AC generator maximum voltage

Open circuit voltage

peak                     1/60 volt/RPM 

effective (RMS)   1/120 volt/RPM

Frequency             1/12 hz/RPM

At 6000 RPM, 100 V peak, 50 V RMS, 500 hz

Thanks! Where did you find that info? I couldn't find it in any of the manuals unless I missed something.

So in other words if the regulator fails in the worst way possible (fail closed instead of fail open), the electrical bus would be subject to a maximum of 50 Vrms.

This type of data is outside the scope of the Rotax manuals. It’s from the results of a French university study project of the stator.  You can see the English translation of the results here;

http://contrails.free.fr/elec_alt_rotax_en.php

Open circuit stator voltage is not a failure condition, it’s part of the normal operation. This is a switched bridge regulator, not a shunt regulator. Once the regulator reaches the set point it turns off, open circuit.