Ignition and Starting Systems — FAA A&P Test Questions (ACS AM.III.H)
Ignition and Starting Systems is the Powerplant subject area that asks how a piston engine makes a spark without any help from the aircraft electrical system, and how either engine type gets turning in the first place. Expect magneto theory — breaker points, the capacitor, E-gap, coming-in speed — plus P-lead grounding and runup magneto checks, then the high-energy capacitor-discharge ignition and pneumatic or starter-generator starting used on turbines. These questions reward understanding the sequence of events inside the magneto rather than memorizing a definition of each part.
Written by the AMTprep editorial team · Published · Last reviewed
Written against the FAA primary sources cited at the foot of this page.
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 1,353 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
- 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.
- 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.
- 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.
50 free sample questions from ACS AM.III.H
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In a reciprocating engine magneto ignition system, what is the primary function of the breaker points?
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Answer: To interrupt current flow in the primary coil and induce high voltage
Why: The breaker points are in series with the primary winding. When they open, they interrupt the primary current; the collapsing magnetic field induces a high voltage in the secondary winding that fires the spark plug. Distribution is the distributor's job, and a magneto produces AC by rotation rather than rectifying it.· FAA-H-8083-32 — Ignition Systems (magneto operating principles)
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What is the purpose of the capacitor (condenser) connected across the breaker points in a magneto?
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Answer: To absorb the primary current surge and reduce arcing at the points
Why: The capacitor is wired in parallel with the breaker points. As the points open it momentarily accepts the primary current, speeding the collapse of the magnetic field and minimizing arcing that would otherwise burn the contact surfaces. Voltage step-up is done by the secondary winding, not the capacitor.· FAA-H-8083-32 — Ignition Systems (magneto components)
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Why is the rotating magnet in a high-tension magneto positioned for maximum voltage when the points open?
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Answer: Because the rate of magnetic flux change in the coil is then greatest
Why: Maximum voltage is induced when the rate of change of magnetic flux is greatest, which occurs just past the magnet's neutral position. The points are timed to open at this point (the E-gap), so the primary collapse produces the strongest secondary voltage. Distributor and capacitor positions do not set the induced voltage peak.· FAA-H-8083-32 — Ignition Systems (E-gap and magneto timing)
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What does the term 'coming-in speed' of a magneto refer to?
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Answer: The lowest rpm at which the magneto produces a usable ignition spark
Why: The coming-in speed is the slowest rotational speed at which the magneto generates enough voltage to fire the spark plugs. Below this speed the magneto alone cannot reliably start the engine, which is one reason auxiliary starting aids such as impulse couplings or induction vibrators are used.· FAA-H-8083-32 — Ignition Systems (magneto operation)
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On a typical aircraft reciprocating engine, what is the result of the ignition switch being in the OFF position?
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Answer: The magneto primary circuit is grounded through the P-lead
Why: Turning the ignition switch OFF grounds the magneto primary circuit through the P-lead, preventing the points from interrupting current and stopping spark production. Because the magneto is self-contained, a broken or disconnected P-lead leaves the magneto 'hot,' which is why a propeller must always be treated as live.· FAA-H-8083-32 — Ignition Systems (ignition switch and P-lead)
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During a magneto check at runup, what does an excessive rpm drop on one magneto most commonly indicate?
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Answer: A faulty ignition system condition such as fouled plugs or bad leads
Why: When one magneto is selected, only one spark plug per cylinder fires, so a small rpm drop is normal. An excessive drop points to a problem in that ignition system—fouled or dead spark plugs, defective harness leads, or magneto faults. Governor and mixture issues affect both magneto positions, not one.· FAA-H-8083-32 — Ignition Systems (ignition system maintenance and check)
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If a reciprocating engine continues to run after the ignition switch is turned OFF, the most likely cause is
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Answer: an open or broken P-lead ground connection at the magneto
Why: Turning the switch OFF grounds the primary through the P-lead to stop the magneto. If that ground path is open or broken, the magneto keeps firing and the engine continues to run. A shorted capacitor or open primary winding would instead prevent the magneto from producing spark at all.· FAA-H-8083-32 — Ignition Systems (ignition switch and P-lead faults)
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In a turbine engine ignition system, what type of igniter discharge is most commonly used to start the engine?
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Answer: A high-energy capacitor-discharge spark at the igniter plug
Why: Turbine engines typically use a high-energy capacitor-discharge ignition system that stores energy and releases it as an intense spark at the igniter plug to ignite the fuel-air mixture during start. Turbine ignition is not timed to a specific shaft position the way a reciprocating magneto is, and igniters are used only for starting, not continuous glow.· FAA-H-8083-32 — Turbine Ignition Systems (capacitor-discharge)
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What is the main reason a starter-generator is used on many small turbine engines instead of a separate starter and generator?
