Definition
An aircraft engine starter that uses both an electric motor and a hand crank to spin a heavy internal flywheel up to high speed. Once the flywheel reaches sufficient speed, its stored rotational energy is mechanically engaged with the engine crankshaft to turn it over and start the engine.
Plain English
A starter that builds up speed in a heavy spinning wheel inside it -- using either an electric motor or a hand crank -- and then releases that stored spinning energy into the engine to get it running.
Context Anchor
Seen in descriptions, maintenance procedures, and starting procedures for some older reciprocating-engine aircraft.
Derivation
Combination refers to the two methods of spinning the starter -- electric and manual. Inertia refers to the tendency of a spinning mass to keep spinning; the flywheel stores energy as inertia, then transfers it to the engine when engaged.
Why Pilots Care
It starts large engines reliably while drawing far less current than a direct electric starter, protecting the aircraft battery.
Analogy
Think of a wind-up toy. You wind it up first to store energy, then release it to make the toy move. The starter winds up its flywheel first, then releases that stored spin into the engine.
Intuition Check
Combination does not mean the engine is started by two starters at once. Here it means the inertia starter can usually be spun up in either of two ways: electrically or by hand.
Example Sentence 1
The old radial engine used a combination inertia starter, so the ground crew could hand-crank it if the battery was dead.
Example Sentence 2
During the preflight, the mechanic checked that the combination inertia starter could also operate in direct-crank mode if needed.