Spectrometric analysis of aircraft engine piston oil

14.06.2024

The main features required of aeronautical reciprocating engines include lightweight construction, adaptation to continuous operation in varying positions and altitudes, operational reliability or high propulsive efficiency. Due to these characteristics and operating conditions, these engines are subject to environmental corrosion and wear of moving (lubricated) engine components. These are naturally occurring processes that affect the continued serviceability of the engine, including its components.

Aircraft reciprocating engine and its loads

Whether an engine is used regularly, operated intermittently or stored, the function of its components deteriorates anyway. The use of engines in more heavily loaded applications, such as aircraft designed for basic training, parachute jumping or glider retrieval, can result in faster and increased wear and tear on engine components.

Both the quality of the parts and accessories from which the engine is built, the proper servicing of the engine (inspections, overhauls) in accordance with the guidelines and recommendations contained in the manufacturer’s technical documentation, as well as the use of appropriate tools, have a direct impact on the life and state and condition of the engine.

One additional ‘tool’ for monitoring engine health, among other things, is to perform an oil analysis in an elemental content study using atomic emission spectroscopy.

interval between overhauls of aeronautical piston engines

– new guidelines No 3 of the President of the Civil Aviation Authority

On 17 January 2023, the President of the Civil Aviation Authority issued new guidelines for piston engine overhaul intervals. Compared to the previous guidelines, there is a more detailed distinction for which aircraft the current guidelines apply and, among other things, the oil analysis interval has been changed (50 FH / 4 mths). Some of the contents of the document are quoted below.

Failure to take into account the interval between overhauls of the piston engine

For the disregard of the piston engine overhaul interval referred to in the technical manuals, in terms of continuing airworthiness, the guidelines apply, inter alia, for:

1. Aircraft subject to Part- ML regulations:

• an aircraft with a maximum take-off mass (MTOM) of no more than 2730 kg
• a rotorcraft with a maximum take-off mass (MTOM) of no more than 1200 kg, certified for a maximum of 4 persons
• other aircraft ELA2 (manned European Light Aircraft)

2. Convention category aircraft (K2)

3. Special Category Aircraft (K3)

The document shows that an engine properly operated and maintained in accordance with the technical instructions for continuing airworthiness issued by the Type Certificate (TC) holder may be in a serviceable condition for continued operation, despite having reached the recommended overhaul interval, in order to be released for a further 100 hours or 12 months.

Before any changes are made to the maintenance programme and the maintenance manual to disregard piston engine overhaul intervals, an analysis of the engine’s service history is carried out, among other things, to determine whether the piston engine in question can be operated beyond the recommended overhaul interval.

When introducing a deviation from the Type Certificate (TC) holder’s recommendations for the piston engine overhaul interval, based on these guidelines, the following scheme should be followed.

One of the mandatory points of the verification inspection of a piston engine, according to the guidelines of the president of the civil aviation authority, is the spectrometric analysis of the oil, which makes it possible to determine the normal characteristics and wear rate of lubricated engine components. Detecting the increase in individual metal elements when engine components wear abnormally allows maintenance measures to be taken in good time to prevent engine failure.

It is important to emphasise that the spectrometric analysis of the oil is only one of several requirements and does not replace other maintenance tasks derived from the engine’s maintenance manual and other requirements detailed in these guidelines.

When deciding to apply these guidelines, and therefore to carry out an oil test, it is recommended that the first analysis should start 200 hours or 12 months before the recommended inter-renovation period of the piston engine is reached.

Using the recommended oil from the same manufacturer, spectometric analysis of the oil should be carried out at regular intervals at each oil change and the results collected and stored (preferably in stabelled form and graphs) to determine trends in change.

The first oil sample is considered as a baseline, subsequent samples show a trend which helps to identify deviations from the baseline values. Individual sample results may vary depending on the condition of the engine or the oil batch in question. For the meaningfulness of the analysis and in order to obtain consistent results, it is recommended that the tests are performed by a single laboratory with the appropriate accreditation and the person responsible for interpreting the analysis results should have knowledge of the construction of the piston engine in question, the influence of the external environment and the way the aircraft is used.

The graphic above is an excerpt from the oil analysis report from the LYCOMING O-320 engine. In addition to the elemental results, the report includes, among other things, information on total engine running time, dates and running time since oil changes, number of oil top-ups between changes. This information should be provided by the customer on the sample printout together with the oil sample.

oil diagnostics for aero piston engines

– spectrometric analysis of oil met. ICP OES (Inductively Coupled Plasma Optical Emission Spectrometry)

Aim of the aeronautical piston engine test

As early as the 1970s, the testing of oils in aeronautical piston engines was becoming increasingly common in the general aviation field. Thanks to this, the methods, procedures and interpretations that occurred, as well as the increasingly widespread access to the results, showed how oil analysis could be a useful tool in the operation of aviation piston engines and to help maintain the continued airworthiness of aircraft.

The most important aspect of monitoring engine health through oil analysis is, above all, safety. Regular oil analysis makes it possible to determine normal engine wear characteristics. Early detection and estimation of the increase in the content of individual metal elements such as iron (Fe), chromium (Cr), aluminium (Al), copper (Cu), lead (Pb), which make up the majority of the engine’s components, makes it possible to take timely maintenance action in the event of abnormal wear to prevent engine failure. On the other hand, the detection of contaminants such as silicon (Si), sodium (Na) or potassium (K) can be a prerequisite for checking the condition of filters, the tightness of the air supply system and the cooling system.

spectrometric analysis – description of the test

In the test, particles smaller than 5 microns (particles “dissolved” in oil) are determined. The elements determined are presented as wear metals, additives and impurities. The results are given in the unit ppm (parts per million). The concentration of the individual element groups indicates, among other things, whether the engine is free of excessive wear processes, degradation processes or the correctness of the oil used, as well as whether there are any leaks in the system.

In our laboratory (ECOL), we test the content of 24 elements categorised into three groups:

In contrast, the possible origin of the detected metallic elements, correlated with the metallurgy of the piston engine components, is illustrated below:

advantages of spectrometric testing

In addition to its functions of lubrication, cooling and corrosion protection, among others, oil is also regarded as a structural element, in some systems transmitting power, and as a carrier of information. The condition of the oil tells us what is happening inside the equipment. Based on the results of oil testing, we can determine whether a piece of equipment is operating properly and estimate the rate of wear of lubrication system components.

The progression of engine wear intensity usually occurs gradually, characterised by an increase in the concentration, size and shape of wear particles. In addition to extending the interval between overhauls, resulting from the guidelines of the President of the Civil Aviation Office, testing the content of various types of particles (metallic, impurities) in the oil is a very good method for monitoring engine condition and early detection of excessive wear or corrosion occurring in the engine.

This enables timely action to be taken to avoid a major breakdown or permanent damage to the engine.