Definition 1 of 2
Definition
The rotational speed of a gas turbine engine, reached during the start sequence, at which the engine can continue to accelerate to idle on its own without further assistance from the starter. Once this speed is reached, the starter is no longer needed to drive the rotor, because the energy produced by combustion is sufficient to keep the engine spooling up.
Plain English
The point during engine start where the engine takes over from the starter and continues spinning up by itself.
Context Anchor
Seen in turbine engine starting procedures, especially when deciding whether the start is progressing normally and when starter assistance is no longer needed.
Derivation
‘Self-accelerating’ literally means ‘speeding itself up.’ The name describes exactly what happens: at this RPM the engine no longer needs outside help to keep gaining speed.
Why Pilots Care
Releasing the starter before this speed risks a hung start; waiting until it is reached ensures reliable acceleration to idle.
Analogy
It is like starting a lawn mower: at first you pull the cord to turn it, but once it catches, the engine keeps running and speeds up on its own.
Intuition Check
Self-accelerating does not mean the engine is running away or overspeeding. It means the engine has enough energy from combustion to continue a normal start without the starter doing the main work.
Example Sentence 1
Once the engine passed self-accelerating speed, the starter dropped out and N2 continued climbing toward idle.
Example Sentence 2
In cold conditions the engine took longer to reach self-accelerating speed because of increased internal drag.