Text-only reference. Published from the recorded official FAA Chapter 6 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
6-13 through it. When the circulating oil has performed its function of lubricating and cooling the moving parts of the engine, it drains into the sumps in the lowest parts of the engine. Oil collected in these sumps is picked up by gear or gerotor-type scavenger pumps as quickly as it accumulates. These pumps have a greater capacity than the pressure pump. This is needed because the volume of the oil has generally increased due to foaming (mixing with air). On dry sump engines, this oil leaves the engine, passes through the oil cooler, and returns to the supply tank. A thermostat attached to the oil cooler controls oil temperature by allowing part of the oil to flow through the cooler and part to flow directly into the oil supply tank. This arrangement allows hot engine oil with a temperature still below 65 °C (150 °F) to mix with the cold uncirculated oil in the tank. This raises the complete engine oil supply to operating temperature in a shorter period of time.
Wet-Sump Lubrication System Operation A simple form of a wet-sump system is shown in Figure 6-16. The system consists of a sump or pan in which the oil supply is contained. The oil supply is limited by the sump (oil pan) capacity. The level (quantity) of oil is indicated or measured by a vertical rod that protrudes into the oil from an elevated hole on top of the crankcase. In the bottom of the sump (oil pan) is a screen strainer having a suitable mesh, or series of openings, to strain undesirable particles from the oil and yet pass sufficient quantity to the inlet or (suction) side of the oil pressure pump. Figure 6-17 shows a typical oil sump that has the intake tube running through it. This preheats the air-fuel mixture before it enters the cylinders.
The rotation of the pump, which is driven by the engine, causes the oil to pass around the outside of the gears. [Figure 6-6] This develops a pressure in the crankshaft oiling system (drilled passage holes). The variation in the speed of the pump from idling to full-throttle operating range of the engine and the fluctuation of oil viscosity because of temperature changes are compensated by the tension on the relief valve spring. The pump is designed to create a greater pressure than required to compensate for wear of the bearings or thinning out of oil. The parts oiled by pressure throw a lubricating spray into the cylinder and piston assemblies.
After lubricating the various units it sprays, the oil drains back into the sump and the cycle is repeated. The system is not readily adaptable to inverted flying since the entire oil supply floods the engine. Lubrication System Maintenance Practices Oil Tank The oil tank, constructed of welded aluminum, is serviced (filled) through a filler neck located on the tank and equipped with a spring-loaded locking cap. Inside the tank, a weighted, flexible rubber oil hose is mounted so that it is repositioned automatically to ensure oil pickup during all maneuvers. A dipstick guard is welded inside the tank for the protection of the flexible oil hose assembly. During normal flight, the oil tank is vented to the engine crankcase by a flexible line at the top of the tank. The location of the oil system components in relation to each other and to the engine is shown in Figure 6-18.
Repair of an oil tank usually requires that the tank be removed. The removal and installation procedures normally remain the same regardless of whether the engine is removed or not. First, the oil must be drained. Most light aircraft provide an oil drain similar to that shown in Figure 6-19. On some aircraft, the normal ground attitude of the aircraft may prevent the oil tank from draining completely. If the amount of undrained oil is excessive, the aft portion of the tank can be raised slightly after the tank straps have been loosened to complete the drainage. 6-14 Engine breather1 Oil outlet2 Oil temperature gauge4 Oil pressure gauge5 Oil inlet3 Oil tank6 Oil filter7 Oil tank vent line9 Engine oil pressure line10 Oil tank drain8 1 Supply Pressure Return Drain Vent 2 3 4 5 6 7 8 9 10 After disconnecting the oil inlet and vent lines, the scupper drain hose and bonding wire can be removed. [Figure 6-20] The securing straps fitted around the tank can now be removed. [Figure 6-21] Any safety wire securing the clamp must be removed before the clamp can be loosened and the strap disconnected. The tank can now be lifted out of the aircraft. The tank is reinstalled by reversing the sequence used in the tank removal. After installation, the oil tank should be filled to capacity. [Figure 6-22] After the oil tank has been filled, the engine should be run for at least two minutes. Then, the oil level should be checked and, if necessary, sufficient oil should be added to bring the oil up to the proper level on the dipstick. [Figure 6-23] Oil Cooler The oil cooler used with this aircraft’s opposed-type engine is the honeycomb type. [Figure 6-24] With the engine operating and an oil temperature below 65 °C (150 °F), oil cooler bypass valve opens allowing oil to bypass the core.
