I accept that its an aviation standard to rout all overflow tubes overboard but would argue that ;
The chances of coolant causing a fire is very low.
An overflow is likly to be caused by a sever engine malfunction - the overflow being the least of your worries.
Most of the automotive applications of this system, do not seem to go to the trouble of an overflow tube.
I only have experince of two Rotax 912 ULS - neither even come close to overflowing a correctly installed expansion bottle.
As pilots we take a calculated risk in committing aviation - In the scheme of calculating risk, the chance of an emission from the bottle, the fluid catching fire, is likly less than a fuel/oil hose malfunctioning, with far greater consequences.
Most Rotax 9's are not installed in Certified aircraft - there is a concerning "creep" towards Certified standards and its related legalities/ weight/ costs. I urge recreational pilots to consider the desirability of pursuing fake Certification.
I support any pilot who wishes to go the extra mile and fit a overflow tube.😈
