The cylinder head temperature gage (CHT) helps the pilot protect his engine against the threat of excessive heat. Most General Aviation aircraft take the CHT off the hottest single cylinder of the four, six or eight cylinder powerplants determined by extensive flight tests. Optional installations offer readings from all cylinders. In Lycoming engines, all cylinders are drilled to accommodate a CHT bayonet type thermocouple.
Some operators in the field have been using a spark plug gasket-type installation in order to get cylinder head temperature readings. Textron Lycoming Engineering does not currently approve this method of determining CHT. Not only is the method less accurate than the recommended thermocouple type, but the temperature readings differ noticeably from the approved installations.
Minimum in-flight CHT should be 150ø F (65ø C), and maximum in most direct drive normally aspirated Lycoming engines is 500ø F (260ø C). Some of our higher powered more complex engines have a maximum limit of 475ø F (245ø C). Although these are minimum and maximum limits, the pilot should operate his or her engine at more reasonable temperatures in order to achieve the expected overhaul life of the powerplant. In our many years of building engines, the engines have benefited during continuous operation by keeping CHT below 400ø F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350ø F to 435ø F.
If an engine has bayonet probes in all cylinders, it is not unusual to see variations in CHT readings on fuel injected engines of 100o F between cylinders, and as much as 150ø F on engines with float-type carburetors. With the latter, an important cause of the variation is the kind of distribution of fuel and air to the individual cylinders. Other influences on CHT are such items as cylinder baffles, cowling, cowling flaps, location of engine accessories and of course manual control of fuel mixture.
It is very important that the CHT probes be checked on a regular basis. When these bayonet probes deteriorate, they tend to give readings that are less than the actual temperature of the cylinder head. This can result in operation above the recommended maximum temperature without the pilot even knowing it.
Operators frequently ask about the difference between the CHT and EGT (exhaust gas temperature) systems, and their meaning to the pilot during operation of the engine or engines. The EGT probe is installed in a different location from the CHT, or about four inches from the cylinder head down the exhaust stack. Although the EGT has some troubleshooting ability, it is primarily a fuel management instrument. On the other hand, the CHT is an engine instrument designed to protect the powerplant against its enemy, excessive heat.