Figure 3-10. Reciprocating Engine Oil Cooler
Turbine engine oil coolers. Oil coolers are used in the lubricating systems of some turbine engines to
reduce the temperature of the oil to a degree suitable for recirculation through the system. Two basic types of oil coolers
in general use are the air-cooled oil cooler and the fuel-cooled oil cooler, as shown in figure 3-11. The fuel-cooled oil
cooler acts as a fuel/oil heat exchanger in that the fuel cools the oil and the oil heats the fuel. The air-cooled oil cooler
normally is installed at the forward end of the engine. It is similar in construction and operation to the air-cooled cooler
used on reciprocating engines.
Maintenance. When components of an aircraft transmission or engine oil system have experienced
internal failure, and metal particles are evident in the oil system, the oil cooler shall be removed and replaced. If the -23
maintenance manual of the aircraft requires retirement or condemnation of the metal-contaminated oil coolers, identify
the cooler with tag DD Form 1577-2. If the -23 maintenance manual does not require condemnation of contaminated
coolers, identify the cooler as follows: (a) Tag the oil cooler with a DD Form 1577-2 and show in block "reason for
repairable condition" that oil cooler is being returned for decontamination.
A locally fabricated metal tag made from 0.040 gauge SO aluminum material (size 3/4 by 2-1/2 inches with
hole drilled 3/8 inch in from end of tag to accommodate installation cooler) will be attached to the cooler. Stamp the
words METAL CONT on the tag with a die stamp. The metal tag should be bent to conform to the configuration of the
cooler to prevent tag from being torn off during packaging or handling.
Pressure Devices. Various pressure devices regulate pressure in aircraft oil systems. These include relief
valves, pumps, check valves, surge valves, and pressure switches in oil pressure indicating gauges. Maintenance
consists mainly of adjusting, removing, cleaning, and replacement of various components. Each type is explained in the
Relief valves. Oil pressure relief valves, as shown in figure 3-12, limit oil pressure to the value specified by
the engine manufacturer. The oil pressure must be high enough to ensure adequate lubrication of the engine and
accessories at high speeds and powers, but not too high to cause leakage and damage. Maintenance usually consists of
an oil pressure adjustment. The relief valve is usually adjusted by means of a screw which changes the pressure on the
spring or springs controlling the valve. Make the pressure adjustments, while the engine is idling and tighten the
adjustment screw locknut after each adjustment. Check the oil pressure reading while the engine is running at the rpm
specified in the aircraft maintenance manual. The oil pressure reading should be between the limits prescribed in the
aircraft maintenance manual.
Oil pumps. The oil pump is designed to supply oil under pressure to the parts of the engine that require
lubrication. The pump may be one of several types. The gear and piston oil pumps are the most common. Each of
these pumps has several possible configurations. A gear-type pump is shown in figure 3-13. Maintenance usually
consists of removal and replacement.
Check valves. Check valves are sometimes installed in the oil supply lines of dry-sump oil systems to
prevent reservoir oil from seeping through the oil pump elements and high-pressure lines into the engine after shutdown.
Check valves, by stopping flow in an opposite direction, prevent accumulations of undue amounts of oil in the accessory
gearbox, compressor rear housing, and combustion chamber. Such accumulations could cause excessive loading of the
accessory drive gears during starts, contamination of the cabin pressurization air, or internal oil fires.