This valve begins to close when the oil temperature reaches approximately 65 °C (150 °F). When the oil temperature reaches 85 °C (185 °F), ±2 °C, the valve is closed completely, diverting all oil flow through the cooler core. 6-15 Oil Temperature Bulbs Most oil temperature bulbs are mounted in the pressure oil screen housing. They relay an indication of engine oil inlet temperature to the oil temperature indicators mounted on the instrument panel. Temperature bulbs can be replaced by removing the safety wire and disconnecting the wire leads from the temperature bulbs, then removing the temperature bulbs using the proper wrench. [Figure 6-25] Pressure & Scavenge Oil Screens Sludge accumulates on the pressure and scavenges oil screens during engine operation. [Figure 6-26] These screens must be removed, inspected, and cleaned at the intervals specified by the manufacturer.
Typical removal procedures include removing the safety devices and loosening the oil screen housing or cover plate. A suitable container should be provided to collect the oil that drains from the filter housing or cavity. The container must be clean so that the oil collected in it can be examined for foreign particles. Any contamination already present in the container gives a false indication of the engine condition. This could result in a premature engine removal. 6-16 A B to the torque value specified in the applicable maintenance manual. Filters should be safetied as required. Oil Pressure Relief Valve An oil pressure regulating (relief) valve limits oil pressure to the value specified by the engine manufacturer. Oil pressure settings can vary from around 35 psi minimum to around 90 psi maximum, depending on the installation. The oil pressure must be high enough to ensure adequate lubrication of the engine and accessories at high speeds and power settings.
On the other hand, the pressure must not be too high, since leakage and damage to the oil system may result. Before any attempt is made to adjust the oil pressure, the engine must be at the correct operating temperature and a check should be made to assure that the correct viscosity oil is being used in the engine. One example of adjusting the oil pressure is done by removing a cover nut, loosening a locknut, and turning the adjusting screw. [Figure 6-27] Turn the adjusting screw clockwise to increase the pressure, or counterclockwise to decrease the pressure. Make the pressure adjustments while the engine is idling and tighten the adjustment screw lock-nut after each adjustment. Check the oil pressure reading while the engine is running at the rpm specified in the manufacturer’s maintenance manual. This may be from around 1,900 rpm to 2,300 rpm. The oil pressure reading should be between the limits prescribed by the manufacturer at all throttle settings.
After the screens are removed, they should be inspected for contamination and for the presence of metal particles that may indicate possible engine internal wear, damage, or in extreme cases, engine failure. The screen must be cleaned prior to reinstalling in the engine. In some cases, it is necessary to disassemble the filter for inspection and cleaning. The manufacturer’s procedures should be followed when disassembling and reassembling an oil screen assembly. When reinstalling a filter or screen, use new O-rings and gaskets and tighten the filter housing or cover retaining nuts 6-17 Increase Decrease Recommendations for Changing Oil Draining Oil Oil, in service, is constantly exposed to many harmful substances that reduce its ability to protect moving parts.
The main contaminants are: • Gasoline, • Moisture, • Acids, • Dirt, • Carbon, and • Metallic particles. Because of the accumulation of these harmful substances, common practice is to drain the entire lubrication system at regular intervals and refill with new oil. The time between oil changes varies with each make and model aircraft and engine combination. In engines that have been operating on straight mineral oil for several hundred hours, a change to ashless dispersant oil should be made with a degree of caution as the cleaning action of some ashless dispersant oils tends to loosen sludge deposits and cause plugged oil passages. When an engine has been operating on straight mineral oil, and is known to be in excessively dirty condition, the switch to ashless dispersant oil should be deferred until after the engine is overhauled.
