Expert: Paul Soderman - 10/22/2007

Question
Ordinarily, the Lycoming 712F engine has a maximum N1 of 107%; however,
as the outside air temperature (OAT) decreases below 0 deg C, the FADEC
limits the max N1 below this value.  For example, at -20 deg C, the max N1
is only 103%.  This appears to be the case whether the aircraft is at sea level
or 10 000 ft.  Why is max N1 limited by OAT?

Answer
Ian
I am an aerodynamicist, so I cannot answer your question definitively.  However, I would assume that the max speed of engines degrade in cold weather because the lubrication becomes sluggish.  That is, viscosity increases.  That is an educated guess.
Paul