Thanks all, for some quality contributions!
Did you tell us what the N2 increase was. I would be surprised if it was more than 0.5%.
Thanks, Buster15. The increase was 0.3 - 0.4%.
How do you know the engine was not operating temperature limited? Was the EGT observed to be below the limit? If the enigne was on the limiter, the limit schedule would override the RPM governing (which ever spool it is).
Thanks, Jetthrust. The engines were well below their ITT limits. There was a smallish ITT rise after switching on the anti-ice.
... the only CF6 hydro engine that controls to N1 is the CF6-80C2 (it controls to both N1 and N2 - N2 at near idle, N1 at higher power - far and away the most complex hydromechanical control I ever dealt with).
Thanks, tdracer. Another Citation in our fleet has FADEC and it too controls N2 at low speeds and N1 at the higher power settings. With FADEC, that is really easy to do, of course.
I have never heard of the "droop slope" governor before, but will now add that to my list of considerations for the explanation! I will try to find out about it.
Since I last posted, it has occurred to me that engine idle speed may have something to do with it. The Pilot Training Manual (PTM) says that the N2 flight idle speed, normally 49.5%, is increased to 60% when engine anti-ice is on. So, I reasoned that, for a mechanical system, the way the idle speed increase is implemented/enforced could explain things.
I spent several hours today trying to find more in the PTM and also talking with a maintenance engineer that works on these engines. The degree of detail in the PTM is very low, unfortunately, and only basic information can be found. The engineer was a bit stumped, but brought out the page from the PW530 Maintenance Manual relating to engine anti-ice. The information there was not at all consistent with that of the PTM, so now I probably have even more questions!
My next flight in this aircraft may present some more clues and opportunities. I don't know when that will be.
Thanks, again, to everybody here for their thoughtful and quality contributions.