When changing from straight mineral oil to ashless dispersant oil, the following precautionary steps should be taken: 1. Do not add ashless dispersant oil to straight mineral oil. Drain the straight mineral oil from the engine and fill with ashless dispersant oil. 2. Do not operate the engine longer than 5 hours before the first oil change. 3. Check all oil filters and screens for evidence of sludge or plugging. Change oil every 10 hours if sludge conditions are evident. Repeat 10-hour checks until clean screen is noted, then change oil at recommended time intervals. 4. All turbocharged engines must be broken in and operated with ashless dispersant oil.
Oil & Filter Change & Screen Cleaning One manufacturer recommends that for new, remanufactured; or newly overhauled engines and for engines with any newly installed cylinders, the oil should be changed after the first replacement/screen cleaning at 25 hours. The oil should be changed, filter replaced, or pressure screen cleaned, and oil sump suction screen cleaned and inspected. A typical interval for oil change is 25 hours, along with a pressure screen cleaning and oil sump suction screen check for all engines employing a pressure screen system. Typical 50-hour interval oil changes generally include the oil filter replacement and suction screen check for all engines using full-flow filtration systems. A time maximum of 4 months between servicing is also recommended for oil system service.
Oil Filter Removal Canister Type Housing Remove the filter housing from the engine by removing the safety wire and loosening the hex head screw and housing by turning counterclockwise and removing the filter from the engine. [Figure 6-7] Remove the nylon nut that holds the cover plate on the engine side of the filter. Remove the cover plate, hex head screw from the housing. To remove the spin-on type of filter, cut the safety wire and use the wrench pad on the rear of the filter to turn the filter counterclockwise, and remove filter. Inspect the filter element as described in the following paragraph. Discard old gaskets and replace with new replacement kit gaskets.
Oil Filter/Screen Content Inspection Check for premature or excessive engine component wear that is indicated by the presence of metal particles, shavings, or flakes in the oil filter element or screens. The oil filter can be inspected by opening the filter paper element. Check the condition of the oil from the filter for signs of metal contamination. Then, remove the paper element from the filter and carefully unfold the paper element; examine the material trapped in the filter. If the engine employs a pressure screen system, check the screen for metal particles. After draining the oil, remove the suction screen from the oil sump and check for metal particles. [Figure 6-28] If examination of the used oil filter or pressure screen and the oil sump suction screen indicates abnormal metal content, additional service may be required to determine the source and possible need for corrective maintenance. To inspect the spin on filter the can must be cut open to remove the filter element for inspection.
6-18 A B C Using the special filter cutting tool, slightly tighten the cutter blade against filter and rotate 360º until the mounting plate separates from the can. [Figure 6-29] Using a clean plastic bucket containing varsol, move the filter to remove contaminants. Use a clean magnet and check for any ferrous metal particles in the filter or varsol solution. Then, take the remaining varsol and pour it through a clean filter or shop towel. Using a bright light, inspect for any nonferrous metals. Assembly of & Installation of Oil Filters After cleaning the parts, installation of the canister or filter element type filter is accomplished by lightly oiling the new rubber gaskets and installing a new copper gasket on the hex head screw. Assemble the hex head screw into the filter case using the new copper gasket. Install the filter element and place the cover over the case, then manually thread on the nylon nut by hand. Install the housing on the engine by turning it clockwise, then torque and safety it. Spin-on filters generally have installation instructions on the filter. Place a coating of engine oil on the rubber gasket, install the filter, torque and safety it. Always follow the manufacturer’s current instructions to perform any maintenance.
Troubleshooting Oil Systems The outline of malfunctions and their remedies listed in system. The purpose of this section is to present typical troubles. It is not intended to imply that any of the troubles are exactly as they may be in a particular aircraft. Requirements for Turbine Engine Lubricants There are many requirements for turbine engine lubricating oils. Due to the absence of reciprocating motion and the presence of ball and roller bearings (antifriction bearings), the turbine engine uses a less viscous lubricant. Gas turbine engine oil must have a high viscosity for good load-carrying ability but must also be of sufficiently low viscosity to provide good flowability. It must also be of low volatility to prevent loss by evaporation at the high altitudes at which the engines operate. In addition, the oil should not foam and should be essentially nondestructive to natural or synthetic rubber seals in the lubricating system. Also, with high-speed antifriction bearings, the formation of carbons or varnishes must be held to a minimum. Synthetic oil for turbine engines are usually supplied in sealed one-quart cans.
