Definition
An aircraft engine starter in which the pilot or ground crew turns a hand crank to spin up a heavy internal flywheel to high speed. Once the flywheel is spinning fast enough, it is mechanically engaged to the engine crankshaft, and the stored rotational energy of the flywheel is used to turn the engine over and start it.
Plain English
A starter you wind up by hand. Turning the crank spins a small heavy wheel inside the starter faster and faster. When it is spinning fast enough, you connect it to the engine, and the spinning wheel's energy turns the engine over to start it.
Context Anchor
Seen on some older aircraft engines and in maintenance discussions about starting systems.
Derivation
Inertia comes from the Latin iners, meaning 'inactive' or 'sluggish.' In physics, inertia is the tendency of a spinning object to keep spinning. The starter uses that stored spinning energy of the flywheel to crank the engine, so the name describes exactly how it works.
Why Pilots Care
It provides a reliable way to start engines without electrical power or ground support, though it demands physical effort and correct engagement technique to avoid gear damage.
Analogy
Think of winding up a spinning top as fast as you can, then dropping it onto a gear. The top's stored spin briefly turns the gear with much more force than your hand alone could apply.
Intuition Check
Do not read “inertia starter” as a starter that works automatically because of inertia. The energy must first be put into the starter by cranking it, then that stored motion is used to turn the engine.
Example Sentence 1
Two ground crew took turns on the hand-cranked inertia starter until the flywheel was whining at full pitch, then the pilot pulled the engage handle and the radial engine coughed to life.
Example Sentence 2
After engaging the clutch on the hand-cranked inertia starter, the radial engine coughed once and settled into a steady idle.