15.10 Lubricating Systems Flashcards
What’s the main purpose of a Lubricating system?
Distribute oil to the engine bearing components and the gearboxes.
What are the two types of oil sump?
- Wet sump.
- Dry sump.
What’s a wet sump?
Where the oil is stored in the engine.
What’s a dry sump?
Where the oil is stored in an external supply tank, located on or near the engine.
What are the two classifications of lubrication systems?
- Full-flow system.
- Constant pressure system.
What’s the main charactersitic in a full-flow system, in terms of pressure?
There’s no way of regulating the pressure.
What factors affect the oil pressure, in a full-flow system?
- N2 rotor speed.
- Oil temperature.
- The flow in areas downstream of the supply pump.
What is the effect of a higher N2 rotor speed on oil pressure, in a full-flow system?
Higher N2 speeds means higher oil pressure.
What is the effect of a higher oil temperature on oil pressure, in a full-flow system?
A higher oil temperature means a decrease in oil pressure.
(Oil is less viscous at higher temps, and thus has less resistance and flows easier, resuting in lower pressures).
What is the effect on oil pressure with a decrease in the flow area, in a full-flow system?
A decrease in the flow area will result in an increase in oil pressure.
(Flow area can be decreased by clogged oil nozzles).
What special type of oil leakage is hard to identify?
When oil pressure decreases but oil quantity remains the same.
(Caused by very low oil temp with very high N2 speeds)
What’s another name for the Constant Pressure lubricating system?
The regulated pressure system.
What’s the key component of the Constant Pressure type lubrication system?
The pressure regulating valve (PRV).
What’s the main function of the PRV?
Holds oil pressure to a given design value, at all N2 rotor speeds from idle to max.
How does oil temperature affect oil pressure in a Constant Pressure system?
It Doesn’t. Oil temperature has no effect on oil pressure in a Constant Pressure system.
What does the PRV ensure, with regards to pressure differential?
Ensures that the difference between supply pressure and vent pressure is held constant, giving the same flow level at all engine speeds.
What are the advantages of full-flow systems? (2)
- Simpler.
- No adjustments necessary during maintenance.
What are the disadvantages of full-flow systems? (2)
- High oil pressures required at engine start means very high oil pressures develop at high engine speeds.
- Significant rise in oil temp after power reduction, as flow rate decreases with engine speed.
What are the advantages of Constant Pressure systems? (2)
- Pressure remains relatively low throughout.
- No oil temp rise after power reduction.
What are the disadvantages of Constant Pressure systems? (3)
- More complex.
- Requires adjustments during maintenance.
- PRV is often a source of trouble.
What component is a critical lubrication point, due to high temperatures?
Exhaust turbine bearing.
What can provide air cooling?
Cool bleed air from the early compressor stages.
What can air cooling help to reduce, in terms of oil?
It helps to reduce the oil quantity required for cooling.
What are the four sub-systems of a lubricating system?
- Oil reservoir and supply system.
- Scavenge system.
- Vent system.
- Monitoring system.
What are the 3 sub-systems of the oil system?
- Storage.
- Distribution.
- Indicating.
What is an oil tank usually constructed of?
Aluminium alloy with a silicone coating for fire resistance.
What are the two methods for filling an oil tank?
- Gravity.
- Pressure.
What part of the oil filler catches foreign particles?
The strainer.
What’s the primary source of oil cooling?
The fuel-cooled oil cooler.
What does the check valve in the oil filter section do?
Stops oil and air from flowing back out of the filler port if the cap isn’t reinstalled correctly by Korey
What two types of elements can oil pumps have?
- Supply element.
- Scavenge element.
- (Or both, usually).
What is the number of elements used in an oil pump, dependent on?
The type and model of the engine.
What element type has a greater pumping capacity, supply or scavenge?
The scavenge element has a greater pumping capacity.
What are the two types of oil pump?
- Gear pump.
- Gerotor pump.
What does a gear pump have to protect from overpressure?
Pressure relief valve.
How is the correct output maintained in a gear pump?
The clearances between the gear-teeth, and sides of the pump wall and plate.
What protection method does a gear pump employ, if it were to sieze?
It’s driven by a ‘shear-shaft’. The shaft will shear-off if the pump siezes.
What elements does a gerotor pump usually consist of?
A single supply element and several scavenge elements.
