27. 09. 2023.

We lubricate your aircraft: Oil 

It is not a secret that preventive maintenance is the key to keep an aircraft safe and airworthy while also reducing operating and replacement costs. Choosing suitable oil and right oil maintenance are absolute musts for prevention. As simple as it seems, there are often misconception and confusion about what type of oils should you use, how often the engine should be serviced and what best practices can you use. We gathered the fundamentals of oil maintenance that will help you navigate in this area. 

certification.  

Using an oil which is approved to the specifications stipulated by your engine manufacturer is pretty evident. Unfortunately, some of the oils on the market do not hold approval. That may make them cheap, but those oils have not undergone the rigorous testing carried out by competent authorities necessary to gain approval. The problems occur when you refer to your engine manual and it advises the use of oils approved to that specification X. We can’t think of a quicker way to void your engine warranty than by using oil not approved for that particular specification or generally anything than recommended approved aviation lubricants. In such case, if you experience mechanical problems, you will pay the price. Literally. aps offers a great variety of aviation oils from all leading manufacturers that are fully approved. 

viscosity. 

The primary function of oil is as simple as it can get: an engine oil should separate moving metal parts within the engine. However, there are trade-offs when selecting lubricant viscosity. If you choose an oil that is too thin, you will risk increased wear of engine parts and potential metal-to-metal contact in engine parts such as journal bearings, which require hydrodynamic lubrication. If the lubricant film is too thick, it can lead to increased fluid friction or “drag”. As a result, power requirements increase, and you end up using more fuel and overheating causing accelerated oxidation of the oil. Make sure you are using the correct oil viscosity fitting the engine’s ambient operating temperature. Forget all notion of “that will do too”. 

oil changes. 

By getting dirty, oils keep your engine clean. This is what they are designed for. This only works though if you change the oil when you are supposed to. Depending on what kind of oil you use, you might experience differences. If you use semi-synthetic blend f.e. you might observe slightly lower oil pressure because the oil flows more easily, and slightly lower oil temperatures on the gauge due to properties reducing heat. Higher time engine might also notice a slightly higher engine consumption rate due to blow-by because the oil flows better than straight grade products.  

Firstly, keep the recommended interval. In the case of an engine with full-flow filtration, this means every 50 hours or every 4 months (whichever comes first). Without full-flow filtration, you should change the oil every 25 hours. Secondly, allow for oil temperature to stabilize after engine shut down but change oil when it is still hot since water and contaminants settle to the bottom of cold oil. If you drain when it is cold, you probably compromise the fresh oil. You will achieve better results and remove a higher percentage of the contaminants when the engine is fully warmed before you drain it. Thirdly, change the oil filter when you change the oil. Always. Otherwise, you could contaminate that lovely fresh oil with the old dirty one. Additionally, filter life has been reduced by the first drain interval and may plug or fail during the next interval, leaving the engine unprotected.   

temperature & consumption. 

For an effective prevention, we recommend you to monitor the oil temperature and oil consumption. Maintain the temperature in the 80-85 °C range during your light. This range will allow moisture that has accumulated in the oil on the ramp to boil away during flight. Keep in mind: oil temperatures that are “in the green” are not necessarily hot enough to boil moisture away. Take a look at the engine temperature gauge for accuracy. If the gauge is marked, it should read approximately 100 °C when the probe is placed in boiling water. If your gauge is not marked, a good practice is to mark your oil temperature gauge with a reference mark at 80-85 °C. 