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Answer: It saves weight by performing both starting and generating functions
Why: A starter-generator is a single unit that acts as a motor to spin the engine for starting and then, once running, functions as a generator to supply electrical power. Combining both roles in one machine saves weight and space. It does not replace the ignition exciter and still requires a power source to crank.· FAA-H-8083-32 — Engine Starting Systems (turbine starter-generator)
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In a capacitor-discharge turbine ignition exciter, the bleed (discharge) resistors are provided to
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Answer: allow the capacitors to discharge when the unit is switched off.
Why: A capacitor-discharge ignition exciter stores a lethal charge on its capacitors. The bleed resistors provide a safe path that drains the stored charge after the unit is switched off, protecting maintenance technicians (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In a turbine ignition exciter, the function of the choke (inductor) is to
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Answer: prolong the duration of the ignition spark.
Why: The choke opposes a rapid change of current, so when the capacitor dumps its charge the choke stretches the discharge out in time. Prolonging the discharge produces a longer, hotter spark that more reliably ignites the fuel-air mixture (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In an electric turbine starting system that uses a series resistor to limit inrush current, the resistor is short-circuited by the
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Answer: centrifugal speed switch.
Why: A series resistor limits the initial inrush current to give a controlled slow start. Once the starter is turning fast enough, a speed-sensing centrifugal switch closes to short out the resistor and apply full power for the remainder of the start (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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Compared with other turbine starters, a principal advantage of an air (pneumatic) starter is that it
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Answer: is light, simple, and economical to operate.
Why: A pneumatic starter delivers high torque from a small, light turbine driven by bleed or ground air, with few parts and no heavy batteries or generators. Its chief advantage is that it is light, simple, and economical to install and operate (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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An advantage of a gas-turbine (jet-fuel) starter is that it
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Answer: provides high cranking power for low installed weight.
Why: A gas-turbine starter is itself a small turbine engine, so it produces a large amount of cranking power for very little weight, which is its principal advantage. It still requires its own fuel supply and connections, so those are not benefits (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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If a turbine engine fails to light off during a start attempt, the start cycle is automatically terminated by the
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Answer: start-cycle time switch.
Why: Allowing motoring to continue indefinitely would overheat the starter and pool unburned fuel. A start-cycle time switch limits motoring to a preset duration and de-energizes the starter, even if light-off never occurs (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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During the start of a turbine engine, the high-pressure fuel shutoff valve should initially be
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Answer: closed until the correct motoring speed is reached.
Why: The HP fuel shutoff valve is kept closed at the start so the engine is first motored to establish airflow and purge the combustion chamber. Fuel is introduced by opening the valve at the correct light-off speed, preventing a fuel-rich hot or wet start (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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When a turbine engine lights off during start, the exhaust gas temperature typically will
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Answer: rise rapidly, then fall as the engine accelerates toward idle.
Why: At light-off, fuel burns in a relatively low airflow, so the gas temperature spikes rapidly. As combustion and the starter accelerate the engine, increasing airflow cools the gases, so the temperature peaks and then falls as the engine settles toward idle (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In turbine engine starting, the term self-sustaining (self-accelerating) speed means that the
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Answer: engine will continue to run independently of the starter.
Why: Self-sustaining speed is the RPM at which the compressor supplies enough air for combustion to keep the engine accelerating on its own, allowing the starter to be cut out. It does not mean the engine can reach full power instantly or taxi (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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A dry-motoring cycle on a turbine engine is performed in order to
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Answer: clear residual fuel from the engine after a wet start.
Why: A dry-motoring cycle turns the engine on the starter with fuel and ignition off, blowing air through the gas path. It is used to clear residual fuel and vapor after a wet (failed) start, removing the fire hazard before another attempt (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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What type of turbine blading is most commonly used in a pneumatic (air) starter motor?
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Answer: Impulse blading.
Why: An air starter must produce high torque from a simple, robust turbine fed by a high-velocity air jet. Impulse blading, where the gas does its work by changing direction across symmetrical buckets at roughly constant pressure, suits this jet-driven application and is the type most commonly used (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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Turbine engine igniter plugs are properly cleaned by
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Answer: blowing with compressed air and brushing lightly with a soft brush.
Why: Igniter firing surfaces are easily damaged, so cleaning is limited to blowing off deposits with compressed air and a light brush with a soft brush. Abrasives such as sandblasting or steel wool would erode the electrodes and embed conductive particles, shortening plug life (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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A high-energy capacitor-discharge turbine ignition system produces its spark by
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Answer: suddenly discharging a charged capacitor across the igniter.
