FAA Powerplant Written Practice Test

What is the purpose of an impulse coupling in a magneto?

1. It reduces fuel consumption

2. It creates a high momentary speed of the magneto

3. It provides spark at lower RPMs

4. It acts as a safety feature

The correct answer is: It creates a high momentary speed of the magneto

The purpose of an impulse coupling in a magneto is to create a high momentary speed of the magneto. This mechanism operates by utilizing a spring-loaded weight that, when the engine is cranked, briefly disconnects the magneto from the engine. As the engine continues to turn, the spring compresses until it reaches a certain point, releasing and instantly engaging the magneto. This action generates rapid rotation of the magneto’s rotor momentarily, which leads to a substantial increase in the speed of the magneto. The enhanced speed results in a higher voltage being produced, allowing for a stronger spark at the spark plugs. This feature is especially important during engine start-up, as it helps ensure that sufficient ignition occurs even at lower RPMs where the engine might otherwise struggle to produce the necessary spark. The impulse coupling effectively enhances engine performance during these crucial moments.