Fly Lah!

Hye...
Can anyone in this forum explain to me what are the differences between idle and sub-idle engine condiotion?

You do not want a sub-idle engine condition. This is what is known as rotary or rotating stall.
This is particular to engines having variable area exhaust.
Sensors sensing such a condition will make the system increase the exhaust area and if required, retards fuel flow just above minimum flow to arrest the impending stall.
Fuel flow and nozzle area will be restored once the situation normalizes. Obviously this is done by a 'smart box' aka CPU/ECU/etc.This is an ANTI-STALL system and this is normally found on gas turbine engines with variable area exhaust. Therefore it is more a millitary aircraft phenomenon rather than civil/commercial aircraft situation.
We all know what IDLE is, right?

So, to answer your question:
Sub-idle is a type of stall,an abnormal temporary condition as opposed to idle, which is normal.It is not preferable and an 'anti-stall' device is used to prevent it.

torx wrote: .. .. So, to answer your question:
Sub-idle is a type of stall,an abnormal temporary condition as opposed to idle, which is normal.It is not preferable and an 'anti-stall' device is used to prevent it.

Sometime I prefer the plain English Dictionary as opposed to the Technical .....

Example:-

IDLE means
a. Not employed or busy: idle carpenters. See Synonyms at inactive.
b. Avoiding work or employment; lazy: shiftless, idle youth. See Synonyms at lazy.
c. Not in use or operation: idle hands.

Cheers.