Bear in mind while you're watching it that the engine doesn't simply use N2 as it's sole-source input for establishing idle speed. The idle power settings are established as part of the maintenance trim-in procedure, but the engine will set it's operating minimum based on multiple inputs...the speed isn't the only parameter it must consider.

Even on the ground, with the same engine on various given days, the idle speed may vary slightly, and for a given condition, the speed an engine must go to do what's required of it varies with wear and tear, compressor efficiency, engine load or demand, etc. Bleed demand loads, electrical and CSD loads, hydraulic pump loads and other accessory demands, etc, change the requirements on an engine to change the operating RPM, temperatures, etc, within a given set of environmental conditions...which also vary with airspeed (ram air input), affecting EPR, etc.

Regardless of the variances in the engine RPM within a given unit, or in comparison with several other engines on the wing, the bottom line is that the engine can be reduced in operating RPM only so far without compromising the job it must do (load), safety (flame-out), and integrity (flame propogation, compressor stall, etc). A slow turning turbine is not an efficient turbine.

Fair enough.. now that I think of it, if one of the engines has a high-stage bleed locked out or CSD disconnected, for example, it will indeed idle slower (and have a much lower EGT)

:ooh:

Your car is the same way, albeit a smaller, piston engine. Engage the air conditioning, and see how this affects your idle?