Why: A capacitor-discharge ignition system stores electrical energy in a capacitor and then suddenly discharges it across the igniter plug to produce a high-energy spark. It is a capacitor-discharge device, not a magneto-type contact-breaker or AC-bus system (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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During normal operation of a gas turbine engine, the ignition system is used
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Answer: only during engine start and for in-flight relight.
Why: Gas-turbine combustion is continuous and self-sustaining once the engine is running, so ignition is needed only to light the fuel during start and to re-establish the flame after a flameout (relight). It is not operated continuously in normal flight (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In a high-energy turbine ignition system, the energy for the spark is supplied directly by
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Answer: a charged storage capacitor.
Why: A high-energy ignition exciter stores charge in a capacitor and releases it in a single rapid discharge to produce the spark. The spark energy therefore comes from the capacitor, not from a contact breaker or directly from the AC bus (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In turbine engine operation, self-sustaining speed is best defined as the
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Answer: RPM at which the engine continues to run without external assistance.
Why: Self-sustaining speed is the RPM at which the compressor supplies enough air for combustion to keep the engine running and accelerating on its own, allowing the starter to be disengaged. It is unrelated to airspeeds such as V1 or takeoff velocity (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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During normal running of a gas turbine engine, combustion is
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Answer: self-supporting and requires no continuous ignition.
Why: Once a gas turbine is running, the flame in the combustion chamber is continuous and re-lights the incoming fuel-air mixture itself, so combustion is self-supporting. Ignition can then be switched off and is reapplied only for start or relight (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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High-energy ignition is required for gas turbine engines primarily because of the
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Answer: relatively high flash point of the turbine fuel.
Why: Aviation turbine fuel (kerosene/Jet A) has a relatively high flash point, so it does not vaporize and ignite easily, especially when cold and finely atomized. A high-energy spark is needed to reliably ignite this fuel (FAA-H-8083-32, Ignition and Starting Systems).· FAA-H-8083-32 — Ignition and Starting Systems
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In a turbine engine high-energy ignition exciter unit, the bleeder (discharge) resistors are provided to
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Answer: allow the storage capacitors to discharge safely when the unit is switched off.
Why: The bleeder resistors in a high-energy capacitor-discharge ignition exciter give the dangerous stored capacitor charge a safe path to dissipate after the unit is switched off, protecting the technician. They are a safety feature, not a means of storing relight energy or limiting voltage.· FAA-H-8083-32 — Ignition and Starting Systems
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The approximate rate of spark discharge of a turbine engine high-energy ignition exciter is
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Answer: 60 to 100 sparks per minute.
Why: A high-energy capacitor-discharge ignition exciter charges its capacitor and then discharges it across the igniter at a deliberate rate of roughly 60 to 100 sparks per minute. This is fast enough for reliable light-up yet allows the unit to recharge fully between discharges.· FAA-H-8083-32 — Ignition and Starting Systems
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Why do turbine engine ignition systems require high energy?
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Answer: To ignite the fuel under conditions of high altitude and low temperature.
Why: High-energy ignition is needed so the engine can be started or relit at high altitude where the air is thin and very cold and the fuel is difficult to vaporize. The strong spark overcomes these low-temperature, low-density conditions; the gas path at altitude is cold, not hot.· FAA-H-8083-32 — Ignition and Starting Systems
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The type of ignition system used on most turbine aircraft engines is the
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Answer: capacitor-discharge type.
Why: Most turbine engines use a capacitor-discharge (high-energy) ignition system, which stores energy in a capacitor and releases it as a powerful spark. This gives the intense, reliable spark needed to light kerosene fuel, unlike low-tension or high-resistance arrangements.· FAA-H-8083-32 — Ignition and Starting Systems
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A safety feature commonly built into pneumatic starters for use if the clutch does not release from the engine drive at the proper time during the start is the
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Answer: shear point built into the starter drive shaft.
Why: If the starter clutch fails to release after light-off, the accelerating engine drives the starter turbine toward burst speed. Shutting off the starter air cannot prevent this, because the engine rather than the air is now turning the starter. Pneumatic starters therefore have a designed shear point in the drive shaft that fails and mechanically disconnects the starter from the engine. The flyweight cutout switch only signals the air valve to close at cutout speed.· FAA-H-8083-32 — Ignition and Starting Systems
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When a pneumatic starter is supplied from a ground air unit, starter overspeed during engine start is normally prevented by
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Answer: activation of a flyweight cutout switch at a preset speed.
