Part 91 of the FAR's apply to all U.S. aircraft, unless the specific
reg says otherwise. Setting aside the ambiguity that the Rotax is
both water and air cooled, where 91.205 says temp gauge for
water-cooled engine, I agree FAA will want to see one. The question
is whether CHT alone is sufficient.
Combustion heats the cylinder head, which heats the coolant, so one
lags the other, with CHT in the lead. Rotax does not provide a means
for installing a coolant probe, nor do they specify max coolant temp.
If FAA were to raise the issue, one could then show the inspector the
Rotax documentation. It's all up the each individual FAA inspector or
DAR, and judgment can vary widely. But it seems senseless for FAA to
prefer coolant temp over (Rotax recommended) CHT, or insist on both.
Besides having no idea what red line coolant temp should be for one's
chosen probe position, I believe you'll see red line CHT long before
excess coolant temp, if ever the latter (depending again on probe
position and its design, or whether you've lost coolant).
Regards,
Fred F., A063
Rob Housman wrote:
>
> OK, point well taken. However, if a "temperature gauge for each
> liquid-cooled engine" is required, but an "oil temperature gauge for each
> air-cooled engine" is likewise a requirement, one can assume that the FAA is
> not going to accept an oil temperature gauge alone on a liquid cooled
> engine, which leaves us to ponder whether EGT or CHT would satisfy the FAA.
> It might, but prudence would suggest that coolant temperature should be
> measured on a liquid cooled engine with the probe low enough in the system
> to always be immersed in coolant (as opposed to those old Austin Healeys
> where it was at the top of the radiator!).
>
> Perhaps someone who has experience with the FAA or PFA (or others) on this
> issue, could give us a more official answer than my speculation.
>
> Best regards,
>
> Rob Housman
> A070
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