OIL SERVICING -
TECHNICAL INSIGHTS FOR OWNERS AND OPERATORS.
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 owner/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 the 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 underservicing 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 preclude 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 1980’s resulting in tighter clearances and less air ingress. The design of centrifugal separators has improved too. 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% effective and the ejected air will contain a mist of fine oil particles.
Monitoring 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 alex@alexbest.ca and I’ll try to help.
There is an up-side 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 in 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: alex@alexbest.ca
This article is protected by Copyright and may not be reproduced, stored or published in whole, or in part, without the written permission of Alex Best Consulting.
ENGINE CORROSION –
TECHNICAL INSIGHTS FOR OWNERS AND OPERATORS.
Corrosion is a topic I’ve frequently discussed with operators at conferences and M&O presentations. In this article I will review some of the technical aspects of corrosion as it relates to the materials used in gas turbine engines and the maintenance actions that can be taken to slow its progress. Choosing the right preventative maintenance action and schedule will help avoid runaway overhaul costs, premature engine repair and loss of asset value. And let’s remember that some maintenance cost guarantee programs exclude corrosion.
Why do engines corrode – Can’t the manufacturers prevent it?
Most materials in a gas turbine engine are subject to corrosion of one type or another. The titanium alloys used in cooler sections of the engine being the rare exception due to the incredibly tough oxide layer that forms on the surface preventing further degradation. Fundamentally, corrosion is a chemical reaction in which a manufactured metal returns to its natural oxide state. There is plenty of oxygen in the atmosphere to aid the process when combined with other catalysts such as water, salt or acidic atmospheric pollution. Manufacturers do apply coatings to many metals to combat this process. The effectiveness is a function of the operating environment and maintenance activities carried out.
Magnesium casings that suffer from deterioration of paint to expose the base metal will show visible signs of corrosion quickly. Although the chemistry books say an oxide layer will form and prevent further corrosion, this effect is short-lived in the environment inside the engine. Contact with other metallic components can set up a galvanic cell in which the magnesium is invariable the loser. In the presence of water, an unprotected magnesium surface will be seen to bubble and react almost immediately. (We all remember that experiment from high school chemistry class.) The importance of regular treatment, minor repair and paint touch-up can’t be over-stressed. Repair techniques have advanced over the years. On-wing coating touch up and local epoxy repair to replace material in non-structural areas will get most operators through to scheduled overhaul. When there, repair suppliers can do extensive weld build-up and are normally able to save most casings. Keeping salt and other contaminants away from the magnesium is a key factor. Compressor washes are the first line of defense and application of corrosion inhibitors, such as corrosion X, will have a definite benefit in areas that can’t be accesses on-wing. How frequently? This depends on many factors: First and foremost, geographical area. Engines operated in coastal or highly polluted industrial areas will require more frequent attention. Is the aircraft hangered or parked outside? Non-coastal operators who hangar the aircraft can probably limit with compressor washes to minor inspection intervals.
Aluminum alloys used on accessories and casings will typically have an anodized finish and, compared with magnesium components, do not normally present a significant corrosion concern.
Ferrous alloys (Steel.) Rusting occurs on all ferrous alloys when exposed to a sufficiently corrosive environment. High strength steels intended for use in bearings and gears will start to corrode within minutes if exposed to water rather than the normal coating of oil. Avoiding contamination of the engine oil is essential. The most common cause is allowing compressor wash fluids to enter the oil system by failing to follow the recommended procedures. Condensation allowed to accumulate in an engine left inactive without appropriate preservation can cause similar issues.
Not all ferrous alloys are so sensitive. Different grades of steel will have varying levels of corrosion resistance, in large part depending on the percentage of chromium in the alloy. However, the designation of a steel as stainless does not equate to “will never corrode.” And generally, the grades of “stainless” steel used on casings and external brackets will tarnish and accumulate a surface corrosion with time. As with Magnesium, time to initiate and speed of progress depends in the operating environment and maintenance activities employed.
