by a capillary tube. The motion produced by the diaphragm’s  expansion and contraction is amplified through a lever and  gear arrangement. The gear varies the electrical value of the  indicating circuit, which in turn, is reflected on the indicator in  the cockpit. This type of indicating system replaces long fluid- filled tubing lines with an almost weightless piece of wire. In dry-sump lubricating systems, the oil temperature bulb  may be anywhere in the oil inlet line between the supply  tank and the engine. Oil systems for wet-sump engines have  the temperature bulb located where it senses oil temperature  after the oil passes through the oil cooler. In either system, the  bulb is located so that it measures the temperature of the oil  before it enters the engine’s hot sections. An oil temperature  gauge in the cockpit is connected to the oil temperature bulb  by electrical leads. The oil temperature is indicated on the  gauge. Any malfunction of the oil cooling system appears  as an abnormal reading. The cooler, either cylindrical or elliptical shaped, consists  of a core enclosed in a double-walled shell. The core is built  of copper or aluminum tubes with the tube ends formed to  a hexagonal shape and joined together in the honeycomb  effect.  [Figure 6-11]  The ends of the copper tubes of the  core are soldered, whereas aluminum tubes are brazed or  mechanically joined. The tubes touch only at the ends so  that a space exists between them along most of their lengths. This allows oil to flow through the spaces between the tubes  while the cooling air passes through the tubes. The space between the inner and outer shells is known as  the annular or bypass jacket. Two paths are open to the flow  of oil through a cooler. From the inlet, it can flow halfway  around the bypass jacket, enter the core from the bottom,  and then pass through the spaces between the tubes and out  to the oil tank. This is the path the oil follows when it is hot  enough to require cooling. As the oil flows through the core,  it is guided by baffles that force the oil to travel back and  forth several times before it reaches the core outlet. The oil  can also pass from the inlet completely around the bypass  jacket to the outlet without passing through the core. Oil  follows this bypass route when the oil is cold or when the  core is blocked with thick, congealed oil. As discussed previously, the viscosity of the oil varies with  its temperature. Since the viscosity affects its lubricating  properties, the temperature at which the oil enters an engine  must be held within close limits. Generally, the oil leaving  an engine must be cooled before it is recirculated. Obviously,  the amount of cooling must be controlled if the oil is to return  to the engine at the correct temperature. The oil cooler flow  control valve determines which of the two possible paths the  oil takes through the oil cooler.  [Figure 6-12] There are two openings in a flow control valve that fit over  the corresponding outlets at the top of the cooler. When the  oil is cold, a bellows within the flow control contracts and lifts  a valve from its seat. Under this condition, oil entering the  cooler has a choice of two outlets and two paths. Following  the path of least resistance, the oil flows around the jacket  and out past the thermostatic valve to the tank. This allows  the oil to warm up quickly and, at the same time, heats the  oil in the core. As the oil warms up and reaches its operating  temperature, the bellows of the thermostat expand and closes  the outlet from the bypass jacket. The oil cooler flow control  valve, located on the oil cooler, must now flow oil through the  core of the oil cooler. No matter which path it takes through  the cooler, the oil always flows over the bellows of the  thermostatic valve. As the name implies, this unit regulates  the temperature by either cooling the oil or passing it on to  the tank without cooling, depending on the temperature at  which it leaves the engine.  When oil in the system is congealed, the scavenger pump  may build up a very high pressure in the oil return line. To  prevent this high pressure from bursting the oil cooler or  blowing off the hose connections, some aircraft have surge  protection valves in the engine lubrication systems. One type  of surge valve is incorporated in the oil cooler flow control  valve; another type is a separate unit in the oil return line.  [Figure 6-12] Surge Protection Valves Oil Cooler Oil Cooler Flow Control Valve Oil Temperature Indicator Bypass jacket Core Baffles Inlet from engine Outlet from core Outlet from bypass jacket Figure 6-11.  Oil cooler. 6-10