Selecting Your Turbine Engine Oil

Turbine engine oils, sometimes referred to as jet oils, turbo oils or abbreviated to TEOs, are speciality lubricants designed to support the operation of turbine engines. First developed during the 1940’s, turbine engines can now be seen on almost all commercial aircraft currently in operation. While aviation is the main market place for these oils, there are other applications, especially in the power industry, where turbine engines are used.

Unlike piston engines, that convert an up and down movement from a piston into motion, turbine engines combine compressed air with the chemical reaction from the fuel within the engine to drive fan blades. This means that turbine engines reach much higher temperatures, and this has driven innovation in mineral TEOs, and subsequently synthetic TEOs.

Why do we need Turbine Engine Oils?

There are three main functions of a TEO. Firstly, they lubricate the engine, including the gearbox, shafts and associated accessories, protecting them from damage and ensuring smooth operation while running.

In addition to this, the various additive packages protect the components from corrosion, and filter contaminants in the engine such as carbon deposits formed from a process known as coking.

Finally, and probably the most important benefit from a TEO is their use as a heat transfer fluid, cooling the engine in temperatures reaching 1000°c, in turn extending the life of the oil up to 25,000 hours of operation.

Turbine Engine Oil Applications

The engine is the most expensive single piece of equipment on an aircraft. However, each engine system is made up of various parts, and due to this there are different turbine engine oils available depending on the area of application:

Auxiliary Power Unit (APU) – due to the reliability requirements on cold-soak startup at high altitude, 3cSt oils are often preferred here;
Fuel and Oil Circuit preservation – when the engine is removed from the wing for storage and some maintenance activities, 3cSt mineral oils are preferred.

In addition to traditional fixed wing commercial aircraft applications, turbine engine oils can also be used in:

Aero derivative gas turbines (ADGT) – used to power infrastructure such as hospitals, factories and plants;
Helicopters and gearboxes – often a high load carrying oil is used for this application.

Specifications and Approvals for Turbine Engine Oils

Two main specifications govern turbine engine oils on modern, fixed wing, commercial aircraft: MIL-PRF-23699 and SAE-AS-5780.

MIL-PRF-23699

The original US Military specification, this has been in existence for over 50 years and was formerly known as MIL-L-23699.

This specification is divided into two further subcategories, Type STD - Standard and Type HTS – High Thermal Stability, defining differences in high temperature performance and thus their ability to reduce coking – the formation of carbon deposits within the engine.

Turbine engine and wing of plane on tarmac

SAE-AS-5780

Innovations in engine design, including the Trent 1000 by Rolls Royce, created further temperature problems that the MIL-PRF-23699 HTS grade could no longer adequately cover. This coincided with a shift in focus from military specifications, to commercial ones, namely those developed by the Society of Automotive Engineers (SAE).

Their SAE-AS-5780 specification provides a more stringent framework for testing of such oils including their thermal stability, lower coking properties and improved load carrying capacity.

These oils are again categorised under two different subheadings – standard performance and high performance.

It is important to note that receiving an SAE or MIL qualification does not complete the approval process, and separate approval should be sought from the engine manufacturer, through either engine development projects or a complete flight evaluation.