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Spark timing relative to top dead center

Aviation Mechanic Powerplant (AMP) practice question 7 of 11 on Ignition and Starting Systems. ACS element AM.III.H.K1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a reciprocating engine ignition system, when does the spark occur in the cylinder relative to the position of the piston?

  1. AA set number of degrees before top dead center
  2. BExactly at top dead center of the compression stroke
  3. CA set number of degrees after top dead center
Show the answer and explanation

Correct answer: A. A set number of degrees before top dead center

  • A — correct The spark occurs at a predetermined number of degrees ahead of the top dead center position of the piston, measured by crankshaft travel in degrees of rotation.
  • B — incorrect The handbook places the spark a predetermined number of degrees ahead of the top dead center position, not at that position.
  • C — incorrect Ignition after top dead center is late ignition, not the normal design condition; the spark is delivered ahead of top dead center.

BACKGROUND

All reciprocating engine ignition systems share the same basic requirement: deliver a high-tension spark across the electrodes of every spark plug, in the correct firing order, at a predetermined number of degrees ahead of the piston's top dead center position. That lead angle is measured as crankshaft travel in degrees of rotation. The output voltage of the system must also be high enough to arc the spark plug gap under all operating conditions. Ignition systems fall into two classes: magneto-ignition systems and electronic Full Authority Digital Engine Control (FADEC) systems.

STUDY TIP

Fix the phrase "a predetermined number of degrees ahead of top dead center" in memory, and remember the lead is measured in degrees of crankshaft rotation, not piston travel.

KEY POINTS

  • The spark is delivered a set number of degrees before top dead center.
  • The lead angle is measured in degrees of crankshaft rotation.
  • System voltage must arc the plug gap under all operating conditions.

SOURCES

  • sources/amp-powerplant/ch04.txt "Reciprocating Engine Ignition Systems"
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MORE ON IGNITION AND STARTING SYSTEMS

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.