Aircraft Lubrication Systems

Introduction to Lubrication Systems

• Lubrication systems provide lubrication and cooling for gears, bearings, and splines.
• These systems collect foreign matter from oil tanks, bearing housings, and gearboxes.
• Lubrication systems protect lubricated components made from non-corrosion resistant materials.
• The oil used should not significantly deteriorate while performing its functions.

Purpose of Turbine Engine Lubrication

• Turbine engine lubrication reduces friction and wear between mating parts.
• Lubrication systems clean moving parts by removing metal particles formed during wear and tear.
• It cools hot parts to prevent material property degradation and potential seizure and reduces vibration through oil damping.

Requirements of Aviation Lubricants

• Aviation lubricants should have low volatility to minimize evaporation at high altitudes.
• Aviation lubricants need anti-forming characteristics to support positive lubrication.
• High flash point and a wide temperature range are essential for aviation lubricants.
• Aviation lubricants need excellent film strength qualities, including cohesion and adhesion.
• A high viscosity index ensures that the oil retains its proper viscosity within the operating temperature range.

Types of Oil Systems

• There are two main types of oil systems- wet sump and dry sump

Wet Sump Lubrication Systems

• Wet sump systems happen to be the oldest lubrication design and are rarely used in modern engines.
• In wet sump configurations, the oil is contained within the engine sump.
• The components needing lubrication are either partially or fully immersed in an oil bath.

Dry Sump Lubrication Systems

• Most modern turbine engines with an axial flow configuration utilize dry sump lubrication systems.
• A dry sump lubrication subsystem has pressure and scavenge components along with a breather vent.
• Oil is supplied to the lubricating parts via a pressure pump then it goes back to a tank by scavenge pumps.

Full Flow Type Oil Systems (Dry Sump)

• Full flow systems maintain the desired oil flow rates by dispensing oil based on maximum RPM.
• Excess oil returns through a pressure relief valve to the supply pump's feeding point.
• The size of the pressure pump is determined by the amount of flow needed at maximum engine speed.

Operation of a Full Flow Lubricating System (Supply Line)

• Oil flows from the tank to the pressure stage of the lubrication and scavenge oil pump.
• Next pump pressurizes the oil, sending it to the main oil filter.
• If the oil pressure exceeds a set limit before the filter, it passes through a pressure relief valve and returns to the oil tank.
• From the main oil filter, the oil is routed to the engine air/oil heat exchanger.
• Then it is sent to the fuel oil cooler.
• At the top of the fuel oil cooler, a manifold distributes the oil.
• Finally, the manifold sends oil through the last chance strainers/filters, reaching the bearing compartments and gearboxes.

Operation of a Full Flow Lubricating System (Scavenge Line)

• Scavenge oil pumps remove the oil from the bearing compartments and gearboxes.
• Before oil reaches the scavenge pump, a magnetic chip detector removes ferrous metal chips/powder from the bearings (if any).
• After the scavenge pump, oil flows through the electric magnetic chip detector and the oil temperature sensor before it returns to the oil tank.

Operation of a Full Flow Lubricating System (Breather Line)

• Oil mist is removed from the breather air by a deoiler (centrifugal breather).
• From the deoiler, oil goes to the tank and breather air is vented overboard.
• Oil mist at the LP Turbine is flushed out from the exhaust.

Components of a Typical Oil System

• Oil Tank
• Pressure & Scavenge Pumps
• Pressure Relief Valves
• Fuel Oil Heat Exchanger / Oil Cooler
• Air Oil Heat Exchanger
• Deoiler / Centrifugal Breather
• Magnetic Chip Detectors
• Filters

Oil Tank Details

• The oil tank can be usually mounted on the engine, and is normally a separate unit, but it may also be contained within the external gearbox.
• Tanks must have a way to drain and replenish the lubrication system, and a sight glass or dipstick to check the oil level manually.

Oil Pump Details

• Oil pumps are vital for efficient engine operation.
• Oil pump failures will cause the engine to shut down rapidly.
• Oil pump driveshafts do not have weak shear necks, ensuring oil supply is maintained as long as possible even if there is damage.
• Supply and scavenge pumps are commonly integrated into a single oil pump assembly.

Gear Type Pump Details

• Gear pumps include intermeshing steel gears housed in a close-fitting aluminum casing.
• When the gears rotate, oil is drawn into the pump.
• The oil travels between the teeth and casing and exits through the outlet.
• Gear pumps handle pressure (feed) and/or scavenge (return) functions.
• The accessory drive system drives the oil pumps’ pack.

Pressure Relief Valve Systems

• Relief valves control oil flow to bearing chambers by limiting pressure in the feed line to a specific design value.
• A spring-loaded valve opens when the design value is exceeded, allowing oil to return to the tank or pressure pump inlet directly.

Fuel-cooled Oil Cooler

• Fuel flows through a large number of tubes in the matrix, while oil is directed across the tubes by baffle plates.
• Heat transfers from the hot oil to the fuel, thus lowering the oil temperature.

Air-Cooled Oil Cooler

• Air-cooled oil coolers are similar in construction and operation to fuel-cooled types.
• The main difference is that it uses atmospheric or ram air as a cooling medium.

Magnetic Chip Detectors

• Magnetic plugs or chip detectors are located on the scavenge (return) side to collect ferrite debris from each bearing chamber.
• They are permanent magnets placed in the oil flow and housed in self-sealing valve housings.
• Upon inspection, they can indicate looming of bearings and splines without filter removal.
• These are designed to be removed during maintenance inspection to conduct condition monitoring without oil loss.

Filter Details

• Filters and strainers are placed within the lubricating system which prevents foreign matter from traveling through lubricating system.
• Coarse strainers are installed at the oil tank outlet or right before the inlet of the oil pumps to prevent debris from damaging the pumps.
• A fine pressure filter is fitted at the pressure pump outlet and is designed to catch small particles that could block oil feed jets.
• A "pop up indicator" on the filter housing serves when it gives a visual warning when the filter becomes partially blocked

Thread-Type Oil Filters

• Thread-type filters are often fitted as a 'last chance' filter right before the oil jets.

Electronic Cleaning of Filters

• Cleaning with solvent by hand is a traditional practice that is still widely used and accepted.
• Ultrasound cleaning devices are use to clean filters, the systems might require multiple rounds.
• It’s normal to see metal particles on filter surfaces.
• If the level of contamination goes beyond the limits set by the manufacturer, it could be a sign of impending engine failure.
• The contaminated oil should be drained.
• The lubrication process should be restarted and engine inspected for a short period of time.

Important Facts on Oil Systems

• Modern gas turbine engines utilize synthetic oil for lubrication, like Mobil jet type II oil, and exxon 2380.
• Oil tanks should be checked 10 minutes after the engine is shut down and filled up to the neck.
• Oil quantity can be monitored via a sight glass or gauge in the cockpit or on an LED screen engine page.
• Red light usually indicates low oil pressure warning.
• Both low oil pressure and high oil temperature are causes for immediate engine shutdown.

Recap

• The requirements of lubrication are important.
• The four principles and purposes of engine lubrication should be considered.
• Description about the distinction between Wet sump and Dry Sump engine.
• Draw out the Hot Oil Tank Full Flow Lubrication System.
• Explain the functionality of all major components that appear in the diagram.
• Essential facts on oil system; specifically on crucial instances that cause the engine to halt.

Explore aircraft lubrication systems, their purpose in turbine engines, and the requirements of aviation lubricants. Learn how these systems reduce friction, cool hot parts, and maintain material properties. Understand the importance of low volatility and anti-forming characteristics in aviation lubricants.