The many requirements for lubricating oils are met in the synthetic oils developed specifically for turbine engines. Synthetic oil has two principal advantages over petroleum oil. It has a lower tendency to deposit lacquer and coke (solids left after solvents have been evaporated) because it does not evaporate the solvents from the oil at high temperature. Oil grades used in some turbine engines normally contain thermal and oxidation preventives, load-carrying additives, and substances that lower the pour point in addition to synthetic chemical-base materials. MIL-L-7808, which is a military specification for turbine oil, is a type I turbine oil. Turbine synthetic oil has a viscosity of around 5 to 5.5 centistokes at 210º F that is approved against the military specification MIL-PRF-23699F. This oil is referred to as type II turbine oil. Most turbine oils meet this type II specification and are made with the following characteristics: 1. Vapor phase deposits—carbon deposits formed from oil mist and vapor contact with hot engine surfaces.
2. Load-carrying ability—provides for heavy loads on 6-19 Trouble Isolation Procedure Remedy Oil line leakage. Check external lines for evidence of oil leakage. Replace or repair defective lines. Accessory seal leakage. Check for leak at accessories immediately after Replace accessory and/or defective engine operation. accessory oil seal. Low grade of oil. Fill tank with proper grade oil. Failing or failed bearing. Check sump and oil pressure pump screen for Replace engine if metal particles are found. material particles. Defective pressure gauge. Check indicator. Replace indicator if defective.
Improper operation of oil Erratic pressure indications either excessively Remove, clean, and inspect relief valve pressure. high or low. accessory oil seal. Inadequate oil supply. Check oil quantity. Fill oil tank. Diluted or contaminated oil. Drain engine and tank; refill tank. Clogged oil screen. Remove and clean oil screen. Oil viscosity incorrect. Make sure correct oil is being used. Drain engine and tank; refill tank. Oil pump pressure relief Check pressure relief valve adjustment. Make correct adjustment on oil pump valve adjustment incorrect. pressure relief valve. Defective temperature gauge. Check indicator. Replace indicator if defective.
Inadequate oil supply. Check oil quantity. Fill oil tank. Diluted or contaminated oil. Drain engine and tank; refill tank. Obstruction in oil tank. Check tank. Drain oil and remove obstruction. Clogged oil screen. Remove and clean oil screens. Obstruction in oil cooler Check cooler for blocked or deformed passages. Replace oil cooler if defective. passages. Diluted or contaminated oil. Drain engine and tank; refill tank. Oil level in tank too high. Check oil quantity. Drain excess oil from tank. Excessive oil consumption1 High or low indicated oil pressure2 High or low indicated oil temperature3 Oil foaming4 the bearing systems of turbine engines.
3. Cleanliness—minimum formation of sludge deposits during severe operation. 4. Bulk stability—resistance to physical or chemical change resulting from oxidation. Permits long periods of service operation without significant increase in viscosity or total acidity, the main indicators of oxidation. 5. Compatibility—most turbine oil is compatible with other oils that meet the same military specification. But, most engine manufacturers do not recommend the indiscriminate mixing of approved oil brands and this is not a generally accepted practice. 6. Seal Wear—essential for the life of engines with carbon seals that lubricant properties prevent wear of the carbon at the carbon seal face.
Turbine Oil Health & Safety Precautions Under normal conditions, the use of turbine oil presents a low health risk for humans. Although each person reacts somewhat differently to exposure, contact with liquids, vapors, and mist of turbine oil should be minimized. Information on established limits on exposure to turbine oil can generally be found in the material safety data sheets (MSDS). Prolonged breathing of hydrocarbon vapor