In turbo-charged engines, high temperatures are often a concern. So, if engine oil temperature is significantly above 85°C check baffles to make sure there is good airflow to cool the cylinders. Keep an eye on CHT, EGT, and lean the engine appropriately while flying. You should also monitor the oil consumption. Some oil consumption is good. It indicates that the oil is providing a seal at the compression ring. If engine oil consumption exceeds the oil consumption test limits as defined by the OEM, then it’s too high and there may be a problem with an unseated or broken ring. Then oil-soaked carbon forms at a fast rate. At the same time, the presence of oil in the combustion chamber has the effect of lowering the octane rating of the fuel. Operating temperatures go up. We have now set up conditions inviting detonation and/or preignition. Unless there is a problem, engine oil consumption should stabilize and remain somewhere in the range of between 0.08 quarts per hour on the low end, and the oil consumption test limit should be as defined by the OEM on the high end. Use straight mineral oil in new engine until the oil consumption stabilized. Check the oil consumption continuously until you have determined the engine is constantly using in a known number of hours. This period should not exceed 50 hours of operation. 

oil analysis: professional or DIY. 

Small amounts of wear in your engine is normal. This wear metal appears as minuscule amounts of metal held in suspension in the oil. These particles are much too small to be removed by your oil filter and they are also harmless. However, those particles in used oil can help you discover engine problems before they become severe. The oil sample can be analysed by a laboratory using a very sensitive technique and particles can be detected down to levels of less than one part in every millions of oil. The so-called spectrographic analysis can not only detect wear metals, but also water content, fuel dilution, acid content of the oil and other characteristics.  

So, what are your benefits?    

  

• The level of microscopic wear metal present in used oil will normally increase if a component starts to wear excessively. This will normally occur prior to any particles being present in the filter and is a good way of predicting a failure. 

• The data can also can be an indication that the engine is not set up correctly or a clue that the engine is being operated incorrectly. For example, high aluminium content can indicate improper engine warm-up prior take off; high silicone value points to filter problems; high chrome content is a sign of excessive piston ring or cylinder bore wear. And so on. 

• Oil analysis has also means of assessing the condition of the oil. As oil is used in an engine several things happen to it to cause it to degrade. Once the anti-oxidants have been consumed, the oil itself will start to oxidise resulting in improper lubricating properties. clues as to whether the correct oil choice has been made, whether the oil has been thermally stressed in use, and may even be a sign that the oil change period needs to be reduced. 

• The viscosity characteristics of an oil can also be analysed and give some indication of how effective the oil is in an engine. 

This is a very powerful tool for preventive maintenance. You cannot only reduce maintenance costs, you can also prevent catastrophic outcomes caused by in-flight engine failure. Your engineer knows which part he needs to replace before he dismantles the engine, so the labour costs are reduced, and he also needs to replace just one part rather than the whole engine as might be the case if it failed. You also have the satisfaction of knowing that you have a reliable engine which is less likely to stop working when you need it most.  

How to collect the oil sample? 

• Fill the sample container mid-way through your drain! This will ensure you get a representative sample rather than the dirt on the bottom of the pan. (Keep in mind to do the drain when the engine is warm before all the dirt settles on the bottom of cold oil). 

• Be consistent! You will get the most reliable results when your oil change and sampling interval are consistent. Trends take to develop. The absolute values or quantity of deposit is less important than how the values have changed from previous samples – every engine will have a different level of what is normal and how this change over time is the important factor. Therefore, we recommend a long-term sampling-program rather than one isolated oil analysis. 

• Wear protective clothing and gloves when dealing with used oil! Tt can be carcinogenic and therefore represents a health hazard. 

DIY oil analysis Even if you decide not to involve a laboratory, it is a good idea to do some basic analysis yourself. Rather than throwing the old oil filter away (or just cleaning it if you have a screen filter), take a look at it first. This method will not be as accurate as a professional analysis if you know the normal level of deposition for your engine, and what types of particles are normally there when the oil is changed, then you may notice a change when things start to go wrong. When doing a DIY oil analysis use the proper tool to open the filter! Do not cut it open using a saw as the metal produced by sawing will contaminate the filter and give a false indication of deposits. There are tools available that work rather like a can opener and do not produce any metal swarf. If you need such tools, ask your aps | Contact for advice.

We lubricate your aircraft: we can provide you lubricants of all popular brands »  

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