Lubrication System


Lubrication Systems:

  • Lubrication systems consist of either a wet-sump or Dry-sump system
  • The difference between the two systems can be remembered as if the engine were off
    • Wet sump systems maintain oil in reservoirs integral to the engine while dry sumps do not, leaving the sump "dry"
  • 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
    • Pilot Handbook of Aeronautical Knowledge, Wet-Sump Oil System
      Pilot Handbook of Aeronautical Knowledge, Wet-Sump Oil System
  • 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
    • When checking the oil level on an dry-sump engine that has been sitting idle, readings may be inaccurate unless rotated (or "burped")
      • Refer to your POH
  • The oil filler cap/dipstick used to measure oil quantity is usually accessible through a panel in the engine cowling [Figure 2]
  • 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

Heat Management:

  • A critical purpose of engine oil is to help manage cooling
  • This occurs by cool oil moving through warm areas, picking up the heat, and dissipating it through a radiator

Oil System Gauges:

  • Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge
    Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge
  • 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]
    • It is usually measured after passing through the oil cooler
    • 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

Oil/Lubrication Malfunctions/Emergencies:

  • Oil consumption depends primarily upon the efficiency of the seals
  • Oil can be lost through internal leakage, and, in some engines, by malfunctioning of the pressurizing or venting system
  • Increases in oil temperature are not always associated with a drop in oil pressure, nor a rise in CHTs
  • Low Oil Pressure:

    • Low oil pressure can be caused by an oil leak which leads to lack of oil in the system, or an ineffective oil pump
    • These emergencies can be particularly detrimental when flying an aircraft utilizing a constant-speed propeller
    • Low Oil Pressure Primary Indications:

      • Oil pressure will indicate low
    • Low Oil Pressure Secondary Indications:

      • Rising Cylinder Head Temperatures (CHT)
      • Oil temperature may rise (if the pressure drops rapidly then it is less likely you will have a corresponding temperature indication
      • Rough engine indications
  • Low Oil Temperature:

    • Primary Indications:

      • Oil temperature will indicate low
    • Secondary Indications:

  • High Oil Temperature:

    • High Oil Temperature Primary Indications:

      • Oil temperature will indicate high
    • High Oil Temperature Secondary Indications:

      • Other temperatures will indicate high
      • Possible smoke
      • Low oil pressure
      • High RPM
    • High Oil Temperature Considerations:

      • Open cowl flaps, if equipped
  • Oil Leaks:

    • Oil on windscreen may come from engine or propeller

Oil System Inspections:

  • Inspecting the oil system is not a routine check, but rather a detailed review of aircraft engine health
  • Oil Inspection:

    • Pilots should inspect the oil as part of the pre-flight checklist
    • Inspecting the oil is critical in determining the quantity and condition of oil
    • Oil should never go below the Pilot Operating Handbook specified minimum quanty
    • The color of the oil is a reflection of oil age and engine health
      • New, clean oil should be light in color
      • Dark oil is generally the result of contaminates and oxidation after many hours of operation
        • Too dark is a judgement call based on the amount of hours of use
        • Dark oil may be due to bad piston seals
    • Larger oil sumps tend to absorb contaminents better as they hold more in suspension
      • Smaller sumps should therefore be changed/drained more often
  • Oil Filter Inspection:

    • It is never required, but pilots who own their airplane may elect to inspect the oil filter for tell-tale signs of wear
    • Oil filters are designed to bypass, if clogged, but are only meant to last for the life of the oil
      • Oil filters should be changed with every oil change
      • Changing an oil filter often also allows for consistent engine health trend analysis

Lubrication System Airman Certification Standards: