Answers - Powerplant Lubrication Systems | Aircraft Systems

Answers - Powerplant Lubrication Systems

1. c
The viscosity, or the resistance of oil to flow, changes with temperature. For example, as the temperature drops, highly viscous oils become extremely thick and do not circulate. Therefore, low viscosity oils are typically recommended for use in reciprocating engines operating in cold climates. However, if a low viscosity oil is used when an engine is operated in extremely high temperatures, the oil can become so thin that a lower than normal oil pressure can result.

2. c
Because of the unique operating requirements, synthetic oils are typically used in gas turbine engines. Furthermore, because of the differences between turbine and reciprocating engine oils, they should not be mixed or used interchangeably.

3. b
A wet-type engine-driven vacuum pump uses an air-oil separator to separate the oil that mixes with the air that passes through the pump. The separator is installed in the pump's discharge line where it removes oil from the air and returns it to the engine sump.

4. c
The viscosity of commercial aviation oils is measured by a testing instrument called a Saybolt Universal Viscosimeter. This instrument consists of a tube that holds a specific oil quantity at an exact temperature and a calibrated orifice. The time in seconds required for exactly 60 cubic centimeters of oil to flow through the orifice is recorded as the oil's viscosity.

5. b
Viscosity index is a measure of the change in an oil's viscosity for a given change in temperature. The smaller the change in viscosity for a given temperature change, the higher the viscosity index.

6. a

7. c
Synthetic turbine engine oil has two principle advantages over petroleum oil. It has less tendency to deposit lacquer and coke, and is less likely to evaporate at high temperatures.

8. c
Reciprocating engines require large engine operating clearances because of the relatively large size of the moving parts and high operating temperatures. Therefore, the oil used in reciprocating engines must have relatively high viscosity in order to maintain an adequate film between moving parts.

9. a
The oil selected for aircraft engine lubrication must be thin enough to circulate freely, yet heavy enough to provide the proper film strength at operating temperatures.

10. b

11. b
The flash point of oil is an important characteristic when selecting a lubricant. Flash point is determined by laboratory tests and represents the temperature at which a liquid will begin to give off ignitable vapors. The safest liquids are those that expel ignitable vapors at very high temperatures (high flash point). Therefore, it is desirable for aircraft engine oils to have a high flash point.

12. a

13. a

14. c
Some of the factors considered when determining the proper grade of oil to use in a particular engine include the engine's operating loads, rotational speeds, and operating temperatures.

15. b

16. c
FAR 23.1557 specifies that oil filler openings be marked with "Oil" and the permissible oil designations or reference to the Airplane Flight Manual for permissible oil designations.

17. b
In general, vegetable based oils are chemically unstable at high temperatures, perform poorly at low temperatures, and are unsuited for aircraft engine lubrication. Mineral based oils, on the other hand, tend to be much more chemically stable and suited for aircraft use.

18. a

19. a
EP (extreme pressure) lubricants are intended for use with spur-type gears operating at high speeds and under high pressure loads.

20. a
In an operating turbine engine the oil scavenge pump returns oil from the main bearing galleries to the oil reservoir. However, after an engine is shut down, the oil in the tank tends to seep down to the engine's lower components. This causes the oil level in the reservoir to decrease and give a faulty oil level indication. Therefore, the best way to prevent overfilling the reservoir is to service the oil system within 30 minutes after engine shutdown.

21. b
Most engine manufacturers recommend the use of straight mineral oil for at least the first 50 hours of the break-in period in new or newly overhauled reciprocating engines.

22. b

23. a
The three most common types of oil pumps used on turbine engines are the vane, gerotor, and gear-types. All are classified as positive displacement pumps since they pump a fixed quantity of oil for each revolution.

24. a
Air-cooled aircraft engines rely to some degree on fuel to aid in cooling during high-power operations. Therefore, as a general rule the mixture should be set in the full rich position when operating at or near takeoff power. A rich mixture typically provides more fuel than is needed for combustion, leaving excess fuel to aid in engine cooling.

25. b

26. b

27. b

28. c
In gas turbine engines, fuel and ram air are both used to cool the oil. 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 front of the engine and is similar to those used on reciprocating engines.

29. a

30. b

31. b
In addition to the main oil filters, turbine engine oil systems utilize multiple secondary filters located throughout the system. For example, fine mesh screens called last chance filters are often used to strain the oil just before it enters the main bearing compartment.

32. c

33. c
To ensure proper tank ventilation at all flight attitudes reciprocating engine oil tanks are fitted with vent lines. These lines are usually connected to the engine crankcase and indirectly vent the oil tank to the atmosphere. This indirect venting prevents the tank pressure from rising above or falling below the outside ambient pressure.

