Oil Servicing: Technical Insights for Owners and Operators
by Alex Best, Aero Gas Turbine Specialist
Talking to pilots over the past few years, I think I’ve spent more time discussing engine oil servicing than any other topic. Perhaps this is because it is one of the few regular maintenance activities that owners/operators are allowed to accomplish on their engines. Let’s face it, most pilots like to talk and everyone will have an opinion. In the following discussion, I will try to dispel a few myths and offer some suggestions based on my experience of engine design and the many discussions I’ve had on this subject. Most of all, I want to explain why replenishing oil to the appropriate level is the right thing to do.
Aircraft operating instructions and engine manufacturers’ publications give a procedure to follow. Typically, this will say to check the oil level at a set period after engine shut-down, often 10 to 20 minutes. If the level is below minimum, then oil should be added to bring the level to somewhere between MIN and MAX. Or if the level is above MAX, some oil should be drained.
Simple? “Not always the case,” I’m told. Other imperatives often make it inconvenient to do the check just after shut-down. The publications provide an alternative to account for this. If the engine has been inactive for several hours or several days, the typical recommendation is to motor the engine then wait for the prescribed time before checking the level. This is great until you get caught in traffic on the way to the airport and you have all those other more important things to do before getting off the ground. So, there is often a temptation to let it slip until the next opportunity, or to take another shortcut. I’ve heard some pilots say, “I don’t bother with the motoring runs, just check the level before starting. If it’s where it normally is – I’m good.”
Some conversations were more disconcerting, such as the long debates on the relationship between oil level and oil consumption. Most revolved around the assertion that a lower oil level would result in lower oil consumption. There may be some truth in this – up to a point. Indeed, an engine serviced to the MIN line, even at maximum specified oil consumption, is designed to provide positive oil pressure throughout the aircraft attitude envelope for much longer than it takes to burn all the fuel in the tanks.
The most disturbing conversations involved assertions such as, “I keep the level at the screw head half an inch below the MIN line.”
“Why?” I would ask. The response generally followed along the lines of someone they knew with decades of experience and thousands of hours had told them that it would reduce oil consumption. “It works for me too, and, in any case, I’ve never had a problem.”
This issue with deliberately under-servicing is that you never know how close you are to the minimum usable oil level – the quantity below which the oil pump starts to suck air rather than oil. Moreover, if a leak starts to develop or there is a slope on the ramp that causes the oil level reading to be optimistic, or a host of other possibilities, deliberately under servicing the oil system will reduce the time to reach a level where oil pressure fluctuation will occur. Why needlessly reduce this margin of safety?
Unlike piston engines, a gas turbine will consume oil at a measurable rate. The speed of rotation of the shafts and internal temperatures precludes the use of contact seals that would prevent ingress of air into the bearing cavities. The radial seals used on main rotor shafts must always run with a clearance. Hence there will always be an ingress of air into the oil system. The necessary job of removing this air falls to the “breather,” which consists of a centrifugal separator mounted on one of the shafts inside the accessory gearbox. Seal designs have improved over the years. The old labyrinth seals of early engine designs gave way to the almost universal use of carbon seals in the 1980s, resulting in tighter clearances and less air ingress. The design of centrifugal separators has improved too. The introduction of fillers such as wire mesh or metal foam materials inside the centrifugal breathers has improved their efficiency. Nevertheless, they remain less than 100 percent effective and the ejected air will contain a mist of fine oil particles.
Monitoring the oil consumption rate will give an indication of the health of air seals as well as helping to identify onset of an internal oil leak. External oil leaks, on the other hand, typically become noticeable long before becoming apparent in oil consumption rates. However, diligent consumption trend monitoring will help in deciding how quickly to address a weeping AGB seal.
Maintaining a reliable oil consumption monitoring program requires some diligence and an understanding of rolling average calculations. Manufacturers’ recommendations to calculate this over a 10-hour interval can be difficult to do and more difficult to interpret. If you need assistance with this, email me at email@example.com and I’ll try to help.
There is an upside to the engine’s propensity to expel oil mist: Continual consumption relieves the requirement for oil changes. The gradual flushing ensures that the quality of the oil is maintained.
Beyond oil consumption trend monitoring, oil condition monitoring techniques aim to give early warning of distress to oil wetted components. Spectrometric Oil Analysis Program (SOAP) and other OEM branded oil analysis programs attempt to identify early stages of component degradation by analyzing particles too small to be captured by the oil filter. Effectiveness is a function of sampling frequency, sensitivity of the analysis method and completeness of the historical data that catalogs the progression of the various possible distress mechanisms. Their usefulness is measured in terms of the time reduction in detecting an issue before it manifests as a chip detector indication or worse. Early detection of a bearing, gear or seal issue is obviously highly desirable. Although the intended design reliability of these components is high enough that the cost-effectiveness of oil analysis programs in terms of reduced maintenance burden is debatable, the peace of mind they provide may be worth the additional costs.
In conclusion, diligent oil servicing is a crucial element in ensuring engine serviceability. A better understanding of how the system operates will help you make appropriate decisions on when to add oil and how much. Consumption trend monitoring can give you confidence that the system is operating normally and help identify the early symptoms of leaks.
I hope this brief article has been helpful. If you have any questions or suggestions, you can email me at firstname.lastname@example.org.
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