Thanks all for your contributions so far.
I forgot to include in my initial post the fact that these engines are not FADEC controlled - they have the old fashioned mechanical cable running from the power levers.
Most posters seem to be aware of this, but I should also have mentioned that, when turning on the engine anti-ice, a lot of bleed air is taken from the compressor, which without governing action, would result in decreases to engine and fan speeds. But there is governing action, of course, and it works to keep things "on speed" by boosting the fuel flow slightly.
It is this governing action that is at the heart of my question. According to the Flightsafety Pilot Training Manual, the power lever angle (PLA) determines the N2 speed (consistent with every other twin spool engine I have experience with).
So, without touching the power levers, switching on the engine anti-ice should result in the N2 remaining exactly the same, should it not? The PLA does not change, so the N2 should not change.
But it does - and the N1 stays the same. How on earth?
Last edited by SPUDO; 9th Mar 2019 at 12:54.