   • CTo rectify the alternating current produced by the rotating magnet
2. AM.III.HTap an answer

   What is the purpose of the capacitor (condenser) connected across the breaker points in a magneto?

   • ATo absorb the primary current surge and reduce arcing at the points
   • BTo step up the primary voltage before it reaches the spark plug
   • CTo store charge for the impulse coupling during engine start
3. AM.III.HTap an answer

   Why is the rotating magnet in a high-tension magneto positioned for maximum voltage when the points open?

   • ABecause the rate of magnetic flux change in the coil is then greatest
   • BBecause the capacitor has fully discharged through the primary coil
   • CBecause the distributor rotor is then aligned with a plug terminal
4. AM.III.HTap an answer

   What does the term 'coming-in speed' of a magneto refer to?

   • AThe rpm at which the impulse coupling flyweights disengage from the lug
   • BThe lowest rpm at which the magneto produces a usable ignition spark
   • CThe rpm at which the engine reaches its rated takeoff power output
5. AM.III.HTap an answer

   On a typical aircraft reciprocating engine, what is the result of the ignition switch being in the OFF position?

   • AThe battery is disconnected from the magneto primary coil
   • BThe magneto primary circuit is grounded through the P-lead
   • CThe magneto primary circuit is opened at the breaker points
6. AM.III.HTap an answer

   During a magneto check at runup, what does an excessive rpm drop on one magneto most commonly indicate?

   • AA propeller governor that is failing to maintain the selected rpm
   • BAn excessively rich fuel-air mixture supplied by the carburetor
   • CA faulty ignition system condition such as fouled plugs or bad leads
7. AM.III.HTap an answer

   If a reciprocating engine continues to run after the ignition switch is turned OFF, the most likely cause is

   • Aan open or broken P-lead ground connection at the magneto
   • Ba shorted capacitor across the magneto breaker points
   • Can open primary winding inside the magneto coil

Frequently asked questions

How does a magneto make a spark with the battery off?

A magneto is a self-contained generator. A permanent rotating magnet drives flux through the primary coil, and when the breaker points open, the primary field collapses and induces 20,000 volts or more in the secondary winding, which the distributor routes to the correct spark plug. Nothing in that chain needs ship's power, so the engine keeps running with the master switch off.

What does an rpm drop during a magneto check actually mean?

Some drop is normal, because one magneto firing one plug per cylinder burns the charge more slowly than both together. An excessive drop on a single magneto points at that circuit — fouled plugs, a failed lead, or timing that has drifted. No drop at all is not good news either; it usually means that magneto was never grounded when you selected the other one.

Why is a propeller treated as live even when the ignition switch is off?

Because the OFF position only grounds the magneto primary circuit through the P-lead. If that lead is broken or disconnected, or the switch itself is faulty, the magneto can still fire, and turning the propeller by hand can start the engine. Treat every propeller as hot, and verify P-lead continuity before you call an ignition system safe.

What is the difference between a spark plug and a turbine igniter plug?

A reciprocating spark plug fires continuously at a small, closely controlled gap, using voltage produced by the magneto. A turbine igniter has a much wider gap and is fed a high-energy capacitor discharge strong enough to jump that larger distance and light the fuel spray. It fires only during starting, relight, and selected icing or turbulence conditions.

What do E-gap and coming-in speed mean?

E-gap is the rotating magnet position, a few degrees past neutral, where the rate of flux change is greatest. The points are timed to open there so the induced secondary voltage is highest. Coming-in speed is the lowest magneto rotational speed at which that whole process still produces a spark strong enough to fire the plug.

All 40 ACS areas

Primary sources

• FAA Mechanic Airman Certification Standards (FAA-S-ACS-1)
• 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers