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Turbine Engine Oil Monitoring

We have seen how to select the appropriate Turbine Engine Oil (TEO), but selection is only the beginning. These oils come under huge amounts of strain due to the high temperatures experienced in flight, and this can lead to degradation that needs to be monitored during an oil’s life.

Sources of Degradation

Contamination can occur through a number of scenarios:

  • Hydraulic Fluid contamination
  • Fuel contamination
  • Water contamination
  • Solvent contamination
  • Airborne Debris contamination from solids or runways

Turbine engine oils degrade in several ways, including through thermal, oxidative, and hydrolytic decomposition, improper storage and contamination. Thermal and oxidative decomposition give off a burnt smell and a darker than normal colour, while the oil’s viscosity also increases. In hydrolytic decomposition caused by moisture and heat, the oil gives off a pungent odour and a high total acid number (TAN), while viscosity remains normal.

A darker colour oil is not necessarily a sign of degradation or contamination. However, if this is combined with one of the following additional characteristics, this could indicate an issue:

Dark/Black Oil with...ViscosityTANPossible Cause
No OdourNormalNormalCell
Burnt OdourHighHighOxidation
Rancid OdourNormalHighHydrolysis
Fuel OdourLowNormalFuel Contamination
Hydraulic OdourHighHighHydraulic Fluid Contamination
High Oil AdditionsNormalNormalHigh oil consumption or high breather pressure
Turbine engine close up

Time to Take Action

Contamination or other degradation should not be ignored. It is recommended that users:

  • Take a sample immediately (using a proper procedure and clean equipment). 
  • Drain and flush the oil system and save oil samples before and after this action for future analysis. 
  • Change oil and the oil filter prior to resuming operation of the equipment. 
  • After resuming operation, take a sample as soon as possible. This last step is important because it can be used to confirm that contamination has occurred and to determine the source of contamination. It is important that you keep a clean supply of sample bottles available in case such contamination occurs. The sample bottles must be clean to obtain accurate test results.
The above is useful if there is the suggestion that depredation has occurred, but prevention is always best practice to reduce the risk in the first place:
  • Using placards to identify all turbo oil filling points.
  • Keeping any other fluids away from filling locations.

  • Not using empty drums/pails as shop containers or to hold used fluids.


When servicing equipment that uses turbine engine oils, OEM and airline maintenance manuals are always the controlling documents of how to proceed, and should be followed ahead of any information in this guide. However, the following points can still be useful: 

  • Water - Accumulated water cannot be seen on sight gauge. Water can reduce/prevent oil flow to wick-lubricated bearings.
  • CSD and IDG – Overfilling can result in overheating the oil. Read the gauge carefully if a sight gauge is available or follow pressure-fill procedure carefully if no sight gauge. CSD’s and IDG’s are also more sensitive to water contamination.
  • Starters - Cold starts are stressful to oil meaning oil should be drained and refilled more frequently.