How is the capacity of the elements in a geretor pump controlled?
By varying the size of the gerotor elements.
Which type of fuel pump is most commonly used?
Gear pump.
Where is the oil pressure regulating valve usually located?
On the pressure side of the pressure pump.
How does the oil pressure regulating valve operate?
It uses a spring, held under tension.
What effect does increasing the tension on the spring of the pressure regulating valve have?
The pressure at which the valve opens also increases the system pressure.
When would a pressure relief valve be required in the system? (2)
- If the system does not have a pressure regulating valve.
- Especially required in systems that use an oil cooler.
Under what conditions should the pressure relief valve be closed?
Normal conditions.
What is a ‘last-chance’ oil filter?
Used to protect the oil nozzles if supply filter is bypassed.
What type of filter element is used for supply filters?
Pleated Woven Wire Mesh (25-35 microns).
Can be cleaned and reused.
What type of filter element is used for scavenge filters?
Paper.
Must be disposed of and cannot be reused.
What are the two types of heat exchanger tank system?
- Hot tank system.
- Cold tank system.
What is the hot tank system?
Oil from the bearing compartments is not cooled before returning to the tank.
Where is the hot tank system located?
In the supply line.
What is the cold tank system?
Oil from the bearing compartments IS cooled before returning to the tank.
Where is the cold tank system located?
In the scavenge line.
What are the two types of oil cooler?
- Fuel-cooled.
- Air-cooled.
Which type of oil cooler is primarily used?
Fuel-cooled oil coolers
What can be used as an addtional cooler?
Air-cooled oil coolers.
What types of engine mainly use air-cooled oil coolers?
Turboprop and APUs.
How does the fuel and oil flow through a heat exchanger?
The fuel flows through tubes in the core, and the oil flows in passages circulating around the tubes.
What provides an exit for the fuel and oil, if their passages become blocked?
Bypass valves. One for fuel, and one for oil.
How many scavenge pumps (generalised) remove oil/contaminants from bearing compartments and gearboxes?
7 scavenge pumps.
What is a ‘breather’ vent system?
Removes air from scavenge oil in external gearbox.
What is a ‘de-aerator’ vent system?
Removes air from scavenge oil at the inlet.
Where are quantity transmitters (Qty Tx) usually located, for ease of maintenance?
At the top of the oil tank, so they can be removed/changed without needing to drain all the oil.
What are the two types of Qty Tx?
- Capacitance type.
- Reed type.
How does the capacitance-type Qty Tx measure oil?
Two concentric tubes makes up the capacitance plates.
As the level of oil increases and fills the space between the plates, the capacitance increases.
How does the Reed-type Qty Tx measure oil?
Has a metal tube with a float inside it, and holes to allow the oil to flow in.
The float has permanent magnet that activates a ladder of reed switches.
The switch closest to the magnet closes, and gives an indication.
How do metal particles in oil, increases exponentially?
Metal particles cause more abrasion which results in more metal particles = more abrasion = even more particles etc..
What does SOAP stand for?
Spectrometric Oil Analysis Program
What does SOAP find and analyse in oil?
Analyses internal wear of parts by finding particles in oil that are 0.001mm to 0.02mm in size.
How does SOAP analyse particles?
Uses an electric arc to vapourise particles, which then produce a light spectra that determines the type and size of the metal.
What is the function of Mag-chip detectors?
To catch metal particles by attracting them to a magnet.
How can a mag-chip be checked?
Manually, or automatically using the maintenance panel in the flight deck.
How does the mag-chip give an indication in the flight deck?
The resistance decreases when the mag-chip picks up metal particles, until it reaches a point where it gives an indication.
What must be done before checking the oil, on an aircaft without an anti-syphon system?
Wait 5-10 mins after engine shutdown to check the oil. The oil slowly feeds back into the tank.
Anti-syphon system prevents the slow flow back into the tank from the gearbox.
What type of hose should be used on the overfill port, when pressure-filling oil on an aircraft without a sight glass?
A transparent hose, so that you can easily see when the oil is flowing out of the overfill port.
How long should you wait before removing the oil filler cap after engine shutdown?
Up to 5 minutes, as high pressure my still be present in the system, causing hot oil to spill out of the filler port.
Why are oil overfills dangerous?
Overfills can result in large amounts of oil being pumped through the vent line into the forward beaaring compartment, or the de-oiler.