Why: When a pneumatic starter is fed from a ground unit, a speed-sensing flyweight cutout switch trips at a preset RPM to shut off the air supply and prevent overspeed. Because it reacts to actual starter speed, it works regardless of timing errors in the ground source.· FAA-H-8083-32 — Ignition and Starting Systems
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Air turbine starters are generally designed so that reduction-gear distress or damage may be detected by
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Answer: inspection of a magnetic chip detector in the oil system.
Why: Air turbine starters use an oil-lubricated reduction gear, so distress produces metallic particles in the oil. A magnetic chip detector collects these particles for inspection, giving early warning of gear damage before failure, which audible noise or a shear section could not reliably provide.· FAA-H-8083-32 — Ignition and Starting Systems
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Routine inspection of a pneumatic starter by a maintenance technician usually includes checking the
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Answer: oil level and condition of the magnetic drain plug.
Why: Routine inspection of a pneumatic starter concentrates on its self-contained lubrication: checking the oil level and examining the magnetic drain plug for metal particles that indicate internal gear or bearing wear. Rotor alignment and turbine-blade FOD checks are not normal line tasks for a sealed starter.· FAA-H-8083-32 — Ignition and Starting Systems
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Pneumatic starters are usually designed with which type of airflow-impingement turbine?
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Answer: Radial inward-flow turbine or axial-flow turbine.
Why: Pneumatic starters are driven by a turbine on which the supply air impinges, using either a radial inward-flow turbine or an axial-flow turbine. The other options describe compressors, which is incorrect because the starter element is driven by the air rather than compressing it.· FAA-H-8083-32 — Ignition and Starting Systems
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The primary advantage of pneumatic (air turbine) starters over comparable electric starters for turbine engines is their
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Answer: high power-to-weight ratio for the cranking torque produced.
Why: An air turbine starter spins at high speed and delivers large cranking torque from a small, light unit, so its chief advantage over an electric starter is its high power-to-weight ratio. It still requires reduction gearing, so that is not a benefit.· FAA-H-8083-32 — Ignition and Starting Systems
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The purpose of the undercurrent relay in a starter-generator system is to
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Answer: disconnect power from the starter-generator and ignition when sufficient engine speed is reached.
Why: In a starter-generator system the starting current falls as the engine accelerates. The undercurrent relay senses this drop and, when current has fallen enough to show the engine has reached self-sustaining speed, it disconnects power from the starter circuit and ignition to end the start.· FAA-H-8083-32 — Ignition and Starting Systems
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How does the ignition system of a gas turbine engine differ from that of a reciprocating engine?
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Answer: On a gas turbine, a high-energy spark is required for initial ignition only.
Why: Unlike a reciprocating engine, whose magnetos must spark every cylinder firing stroke with precise timing while running, a gas turbine has continuous self-sustaining combustion. Its high-energy ignition is therefore needed only to light the engine for starting or relight, not continuously.· FAA-H-8083-32 — Ignition and Starting Systems
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In a gas turbine engine DC capacitor-discharge ignition system, where are the high-voltage pulses formed?
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Answer: At the triggering transformer.
Why: In a DC capacitor-discharge ignition system, the stored capacitor energy is released through a triggering (high-tension) transformer, which steps it up to the high voltage that jumps the igniter gap. The high-voltage pulses are therefore formed at that transformer, not at the rectifier or breaker.· FAA-H-8083-32 — Ignition and Starting Systems
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Igniter plugs used in turbine engines are subjected to high-intensity spark discharges yet have a long service life because they
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Answer: do not require continuous operation during engine running.
Why: A turbine igniter fires only during starting and relight, not continuously throughout running like a reciprocating engine spark plug. Because it is used so briefly, the intense sparks cause little cumulative erosion, giving the plug a long service life despite the high energy involved.· FAA-H-8083-32 — Ignition and Starting Systems
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Great caution should be exercised when handling damaged hermetically sealed turbine engine igniter exciter (transformer) units because
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Answer: some contain a small amount of radioactive material.
Why: Some hermetically sealed igniter exciter units contain a small amount of radioactive material used to aid ionization. If the unit is damaged and its seal is breached, the material could be released, so damaged units must be handled with great caution and disposed of correctly.· FAA-H-8083-32 — Ignition and Starting Systems
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When removing a turbine engine igniter plug, to eliminate the possibility of a lethal shock the ignition switch is turned off and the technician then
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Answer: disconnects the igniter lead from the plug and grounds the center electrode to the engine after disconnecting the exciter input lead and waiting the prescribed time.
