Steve,
Yup... that is what I will probably end up doing at some level. Some will be "in sim" and some will be "external.... in the sense that the APU controls will be "real" in the simpit... and then the final "output" of a series of "real" control events will eventually activate some FSUIPC variable in the sim.
Simulating a normal jet engine APU startup with the N1 + N2 + EGT values and the delays until the Gen and Bleed Air will function corectly and all that stuff is complicated. A FS2004 engine just "comes" with all that stuff modeled already.
I just wanted a tad more realism in the sim........ and a bit of an "easier go". The pit is already a "labor of love" without adding more work to it.

best,
.................john

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