FAA Powerplant Written Practice Test

When must magneto breaker points be timed to open?

1. When the engine is at idle

2. At maximum RPM

3. When the greatest magnetic field stress exists

4. When the ignition key is turned

The correct answer is: When the greatest magnetic field stress exists

The timing of magneto breaker points is crucial for ensuring that the ignition system functions properly, and it must occur when the greatest magnetic field stress exists. This moment typically coincides with the point at which the engine is at or near the top dead center (TDC) of the compression stroke for the cylinder being fired. At this point, the magnetic field in the magneto is at its maximum strength due to the rotor's proximity to the coil, which allows for the most efficient transfer of energy to generate a high voltage spark at the spark plugs. If the breaker points open too early or too late, it can result in a weak spark, incomplete combustion, and ultimately poor engine performance. Proper timing at maximum magnetic field stress ensures that the ignition occurs at the optimal moment for combustion, leading to effective engine operation. The other options present situations that do not align with this requirement. Timing at idle or maximum RPM may not correlate with the peak magnetic field condition. Similarly, the act of turning the ignition key does not dictate the optimal timing for the magneto to open breaker points in operational terms.