There are simple truths that have withstood the test of time: airlines will always look at ways to increase efficiencies, both in terms of cost as well as the required level of aircraft maintenance.

In recent years, for example, airlines have increasingly turned towards fleet harmonisation¹—transitioning to fewer aircraft models in order to simplify operations and crew training, reducing cost and maintenance inefficiencies.

This efficiency drive has been heightened by the Covid-19 pandemic. Some industry analysts are now predicting a decade of smaller fleets, with the potential for fewer direct and less frequent routes, which has in turn driven down the production of new aircraft.²

As the industry plays its part in economic recovery and passenger numbers rebuild towards pre-pandemic levels³, one way in which efficiencies can be made is by choosing the right turbine engine oil (TEO), a relatively simple decision that can help optimise maintenance and complement larger operational shifts including fleet harmonisation.

"What we are observing is that effective maintenance is becoming an increasingly critical consideration for airlines, sitting at the intersection between internal drives for efficiencies across fleets and the challenging external industry conditions resulting from the pandemic," Saskia Boeve, GM of Shell Aviation Lubricants, explains.

"Minimising aircraft downtime and unscheduled maintenance is becoming even more vital."

Modern engine challenges

As innovations in airframe and aircraft engine technology enable aircraft to fly further, the requirements for them to run hotter and last longer have increased.

TEOs lubricate and protect the bearings and gears and act as a coolant, cleaner and vibration damper for engines, helping to increase their performance. Turbine oils are highly regulated to ensure that they enable the safe running of engines, although these standards have made TEO development lengthy and costly and only a handful of developers remain.

"Whilst choosing the right TEO might seem a relatively routine decision in the context of maintaining an entire fleet of aircraft, its importance cannot be understated," Boeve notes.

"Using a TEO that is not best suited to either the engine type or the operating conditions it will be used in has the potential to result in a significant impact on engine health."

Selecting the incorrect oil may result in two maintenance challenges: coking and elastomer compatibility.

Coking is the build-up of unwanted carbon deposits, formed when oils break down under high temperatures. If preventative measures are not applied, coking can be costly and unsafe.

Carbon deposits can block oil filters and pipes. In extreme circumstances, blocked filters can lead to oil starvation or carbon in the oil being fed to the bearings, resulting in bearing failures. Blocked oil scavenge tubes can also lead to engine fires.

Elastomer compatibility is the other challenge; some antioxidants can provide good thermal stability and low coking propensity, but some can be aggressive to certain elastomer materials used in some engine oil systems.

If an elastomer is incompatible, cracking or deterioration of rubber 'O' rings can form, leading to high oil consumption and a maintenance headache.

Choosing the right TEO

To develop the lubricants solutions that meet the evolving requirements of the commercial aviation industry, Shell has put a focus on collaboration and partnering with others.

Back in 1939, Shell worked with Frank Whittle when he developed the game-changing jet engine, with Shell's contribution vital to his success.

"For more than a century our strong alliances with OEMs have enabled us to develop lubricants that provide all-round protection and performance, and which meet evolving customer demands," says Boeve.

"AeroShell Turbine Oil 560 (ASTO 560) is one of the outcomes of our collaborative approach. The result is a versatile turbine oil proven to deliver cleaner engines, more reliable operational performance, and longer time between overhaul on some engines."

ASTO 560 provides thermal stability, thus reducing coking propensity, and is additionally fully compatible with elastomer materials in the engine. It therefore achieves the desired balance for an additive system.

It also achieves lower oil consumption. A study of 10 aircraft from one of Shell's customers that switched to ASTO 560 found that average oil consumption over a three-year period was reduced by 10-15% compared to other TEOs they were previously using.⁴ This further helps to reduce the cost of operations.

Simple decisions at a challenging time

This past year has been a difficult reminder of the reality that when it comes to grounded aircraft, time is money.

As the industry continues to focus on optimising fleets and saving on costs where possible, the importance of good maintenance practices has never been more relevant.

"Reducing operational costs is perhaps more crucial now than ever before," Boeve concludes.

"Airlines are already making great strides towards optimising their fleets, and simple maintenance considerations like choosing the right TEO can be an easy way to protect both their aircraft and their bottom lines."

^{References
1See, for example: IAG details harmonisation efforts across A320 fleets, The Air France KLM-Group takes a next step in optimisation of the long-haul fleet,
2Oliver Wyman gauges covid’s damage to future airliner fleets
3https://www.iata.org/en/iata-repository/pressroom/presentations/outlook/
4Based on data from a study of 10 aircraft from a Shell Aviation customer who switched to ASTO 560, with oil consumption measured over a three-year period.}