Took the wife and Mikey the dog up into the mountains this weekend
for a little wife preventive maintenance and had the Caravan "overheat"
again at about 4000 feet. I say overheat but what it really does is
boil over since the boiling temperature is lower at 4000 feet than it
is at sea level. 

I looked carefully at the pressure cap and there is a slight
grove worn in the rubber gasket. This could, in theory, lower the
pressure slightly to perhaps 15 psi instead of 16 psi gage at sea level.
At a hot 4000 feet it could be perhaps around 13 psi gage or 28 psi
absolute. Other than this the cap seemed to work as advertised as 
after the engine cooled off it sucked all the coolant back out 
of the overflow can back into the engine. I also removed the 
rad cap and topped up the system. Very little coolant was 
lost as I caught it early. This is a good reason to use as large 
an overflow tank in your airplane as you have room for. In any 
event I will try a 24 psi gage 39 psi absolute pressure cap and 
confirm that it helps at high altitude.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org