Why a Cylinder Head Temperature Gauge

[Delta Bravo]

Question #1- Could someone tell me what the cylinder head temperature gauge monitors? Why is such a device needed if an oil temperature gauge is already installed?

Question #2- Is it safe to fly with a broken cylinder head temperature gauge providing the oil temperature gauge is working?

[BOHIXON]

A CHT gauge monitors just that, the temp of the head. As I understand it, this is important because at a certain temp, detonation will start to occour. Aside from this, cht is a good indication of how effeciently the oil system is working. It could be possible that the oil temp is normal, but due to a slightly lower than normal quantity the cht would run a little bit higher. For your second question, I would say that it is safe, although I wouldn't go with it on a cross-country flight. Remember that safe is one thing and legal is another.

[skid]

CHT probes screw into a threaded hole in the cylinder head to accurately read the temp of the hottest part of the cylinder. Oil temp probes usually screw into the oil screen housing area to measure oil temp after it goes through oil cooler and right before it goes to oil pump. Some engine installations do not provide adequate cylinder cooling in certain situations(extended slow airspeed climbs,etc.,) that may have an adequate oil cooler and will show normal oil temp as it is possible to run the cylinders too hot for a while before it would overheat the oil system. The other use (of CHT) is to monitor shock coooling of the cylinders on descent with low power settings. The combination of too hot CHT, and too rapid of cool down shortens the life of the top end of an engine tremendously, extreme cases crack cylinders. The particular engine makes a lot of difference. A 150 Lycoming will tolerate tons of abuse unnoticed; high output turbocharged engines need careful operation just to get to TBO.
A little common sense goes a long way. On most smaller engines let it warm up before taking off, climb a little higher airspeed than best rate, make gradual power reductions and plan descents in advance to avoid shock cooling and there should be no problems. Aircooled airplane engines have huge clearances to minimize the chances of sticking one from being to hot. The dimensional changes from hot to cold are considerable and are part of the reason airplanes tend to use more oil than watercooled engines. A single probe digital CHT/EGT combination guage is very cheap(3-400) and will tell you most of what you need to monitor engine characteristics and health.

[BAYALON]

Everything that was written is correct, yet, the most important reason was missing. If the CHT will rise above a certain temperature the metal properties will be effected resulting in metal fatigue.