Nickel alloys used in the combustion and turbine sections, while inert at room temperature and resistant to many chemicals, have their own issues at their typical working temperatures. Sulfidation and thermal and oxidation being the two most relevant to this discussion. Sulfidation is a chemical process that attacks temperature resistant alloys with a high chromium. This is most often seen on the airfoils of turbine blades but can occur in other location. There are two types, operating in two different temperature ranges: Type I sulfidation operates at 1,470 F to 1,740 F (800 C to 950 C) whereas Type II low temperature hot corrosion is prevalent in the range of 1,240 F to 1,380 F (670 C to 750 C,) give or take a few degrees depending on which research you read. Type II is the variety most commonly seen in small gas turbines. The process starts with airborne sea salt (sodium chloride) combining with Sulphur from the fuel to produce sodium sulphate salts (Na2SO4.) If these salts are allowed to accumulate, they will become molten in the sulfidation temperature ranges and leach the chromium content from the blade alloy. With the depletion of the chromium, oxidation attack of the alloy follows quickly. Manufacturers do apply diffused coatings to increase resistance. Diffused aluminum in the surface of the blade material gives good resistance in most applications but it has its limits. Some manufacturers offer an optional platinum aluminide for operators in areas particularly prone to the issue. There will be an incubation period during which the blade coating provides a barrier. Pitting of the coating becomes the first indication of sulfidation attack. Once the coating is compromised the condition will worsen more rapidly as the chromium depleted base material begins to oxidize, bulge and eventually start to flake away. Since sulfidation is temperature dependent, it will be seen in the turbine stages that operate at these temperatures. In larger turbine engines with airfoil cooling, the blades may operate at temperatures above the sulfidation range. In which case the sulfidation may occur farther downstream; in the LP turbine perhaps. The good news, if such exists, is that the sodium sulphate salts are very soluble in water. Therefore, water washing can be effective in removing the salts from the engine gas-path. Think of a desalination wash as one that removes sodium chloride from the compressor and sodium sulphate salts from the turbines.
Washing a little more frequently than you think is necessary will do no harm. Every little helps. Provided it is done properly.
Care needs to be taken when carrying out compressor and turbine washes. Allowing water or cleaning products to enter the engine oil system will cause gears and bearings to rust very rapidly. A prime culprit is failing to properly seal breather discharge lines that exhaust into the bypass duct. If this occurs it must be addressed immediately by oil system flushing. Otherwise rust will start to form and within hours and progress rapidly to a point where there is no alternative but to remove the engine. Manufacturers manuals will indicate which openings into the engine need to be sealed prior to the wash. So, ensure that the recommended steps are followed.
It is also good practice to ensure the engine is dried after the wash. Run it for a few minutes to blow out any water. Heat dissipated after shut-down will also help to ensure everything is dry before installing the covers. Engines with abraidable coatings on blade paths are particularly at risk from a slipshod compressor wash job. If wash water pools, and the engine is inactive for a week or two the residual water can soak into the porous plasma coating. (I’m talking about the grey powder metallic coatings on the Boost, IP and HPC areas, not blue or black material surrounding the fan blades.) Corrosion will subsequently cause it to swell, become embrittled and delaminate from the casing. Swelling can reduce the tip clearance to the point that contact between the casing and the blade tips lock the rotor.
How do I avoid additional maintenance cost?
Even if you have the Gold maintenance cost protection plan, check the fine print. Many programs have clauses that exclude corrosion. Originally intended to ward off grossly negligent maintenance practices, eager customer reps may use this to try boosting their sales margins by adding a charge for any parts rejected with the word “corrosion” on the tag.
In summary:
- Understand your operating environment and act accordingly.
- Take the appropriate precautions when washing.
- Always make sure the engines are thoroughly dried after a compressor wash. Start them up and run for a few minutes to get them fully dried out before putting the covers on.
- Have the maintenance techs check the engine maintenance manual for procedures to apply a barrier type corrosion inhibitor such as Corrosion X.
- Be aware of engine preservation recommendations if the aircraft will be inactive.
At the end of the day, some corrosion is inevitable but reasonable care will help avoid unnecessary component rejection at overhaul. A good record of the maintenance activities will help ward off additional charges from your program provider at the time of overhaul. If you need help, let me know.
I hope this has explained some of the science behind the terminology allowing you to make more informed decisions that ultimately save money and protect your investment. If you have any comments or questions please feel free to email me.
Alex Best
This article is protected by Copyright and may not be reproduced or published in whole, or in part, without the written permission of Alex Best Consulting.
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