APP Chpt 9

• Facts on lubrication system:

  • Provide lubrication & cooling for all gears, bearings & splines
  • Capable of collecting foreign matter which, if left in an oil tank/bearing housing/gearbox
  • The oil must protect the lubricated components which are  manufactured from non-corrosion resistant materials
  • The oil must accomplish these tasks without significant deterioration
• 4 purposes of turbine engine lubrication:

  • Lubrication of Mating Parts – reduction of friction & wear
  • Cleaning of moving parts – metal particles formed during wear & tear should be removed to prevent further damages
  • Cooling of hot parts – heat can cause degradation of material properties & may cause seizure of parts
  • Oil damping – to reduce vibration
• Requirements of aviation lubricants:

  • Low volatility to reduce evaporation at high altitude
  • Anti-forming characteristics to give positive lubrication
  • High flash point
  • Wide temperature range
  • Excellent film strength qualities of cohesion and adhesion
  • High viscosity index meaning the oil will tend to retain its viscosity when heated within the operating temperature
• Types of oil systems:

  • Wet Sump Lubrication System
  • Dry Sump Lubrication System
  • Full Flow Type Oil System
• Wet sump lubrication system:

  • Is the oldest design & is rarely seen in modern engine
  • The oil is contained integrally in an engine sump
  • The parts to be lubricated are immersed partially or fully in a bath of oil
• Dry sump lubrication system:

  • Most modern turbine engines of axial flow configuration use it
  • The subsystem consists of Pressure, scavenge & breather vent
  • Oil is supplied to the lubricating parts by pressure pump & is returned to the tank by scavenge pumps
• A full flow type oil system:

  • Normally applied to the dry sump system
  • Achieves the desired oil flow rates throughout by dispensing oil based on maximum RPM. Excess oil will return through the pressure relief valve to the feeding point of the supply pump
  • The pressure pump size is determined by the flow required at maximum engine speed
• Operation in supply (Pressure) line:

  1. The oil flows from the tank down to the pressure stage in the lubrication & scavenge oil pump
  2. The oil pump pressurizes the oil and sends it to the main oil filter
  3. If the oil pressure before the filter is greater than set pressure, some of the oil goes through a pressure relief valve & is returned to the oil tank
  4. From the main oil filter, the oil is sent to the engine air/oil heat exchanger
• Operation in supply (Pressure) line:

  5. Then the oil goes to the fuel oil cooler

  6. At the top of the fuel oil cooler, a manifold distributes the oil

  7. Then the manifold sends oil through the last chance strainers/filters, & to the bearing compartments & gearboxes
• Operation in scavenge (return) line:

  1. The oil is removed from the bearing compartments and gearboxes by the scavenge oil pumps.
  2. Before oil reaches to the Scavenge pump, Magnetic Chip Detector will help to remove ferrous metal chips/powder from the bearings (if any)
  3. After scavenge pump, oil will travel to the Electric Magnetic Chip Detector & the Oil Temperature Sensor before oil return to the Oil Tank
• Operation in breather line:

  1. Oil mist is removed from the breather air by the deoiler (centrifugal breather)
  2. From the deoiler, oil goes to the Tank and breather air is vented overboard
  3. While Oil mist at the LP Turbine will be flushed out from the Exhaust
• Components for 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 detector
  • Filters
• Oil tank:

  • Usually mounted on the engine & is normally a separate unit although it may also be an integral part of the external gearbox
  • It must have provision to allow the lubrication system to be drained & replenished
  • A sight glass or a dipstick must also be incorporated to allow operator to check the oil level manually
• Oil pump:

  • Are vital to the efficient operation of the engine
  • Failure of the pumps will necessitate a rapid shutdown of the engine
  • For this reason, the oil pump drive-shafts do not incorporate a weak shear neck because they must continue to supply oil for as long as possible, regardless of damage
  • The Supply pump is normally integrated together with the Scavenge pump in an Oil Pump Assembly
• Gear type pump:

  • Consist of a pair of intermeshing steel gears which are housed in a close fitting aluminum casing
  • When the gears are rotated, oil is drawn into the pump, carried round between the teeth & casing & delivered at the outlet
  • Can be used for pressure (feed) pumps and/or scavenge (return) pumps
  • The oil pumps pack is driven by the accessory drive system
• Vane type pump:

  • Is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside a cavity
  • In some cases these vanes can have variable length and/or be tensioned to maintain contact with the walls as the pump rotates
• Pressure relief valve system:

  • The oil flow to the bearing chambers is controlled by limiting the pressure in the feed line to a given design value
  • This is accomplished by the use of a spring-loaded valve which allows oil to be directly returned to the oil tank, or pressure pump inlet, when the design value is exceeded
• Fuel-cooled oil cooler:

  • A large number of tubes convey the fuel through the matrix, the oil being directed by the baffle plates in a series of passes across the tubes
  • Heat is transferred from the oil to the fuel, thus lowering the oil temperature
• Air/oil heat exchanger:

  • Similar to the fuel-cooled type in both construction and operation; the main difference is that atmospheric/ram air is used as the cooling medium
• Centrifugal breather: It helps to separate air from oil by means of centrifugal force from the rotor
• Magnetic chip detector:

  • Fitted on the scavenge (return) side to collect ferrite debris from each bearing chamber
  • They are permanent magnets inserted in the oil flow & are retained in self-sealing valve housings
  • Upon examination, they can provide a warning of impending failure of bearings & splines without having to remove & inspect the filters
  • Designed to be removed during maintenance inspection, for condition monitoring purposes without oil loss occurring
• Filter:

  • To prevent foreign matter from continuously circulating the lubricating system, several filters & strainers are positioned within the system
  • Coarse strainers are usually fitted at the outlet of the oil tank or immediately before the inlet of the oil pumps to prevent debris from damaging the pumps
• Filter:

  • A fine pressure filter is fitted at the pressure pump outlet which retains any small particles that could block the oil feed jets
  • A ’pop up indicator’ may be fitted to the filter housing to give a visual warning of a partially blocked filter
• Threat-type oil filter: Are often fitted as a ’last chance’ filter immediately upstream of the oil jets
• Electronic cleaning of a cleanable filter:

  • Traditional hand cleaning using a solvent is still commonly used & acceptable
  • An ultrasound cleaning device for cleaning of filter. Several rounds are needed to clean the systems
• Electronic cleaning of a cleanable filter:

  • It is normal to find particles of metal on the filter surfaces.
  • However, if the level of contamination hits the limits set by the manufacturer, the engine might be facing an impending failure.
  • The engine oil could be drained, re-serviced & run for a short time to inspect the engine
• Important facts on oil system:

  • Modern gas turbine engines utilize synthetic oil for lubrication. Ex. Mobil jet type II oil, exxon 2380
  • Oil tank is inspected 10 minutes after engine shutdown & usually filled up to the neck
  • Oil quantity may be read on a sight glass or a gauge in the cockpit or on LED screen engine page
• Important facts on oil system:

  • Low oil pressure warning is usually red & is a cause for immediate engine shutdown
  • High oil temperature is also a cause for engine shutdown