34. c

35. c

36. c
The fuel-oil heat exchanger is designed to exchange, or transfer heat from the engine oil to the fuel. This process warms the fuel sufficiently enough to prevent fuel icing.

37. c
According to FAR 33.71, oil filler openings for turbine engines must be marked with the word "oil."

38. a
FAR 33.71 states that pressurized oil tanks may not leak when subjected to their maximum operating temperature and an internal pressure that is not less than 5 psi plus the maximum operating pressure of the tank.

39. a

40. b

41. c
If the oil system pressure should become excessive in a turbine engine, the pressure relief valve would open and direct oil back to the supply pump inlet before the oil reached any moving parts. Therefore, if the pressure relief valve should stick in the open position, system pressure would decrease below acceptable levels and insufficient lubrication be provided to moving parts.

42. b

43. a

44. a
An oil pressure relief valve limits oil pressure to a value specified by the engine manufacturer. If the relief valve should stick closed and allow the oil pressure to become excessive, leakage and damage to the oil system could result. Therefore, high oil pressure should always be limited to the engine manufacturer's recommendations.

45. c
The breather pressurizing system of a turbine engine ensures a proper spray pattern from the main bearing oil jets and furnishes a pressure head to the scavenge system. If the pressure within the bearing housings were allowed to drop as atmospheric pressure dropped with changes in altitude, the flow of oil from the oil jets would change. Therefore, to maintain a relatively constant flow rate the breather pressurizing system maintains a relatively constant pressure within the bearing compartments and oil tank as the aircraft climbs.

46. a

47. a
Some larger reciprocating engines require high oil pressure to force cold oil through the bearings during starting and warm-up. However, after the oil has warmed up, lower oil system pressure is preferred to minimize oil consumption. One way of providing varying oil pressures is with a compensating oil pressure relief valve.

48. c

49. c

50. a

51. b
Reciprocating engines using dry-sump oil systems often have a check valve installed in the oil filter. This check valve is held closed by a light spring load of one to three pounds when the engine is not operating and prevents oil from draining out of the supply tank and into the engine crankcase.

52. a
The factors that affect oil consumption are engine speed, engine temperature, operating clearances, oil condition, and characteristics of the lubricant being used. An engine's mechanical efficiency has little effect on oil consumption.

53. b

54. b
The fuel line for an oil dilution system is never located between the pressure pump and the engine pressure system. The reason for this is that with an oil dilution system, the fuel must be introduced into unpressurized oil so there is almost no chance for oil to flow back through the dilution system and enter the fuel supply.

55. c
Oil dilution systems thin engine oil for cold-weather starting by allowing the pilot to add fuel (gasoline) to the engine oil just prior to engine shut down.

56. c
Initial adjustment on the oil pressure relief valve for a newly overhauled engine is made in the overhaul shop. The adjustment is fine tuned after the engine is installed in a test stand and run. To help eliminate the risk of oil starvation, the initial adjustment should not wait until the engine is run.

57. a
It is important for pilots to know the temperature of the oil just before it enters the engine. Therefore, in drysump lubricating systems, the oil temperature bulb may be located anywhere in the oil inlet line between the supply tank and the engine.

58. a
In most reciprocating engines, the cylinder walls receive oil spray from the crankshaft and crankpin bearings. Some of the oil coming from the crankshaft is also splashed onto the cylinder walls.

59. c

60. c
Some radial engine cylinders and all cylinders in an inverted engine are located at the bottom of the engine. To prevent these cylinders from being flooded with oil and suffering hydraulic lock, extended cylinder skirts are often installed. With these extended skirts, oil that falls into the cylinders is immediately thrown back into the crankcase.

61. b
Oil in an engine is constantly exposed to many harmful substances that reduce its ability to protect moving parts. These contaminants include moisture, acids, dirt, carbon, and metallic particles. Therefore, it is important that the oil be changed at regular intervals.

62. b
A cuno-type oil filter consists of a cartridge consisting of disks, spacers, and a cleaner blade located between each pair of disks. As oil enters the filter it passes through the gaps between the closely spaced cartridge disks, then through the hollow center and on to the engine. The smaller the spacer thickness between the disks the smaller the particles that are filtered out of the oil.

63. a
Some oil tanks have a built-in hopper, or temperature accelerating well, that extends from the oil return fitting on top of the oil tank to the outlet fitting in the sump in the bottom of the oil tank. The primary purpose of the hopper is to separate the circulating oil from the surrounding oil in the tank so the circulating oil warms quickly after the engine is started.

