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Fast rpm drop on the magneto check

Aviation Mechanic Powerplant (AMP) practice question 13 of 13 on Reciprocating Engines. ACS element AM.III.A.K9 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn an ignition system operational check, a fast rpm drop when the switch is moved to a single magneto is usually the result of what?

  1. AIncorrect ignition timing or valve adjustment
  2. BFaulty grounding of one ignition side
  3. CFaulty spark plugs or ignition harness
Show the answer and explanation

Correct answer: C. Faulty spark plugs or ignition harness

  • A — incorrect Those conditions produce a slow rpm drop, because the power loss develops less rapidly than the loss from a dead plug or lead.
  • B — incorrect Faulty grounding of one side of the ignition system shows up as a lack of rpm dropoff, not as a fast drop.
  • C — correct Faulty plugs or leads take effect at once, so the cylinder goes dead or fires intermittently the instant the switch is moved, producing a fast drop.

BACKGROUND

In the ignition system operational check, switching to an individual magneto cuts out the opposite plugs, which slows the rate of combustion and gives the same effect as retarding the spark advance; the drop in engine speed measures that power loss. When recording results, the technician notes how much of the total rpm drop occurs rapidly and how much occurs slowly, because that breakdown pinpoints the part of the ignition system at fault. A fast drop is usually caused by faulty spark plugs or a faulty ignition harness, since those defects act immediately. A slow drop usually comes from incorrect ignition timing or faulty valve adjustment, and lack of any dropoff may indicate faulty grounding of one side of the ignition system.

STUDY TIP

Sort magneto check symptoms into three buckets: fast drop, slow drop, and no drop, and attach one cause to each.

KEY POINTS

  • A fast rpm drop points to faulty spark plugs or ignition harness.
  • A slow rpm drop points to ignition timing or valve adjustment.
  • No rpm drop at all points to faulty grounding of one ignition side.

SOURCES

  • FAA-H-8083-32B ch10 "Ground Check"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.