FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A technician compares two magneto designs: one with a rotating magnet and stationary primary/secondary coils, and another with a stationary magnet and rotating armature (coil). What is the primary advantage of the rotating-magnet design used in most modern aircraft magnetos?
- AIt allows the magneto to operate without an impulse coupling at any starting RPM
- BIt produces a significantly higher output voltage at all engine speeds
- CIt requires no condenser because arcing at the breaker points is inherently eliminated
- DIt eliminates the need to carry primary current through slip rings and brushes to a rotating coil
Show answer & explanationAnswer & explanation
Correct answer: D. It eliminates the need to carry primary current through slip rings and brushes to a rotating coil
In a rotating-armature magneto, the primary current must be carried to the rotating coil through slip rings and brushes, which wear and reduce reliability. The rotating-magnet design keeps the coils stationary and rotates only the magnet, avoiding the need for current-carrying slip rings/brushes, improving reliability and reducing maintenance.
Why the other options are wrong
- A. Impulse couplings are used for slow-speed cranking spark boost regardless of magneto type.
- B. Output voltage depends on coil design/turns, not simply on which element rotates.
- C. A condenser is still required across the breaker points in either design.
Rotating-Magnet Magneto
A magneto design where a permanent magnet rotates within stationary primary and secondary windings, avoiding current-carrying slip rings.
- Most modern aircraft magnetos use the rotating-magnet design.
- Rotating-armature designs require slip rings/brushes to carry primary current, increasing wear.
- Rotating-magnet design improves reliability, especially at high altitude and temperature.
Memory trick: Spin the magnet, keep the coil still — no brushes to wear or spill.