64. a
A flow control valve determines whether the oil passes through or around the oil cooler. When oil is cold, the flow control valve directs the oil around the cooler and back to the tank for circulation. However, when the oil is hot, the flow control valve directs the oil through the oil cooler.

65. a
Some crankshafts are manufactured with hollow crankpins that serve as sludge removers, or chambers. On a crankshaft with sludge chambers, centrifugal motion forces sludge and other foreign material into the sludge chambers during engine operation. This sludge remains in the sludge chamber until the engine is overhauled.

66. c

67. c

68. b

69. a
The scavenger pump in a dry sump lubrication system is responsible for pumping circulated oil from the sump back to the oil tank. However, because the oil thermally expands once it gets to the sump, scavenger pumps must have a greater capacity than the pressure pump to prevent oil from collecting in the sump.

70. b

71. b
Most oil pumps used in aircraft engines provide excessive oil pressure when run at a high rpm and, therefore, a relief valve must be used to maintain a constant pressure. When the relief valve opens, oil is directed back to the oil pump inlet for recirculation. With this type of system, as the clearances between moving parts increase, the pump output remains constant but less oil is returned to the pump inlet by the relief valve.

72. c
The overhead valve assemblies of opposed engines used in helicopters and airplanes are lubricated by a pressure system. In this type of system, pressurized oil flows through the hydraulic tappet body and through hollow pushrods to the rocker arm where it lubricates the rocker arm bearing and the valve stem.

73. c

74. b

75. c
Last chance filters in turbine engines are typically located at the oil jets within the bearing housing. Since bearing housings are not readily available for disassembly and inspection, last chance filters are typically cleaned when an engine is overhauled.

76. b

77. c

78. c
To prevent internal engine damage caused by excessive oil pressure, an oil pressure relief valve is typically installed between the pressure pump and the internal oil system. This way, if the pump output pressure exceeds the recommended system pressure, the relief valve can relieve the excess pressure before the oil enters the engine.

79. a
The temperature control valve, sometimes called the flow control valve, is located at the oil cooler inlet and determines whether or not the oil passes through the oil cooler. When the oil is cold, the flow control valve directs oil around the jacket surrounding the cooler to allow the oil to warm quickly. However, when the oil reaches its operating temperature, the flow control valve closes and directs oil through the oil cooler core.

80. b
The temperature control valve, or flow control valve, is located at the oil cooler inlet and determines whether or not the oil passes through the oil cooler. When the oil is cold, the flow control valve is open and directs oil around the jacket surrounding the cooler to allow the oil to warm quickly. However, when the oil reaches its operating temperature, the flow control valve closes and directs oil through the oil cooler core.

81. a
In most turbine engine oil tanks, a slight pressure buildup is desired to ensure a positive flow of oil to the oil pump inlet. This pressure buildup is accomplished by installing an adjustable check relief valve in the tank overboard vent line. This check valve is set between three to six psig to maintain positive pressure within the oil tank.

82. c
To avoid damage to an engine's internal lubrication system, an oil pressure relief valve is installed after the oil pressure pump outlet that relieves excess pressure by directing some of the excess oil back to the inlet side of the pressure pump.

83. c
If the oil in an oil cooler becomes congealed, the scavenge pump could build up enough pressure in the system to cause damage. To prevent this high pressure from damaging the oil cooler, some engines are equipped with a surge protection valve in either the oil cooler or the oil return line.

84. c
The primary source of oil contamination in a reciprocating engine is combustion by-products that escape past the piston rings (blow-by), and oil carbonizing that occurs when oil becomes trapped in the pores of the cylinder walls and is burned.

85. c
When oil pressure in an engine becomes excessive, the pressure relief valve unseats and excess oil is directed back to the inlet of the pressure pump. If foreign matter causes the relief valve to stick open, oil would continue to be bypassed even when the pressure was not excessive. This would cause a reduced amount of oil to flow to the engine which, in turn, would cause a low oil pressure.

86. c

87. c
In both reciprocating and turbine dry-sump lubrication systems, check valves are installed between the oil tank and the engine. These valves are set at two to five psi, and their purpose is to prevent oil from draining into the engine when the engine is not operating.

88. a
According to the FAR 23.1013, an oil tank must have an expansion space of 10 percent or 0.5 gallon, whichever is greater. In this example, 10 percent of 5 gallons is 2 quarts which is equivalent to 0.5 gallon.

89. a

90. b

91. c
A dwell chamber, sometimes referred to as a deaerator, provides a means of separating entrained air from scavenge oil.

92. c
All bearings require lubrication. However, plain bearings must have oil supplied to them under pressure to prevent metal-to-metal contact.