First, the instrument — Page 65, Lesson 83

I want to walk you through how we monitor engine temperature in a piston engine, and then the operational procedures that keep that temperature under control. First, the instrument. We monitor the temperature of the engine by the use of a Cylinder Head Temperature Gauge. The gauge uses a sensor which is fitted to the engine's cylinder heads. Now, if only one sensor is fitted, it will be fitted to the Hottest Cylinder. This is usually one of the rearmost cylinders. Why the rearmost? Because in a horizontally opposed engine, the rear cylinders get the least cooling airflow — the air has already been heated and disturbed by the front cylinders. So the rearmost cylinder runs hottest, and that's the one we want to watch. The sensor itself is a Thermocouple. The principle of operation of a thermocouple is covered in depth in the electrical and instrument objectives, so I won't go deep into it here. It is sufficient to say that the sensor produces a Voltage which is directly proportional to its temperature. So as the cylinder head gets hotter, the thermocouple produces more voltage. The cockpit indication is displayed by a sensitive moving coil meter called a Galvanometer. The scale reads temperature and not voltage — so the pilot reads degrees, not millivolts. Now, the operational procedures. The cooling arrangements for a particular engine are designed to ensure satisfactory cooling during flight, when the forward speed of the aircraft should give an adequate flow of cool air. But this can sometimes not be the case. Let's look at the two problem phases: climb and descent. During a climb, high power is used, which generates high temperature in the engine. At the same time, forward speed is reduced, and airflow to the engine is reduced. So you have high heat generation and low cooling airflow — the pilot should be aware of the possibility of overheating. The procedure here: climbing at best rate of climb speed, which we call VY, is preferable to prolonged use of best angle of climb speed, which we call VX. Why? Because VY gives you a higher forward speed and therefore more cooling airflow over the engine, while VX is a slower speed with less airflow — fine for clearing obstacles, but not for prolonged climbs if you want to keep the engine cool. Descending can also cause problems, but the opposite kind. Engine power is reduced, so there is less heat generated in the engine. If the aircraft is placed into a dive, this increases the flow of air over the engine, and it will be overcooled. The sudden change in temperature could cause what is known as Thermal Shock. This can cause components to fracture, and is a common problem on the cylinders of engines. So a rapid cooling of a hot cylinder can crack it. Finally, better control of temperature is possible if cowl flaps or gills are fitted — but these are only... and that's where the passage cuts off. So we have the two failure modes: overheating in a climb, and thermal shock from overcooling in a dive. The cowl flaps or gills, when fitted, let the pilot regulate the airflow through the cowling to manage that temperature.