• 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
  • 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
    Figure 1: Cessna 172N POH, Oil Grade Required
    Pilot Handbook of Aeronautical Knowledge, Checking Engine Oil Level
    Figure 2: 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


  • 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


  • 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
Figure 3: Pilot Handbook of Aeronautical Knowledge, Wet-Sump Oil System

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

Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge
Figure 4: Pilot Handbook of Aeronautical Knowledge, Oil Temperature and Pressure Gauge

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