Monitoring Degradation in Aircraft Lubricants

Equipment failure in any industry has negative impacts. Within the aircraft industry, the consequences and risks associated with failure can triple. However, if military operations are now added to the mix, these risks of failure have increased exponentially! In this blog, we take a look at ways of monitoring the aircraft lubricants used in the Royal Netherlands Navy and which of the lab tests can accurately assist in trending ongoing degradation.

Understanding Jet Engine Lubricants

Jet engine lubricants which comprise mainly of polyol ester base oil can easily oxidize. However, these usually have primary oxidation inhibitors such as phenols, aminophenols, aromatic amines and / or secondary oxidation inhibitors such as sulphurized phenols and phosphites. Typically, primary oxidation inhibitors react with free radicals to prevent polymerization. In these reactions, aminic inhibitors are active at higher temperatures when compared to phenolic inhibitors but they can act synergistically if they are combined.

In this case, the 0-160 jet lubricant (Defence Standard 91-100/2 & MIL-L-85734) is polyol ester based blended with phenothiazine antiwear agent and aromatic amine (dioctyldiphenylamine – DOPDA) antioxidant. It is important to firstly determine the composition of the lubricant before trending the degradation patterns. Since the additives have been identified, their concentrations can easily be charted as the lubricant degrades which makes it easier to determine the rate of degradation. This is shown in the RULER graphs below for a normal used sample and an abnormal used sample.

Figure 1a + b – RULER™ graph O-160 standard (normal used oil vs. abnormal used oil)

After some testing, the following was observed;

  • The Phenothiazine (antiwear agent) depletes first and the depletion rate increases with increasing temperature.
  • The depletion of the Phenothiazine is accelerated with the presence of copper and depletes less in the presence of iron.
  • The aromatic amine antioxidant has a slower depletion rate than the Phenothiazine. When the Phenothiazine achieves a low concentration (20%), the aromatic amine begins to deplete as well.

It can be concluded that as the oil reaches 20% of the Remaining Useful Life (RUL%), the antioxidant becomes ineffective which leaves the base oil open for oxidative degradation. Additionally, when an oxidation test was performed at 250°C, it was noted that the TAN and viscosity levels reached critical limits after 2.5 to 3.5 hours. It should also be noted that the RULER test gave the first indication of degradation before the TAN or viscosity levels began to show changes.

Figure 2: Aircraft Turbine Oil Effective Life, Ruler %, Volatility, Viscosity, TAN v Test Duration


The role of RULER in Detecting Failures

In this field trial at the Navy’s base, we took a closer look at the monitoring of the Lynx Helicopters equipped with 2 RR GEM-42 turbine engines. In these engines, a total of 8 liters of lubricant (0-160 type – Defence Standard 91-100/2) is circulated over the main shaft bearings followed by the reduction gearbox of the engine. Fresh oil is topped up regularly where the rate of top up varies from engine to engine. The top up rate for 6 engines was between 0.02 to 0.2 l/h giving us an average of 0.05 l/h.

One of the issues reported by the site included single engine situations caused by planet gear bearing failures of the engine reduction gearbox. This failure creates a larger oil flow due to the bearing damage which is observed as an oil pressure loss by the helicopter crew.

After testing 20 helicopter engines it was established that a top up rate of 0.05 l/ h would be considered the average rate, any top ups below or above this value will be considered less or more than the average respectively. Overall, all engines showed a quick and continuous depletion of phenothiazine additive (antiwear, Add#1) after 125 hours, which stabilizes between 150-200 hours of operation. For the Low oil consumption engines (typically 0.03 l/h), there is a continuous additive depletion of the phenothiazine for 370 hours until the minimum value. However, with the top ups, the additive value stabilizes at 1100 hrs. When this antiwear additive is low, there is an increase in the presence of silica which decreases as the antiwear is replenished.

On the other hand, for engines with high oil consumption (typically 0.11 l/h), for a period of 150 operating hours, the concentration of both additives are high. However, between 1800-1900 hours (on one particular high oil consumption engine), the concentration of the phenothiazine reduces to a RUL rate below 20% and the aromatic amine begins to deplete faster. One reason behind this abnormal additive depletion was the damage of a labyrinth seal of one of the main bearings in the hot section of the engine which resulted in hot air suction and contact with the lubricant and degrading it significantly.

The trend of the aromatic amine antioxidant depletion is dependent on the engine’s performance. However, there is a slight differentiation where a normal, stable oxidation process is seen in the engines with high top up rate (RUL varying between 95-105%). On the other hand, engines with low top up rate experience a stable RUL % for the aromatic amines in the first phase then a continuous decrease in the second phase.

Undoubtedly, it can be concluded that an average oil consumption of 0.05 l/h should provide sufficient protection of the engines by the lubricant. If there is a variation from the normal depletion trend, this could be as a result of higher operating temperatures (hot spots), higher friction, reduced oil flow or the lubricant coming into contact with the hot gases (labyrinth seal defect). These conditions typically result in the rapid depletion of antioxidants.

In this case, temperatures in the planet gear bearings can exceed 150°C which directly impacts on the antioxidant concentrations. This part of the engine remains at risk for being a hot spot which can result in oil degradation. Period replenishment (at every 100 hours) will lead to a higher level of oil consumption. Hence, it is suggested to us a combination of periodic RULER measurements and selective monitoring. RULER analysis can be used as a measure of quality control for the incoming oil batches. Additionally, samples can be taken every 50 hours to trend the antioxidants as well as the current SOAP (Spectrophotometric Oil Analysis Program) and DEBRIS program.

Through the use of the RULER technique, it was easy to monitor the degradation of these aircraft lubricants and predict their failure to prevent negative impacts on the organization. Although regular spectrophotometric analyses were used initially, they did not provide as much detail or advanced knowledge of impending failures. In essence, RULER can be used to help determine the trend of the depletion of the antiwear and antioxidant additives in polyol ester based lubricants used in aircraft turbine engines.