Why: Because the ignition exciter stores a lethal charge, the safe procedure is to switch off, disconnect the exciter input lead, wait the prescribed time for the stored charge to bleed off, then disconnect the igniter lead and ground the plug center electrode to the engine before handling.· FAA-H-8083-32 — Ignition and Starting Systems
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What is the first engine instrument indication of a successful start of a turbine engine?
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Answer: A rise in the exhaust gas temperature (EGT).
Why: The first positive sign that light-up has occurred is a rise in exhaust gas temperature, confirming fuel is burning in the combustion chamber. Fuel flow is selected on by the operator and oil pressure builds gradually, so EGT is the instrument that first signals a successful start.· FAA-H-8083-32 — Ignition and Starting Systems
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In a capacitor-discharge ignition system, the igniter plugs receive their electrical supply from the
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Answer: discharge capacitor in the exciter unit.
Why: In a capacitor-discharge ignition system the energy is stored in a capacitor, then released as a high-energy pulse across the igniter plug to create the spark. The plug therefore receives its supply from the discharge capacitor, not from the starter circuit or a simple inductor.· FAA-H-8083-32 — Ignition and Starting Systems
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The output of a turbine engine high-energy ignition exciter unit is rated in
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Answer: joules of spark energy.
Why: Turbine ignition systems are rated by the spark energy delivered to the igniter, which is expressed in joules; high-energy units typically deliver several joules per spark. The watt is a unit of power and the ampere is a unit of current, so neither expresses the stored spark energy.· FAA-H-8083-32 — Ignition and Starting Systems
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A glow plug ignites the fuel/air mixture
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Answer: by heat from a glowing element.
Why: A glow plug ignites the fuel/air mixture by means of a heated incandescent element rather than by producing an electrical spark. Although electricity heats the coil, ignition itself is achieved by the heat radiating from the glowing element.· FAA-H-8083-32 — Ignition and Starting Systems
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The series resistor in a DC starter motor circuit is used to
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Answer: limit the inrush current while the motor is at low rpm.
Why: At the instant of starting the motor turns slowly and generates little back-EMF, so the current drawn would be excessive. The series resistor limits this initial inrush current at low rpm; as speed builds and back-EMF rises, the resistor is progressively cut out. It is not an overspeed device or a backup.· FAA-H-8083-32 — Ignition and Starting Systems
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On a turbine engine DC starting circuit, if the overspeed relay contacts become open-circuited, the result is that
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Answer: no power is supplied to the starter motor at all.
Why: The overspeed relay contacts are in series with the starter motor supply. If those contacts go open-circuit, the circuit is broken so that no power reaches the starter motor and it will not turn at all.· FAA-H-8083-32 — Ignition and Starting Systems
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The field winding of a DC starter motor used on a turbine engine is wound as
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Answer: a series or compound field winding.
Why: Starter motors must deliver very high torque at low speed to crank the engine, which is the characteristic of a series field. Series or compound field windings provide this high starting torque, whereas a pure shunt motor would not develop the necessary cranking effort.· FAA-H-8083-32 — Ignition and Starting Systems
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The areas that share the most ground with AM.III.H — study them alongside it.
- Reciprocating Engines (AM.III.A)
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- Fundamentals of Electricity and Electronics (AM.I.A)
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
General
- Fundamentals of Electricity and Electronics
- Aircraft Drawings
- Weight and Balance
- Fluid Lines and Fittings
- Aircraft Materials, Hardware, and Processes
- Ground Operations and Servicing
- Cleaning and Corrosion Control
- Mathematics
- Regulations, Maintenance Forms, Records, and Publications
- Physics for Aviation
- Inspection Concepts and Techniques
- Human Factors
Airframe
- Metallic Structures
- Non-Metallic Structures
- Flight Controls
- Airframe Inspection
- Landing Gear Systems
- Hydraulic and Pneumatic Systems
- Environmental Systems
- Aircraft Instrument Systems
- Communications, Light Signals, and Runway Lighting Systems
- Aircraft Fuel Systems
- Aircraft Electrical Systems
- Ice and Rain Control Systems
- Airframe Fire Protection Systems
- Rotorcraft Fundamentals
- Water and Waste Systems
Powerplant
- Reciprocating Engines
- Turbine Engines
- Engine Inspection
- Engine Instrument Systems
- Engine Fire Protection Systems
- Engine Electrical Systems
- Engine Lubrication Systems
- Ignition and Starting Systems
- Engine Fuel and Fuel Metering Systems
- Reciprocating Engine Induction and Cooling Systems
- Turbine Engine Air Systems
- Engine Exhaust and Reverser Systems
- Propellers
Primary sources
Drill ACS AM.III.H inside the full Powerplant bank
Adaptive engine, timed exam simulator, ACS-code remediation, and oral prep — everything you need to pass.