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Lubrication

Introduction:

  • Oil has several functions:
    • Lubricates moving parts in order to reduce friction
    • Creates better seals between cylinder walls and pistons
    • Cools the engine by reducing friction
    • Removes heat from cylinders
    • Carries away contaminants
    • Runs accessories, depending on the aircraft

Oil Systems:

  • Two Systems:
  • The difference in the systems can be remembered as if the engine were off
  • If it didn't have oil in the engine, it would be dry or a dry sump system, where a separate tank was used; but if the engine would be wet with oil, then it is a wet sump system with the sump being integral to the engine
  • The oil filler cap and dipstick are usually accessible through a panel in the engine cowling [Figure 2]
    • The dipstick is used for measuring engine oil quantity
  • If the quantity does not meet the manufacturer's recommended operating levels, oil should be added
  • The type of oil required may vary on numerous atmospheric and operation conditions, as stipulated by the aircraft operations manual [Figure 1]
  • The AFM/POH or placards near the access panel provide information about the correct oil type and weight, as well as the minimum and maximum oil quantity
  • System is monitored through pressure and temperature gauges [Figure 3]
  • Cessna 172N POH, Oil Grade Required
    Cessna 172N POH, Oil Grade Required
    Cessna 172N POH, Oil Grade Required
    Cessna 172N POH, Oil Grade Required
    Pilot Handbook of Aeronautical Knowledge, Checking Engine Oil Level
    Pilot Handbook of Aeronautical Knowledge, Checking Engine Oil Level
    Pilot Handbook of Aeronautical Knowledge, Checking Engine Oil Level
    Pilot Handbook of Aeronautical Knowledge, Checking Engine Oil Level
  • The loss of engine oil pressure would lead to engine vibrations, RPM would decrease, and the engine would eventually seize
  • Viscosity:
  • the ability of a liquid to resist flow
  • Wet-Sump:

    • Oil is carried in a sump, which is an integral part of the engine [Figure 2]
    • The main component is the oil pump, which draws oil from the sump and routes it to the engine
    • After the oil passes through the engine, it returns to the sump
    • In some engines, additional lubrication is supplied by the rotating crankshaft, which splashes oil onto portions of the engine
  • Dry-Sump:

    • Oil is contained in a separate tank, and circulated through the engine by pumps
    • These tanks are always larger than the oil it is meant to contain to compensate for thermal expansion
    • An oil pump also supplies oil pressure in a dry-sump system, but the source of the oil is located external to the engine in a separate oil tank
    • After oil is routed through the engine, it is pumped from the various locations in the engine back to the oil tank by scavenge pumps
    • Dry-sump systems allow for a greater volume of oil to be supplied to the engine, which makes them more suitable for very large reciprocating engines
    • Most jet engines will consist of a dry sump design
Pilot Handbook of Aeronautical Knowledge, Wet-Sump Oil System
Pilot Handbook of Aeronautical Knowledge, Wet-Sump Oil System
Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge
Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge

Oil System Gauges:

  • Oil Pressure Gauge:

    • The oil pressure gauge provides a direct indication of the oil system operation [Figure 3]
    • It ensures the pressure in pounds per square inch (psi) of the oil supplied to the engine
    • Green indicates the normal operating range, while red indicates the minimum and maximum pressures
    • There should be an indication of oil pressure during engine start
    • Refer to the AFM/POH for manufacturer limitations
  • Oil Temperature Gauge:

    • The oil temperature gauge measures the temperature of oil [Figure 3]
    • A green area shows the normal operating range and the red line indicates the maximum allowable temperature
    • Unlike oil pressure, changes in oil temperature occur more slowly
    • This is particularly noticeable after starting a cold engine, when it may take several minutes or longer for the gauge to show any increase in oil temperature
    • Check oil temperature periodically during flight especially when operating in high or low ambient air temperature
    • High oil temperature indications may signal:
      • Plugged oil line or cooler
      • low oil quantity (possible engine failure)
      • Defective temperature gauge
    • High oil temperatures can lead to metal on metal contact as viscosity decreases
    • Low oil temperature indications may signal improper oil viscosity during cold weather operations

References: