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.
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.
Contamination can occur through a number of scenarios:
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... | Viscosity | TAN | Possible Cause |
|---|---|---|---|
| No Odour | Normal | Normal | Cell |
| Burnt Odour | High | High | Oxidation |
| Rancid Odour | Normal | High | Hydrolysis |
| Fuel Odour | Low | Normal | Fuel Contamination |
| Hydraulic Odour | High | High | Hydraulic Fluid Contamination |
| High Oil Additions | Normal | Normal | High oil consumption or high breather pressure |
Contamination or other degradation should not be ignored. It is recommended that users:
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:
