FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A student mechanic asks why most certificated reciprocating aircraft engines are equipped with two spark plugs per cylinder, each fired by a separate magneto. What is the primary reason for this design?
- AIt increases the engine's compression ratio and rated power
- BIt allows the engine to run normally if either magneto fails completely
- CIt provides ignition redundancy and produces more complete, faster combustion of the fuel/air charge
- DIt prevents lead fouling from ever occurring on either spark plug
Show answer & explanationAnswer & explanation
Correct answer: C. It provides ignition redundancy and produces more complete, faster combustion of the fuel/air charge
Dual ignition (two magnetos, two spark plugs per cylinder) is used for safety redundancy and to ignite the mixture from two points simultaneously, producing more complete and efficient combustion and slightly higher power. It does not affect compression ratio, does not prevent fouling, and losing one magneto reduces power/smoothness rather than having no effect.
Why the other options are wrong
- A. Compression ratio is a mechanical design feature unrelated to ignition redundancy.
- B. Losing one magneto causes a noticeable RPM drop and rougher running, not normal operation.
- D. Fouling can still occur on either or both plugs regardless of dual ignition.
Dual Ignition System
A system using two independent magnetos and two spark plugs per cylinder to improve reliability and combustion efficiency.
- Provides redundancy if one magneto fails
- Two flame fronts burn the mixture faster and more completely
- Improves power output slightly over single ignition
- Required on most certificated reciprocating engines
Memory trick: Two sparks, two flames, faster burn, safer flight