A critical step towards a healthy engine
By Alan Baker
Effective equipment condition monitoring is vital to efficient and safe aircraft operation. Costly downtime and potentially dangerous in-flight shut-downs are to be avoided. One way of reducing these problems is to develop methods of diagnosing equipment wear or potential failure through an accurate oil analysis program.
Oil analysis provides a reliable means of checking the elemental content within the oil system and involves taking periodic oil samples.
Concentrations of elements present as a result of a units wear or damage can be determined to an accuracy of 0.01parts per million.
Typically, on arrival at the laboratories, samples undergo a variety of tests. These include analysis of viscosity, dilution, oxidation, and acidity.
Checks on an oil's viscosity, oxidation, and acidity are used to determine whether the oil is compliant with the manufacturer's recommendations on oil effectiveness in the given situation.
Viscosity checks are used to determine the grade of oil in use. Contaminants, such as fuel, hydraulic fluid, or another brand of oil, will alter the viscosity of the original oil. The test involves a viscometer [a capillary tube in an oil bath held at a constant temperature] and a known sample volume of the oil to be tested. The process involves placing the oil into the capillary tube and allowing it to pass through. The flow of the oil between two points is timed by the viscometer and the viscosity determined mathematically.
New and used oils may contain acidic constituents as additives or as degradation products formed during service, such as oxidation products. Oxidation of the oil is therefore a measure of the lubricant degradation in service, and is usually indicated by discoloration or by testing for Total Acid Number [TAN]. The TAN is determined by the use of Ptentiometric Titration and checked against condemning limits which have been empirically established.
The detection and monitoring of constituent elements in oil and hydraulic fluid samples requires highly complex technology.
An atomic absorption test consists of heating the diluted oil or fluid sample so that the ions form a gaseous cloud. Light at the appropriate wave length for a particular element is shone through the cloud. The extent of light absorbed by a certain element in the sample can then be measured to reveal the quantity of that element present.
An alternative method of elemental analysis is the inductively coupled plasma (ICP) test. The ICP analysis involves introducing a sample into an argon plasma induced by high frequency where the temperature can reach 10,000¡C. The sample is drawn from a nebulizer in the form of an "aerosol" into the torch area where it undergoes consecutive decomposition, then atomization. The atoms, excited by the plasma, emit energy in the form of light with varying wavelength characteristics of the elements present. The light then passes through optics and a photomultiplier where it is converted into an electrical signal. The intensity of light transmitted is proportional to the concentration of each of the elements present. This is then passed into a data processing system which interprets the data into a usable form.
The ICP equipment is calibrated before each run using samples with known elemental concentrations. The advantage of ICP analysis is that it enables simultaneous analysis of 23 elements in approximately 3 minutes. Results gained from the ICP are trended on an in-house database which has been developed and programmed with alert levels for specific engine types.
Continuous comparison with limits and rates of change
At the time of adding data to the database, a set of limits specific to the subject unit is loaded, complete with a list of the elements which are considered to be critical for the unit.
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