Definition
A spring-loaded device installed between the engine accessory drive and a magneto that briefly delays magneto rotation during engine start, then releases to spin the magneto rapidly. This produces a hot spark and retards ignition timing, allowing the engine to start reliably at low cranking speeds. Once the engine is running above a set RPM, flyweights inside the coupling disengage and the magneto turns at normal engine timing.
Plain English
A small spring-loaded clutch on the magneto that helps the engine start. It holds back, then snaps the magneto around quickly to make a strong spark while the starter is still slowly turning the engine over. After the engine catches and speeds up, it releases and the magneto runs normally.
Context Anchor
Seen in aircraft engine ignition and magneto maintenance discussions, especially when diagnosing hard starting or engine kickback during start.
Derivation
Impulse means a sudden push or burst, from the Latin impulsus, meaning 'driven against.' Coupling refers to a device that connects two rotating parts. Together the term describes a coupling that delivers a sudden burst of rotation — exactly what it does to the magneto at start.
Why Pilots Care
A functioning impulse coupling enables reliable starts on piston engines by providing the strong, retarded spark needed when the engine turns slowly.
Analogy
Think of winding back a toy car and letting it go. The starter slowly winds the spring; then the coupling releases and snaps the magneto forward fast enough to make a strong spark — much faster than the engine itself is turning.
Intuition Check
Impulse coupling does not mean a simple connector that moves whenever it feels a bump. In this context, it is a specific spring-and-release starting device in the magneto drive.
Example Sentence 1
When the mechanic pulled the propeller through by hand, he heard a clear snap from the impulse coupling, confirming it was working correctly.
Example Sentence 2
A faulty impulse coupling often results in hard starting and requires replacement to restore proper ignition timing.