Aircraft Maintenance: The Five Pillars of Engine Condition Monitoring



The five pillars of engine condition monitoring are:

  • Engine data monitoring
  • Oil Filter and Screen Inspection
  • Oil analysis
  • Cylinder compression
  • Endoscopic inspections

These five areas of inspection and monitoring data give us a pretty comprehensive view of engine health. Taken individually, they each focus on different aspects of engine health. They also each provide two different types of data: objective data and trend data.

This is a very important distinction. Objective data can be thought of as data that provides quantifiable pass/fail or other results that fall outside the bounds of normal operation. These data thresholds act as “alarms” that clearly tell us that something is wrong and that we need to investigate further to identify the source of the problem. Trending data is equally important, as trends can identify issues before they reach the point of triggering objective data alarms. Trending data can also identify changes that affect performance and efficiency, potentially mitigating repairs before excessive wear or damage occurs.

Here is a brief summary of the objective and trend data provided by each of the five main areas of engine inspection.

Engine data monitoring

Engine monitor data can provide detailed data on many parameters including cylinder head temperature (CHT), exhaust gas temperature (EGT), oil temperature, etc. Most systems can be configured to alert the pilot when critical objective limits are exceeded, such as high CHT, high oil temperature, or excessive differences in EGTs. However, engine monitors are even more useful for their ability to provide trending data. By comparing flights over time, we can identify changes in these parameters that may indicate that adjustments or repairs need to be made. Although not all aircraft have engine analyzers, all pilots have the ability to monitor both objective limits (such as high CHTs or oil temperature) and trend data (such as oil usage or comparison of temperatures and performance with previous flights). Regardless of the content of your panel, you have the ability to use your eyes to observe and your kneeboard to record important data for future reference.

Oil Filter and Screen Inspection

Inspecting an oil filter or screen during an oil change has given mechanics insight into an engine’s health for as long as engines and filters have existed. While there is certainly some trend value in the amount and type of contaminants found, most of the value of a filter inspection is objective. We look at the amount and type of material caught in the filter to identify engines that are “making metal” and use the advice provided by the manufacturers to determine the best course of action. As long as the amount of metal is well below these limits, we can also monitor the amount of metal for trending information. However, this is of limited value unless the change in quantity is fairly obvious.

Oil analysis

While the Oil Filter Inspection is primarily an objective data tool, the Oil Analysis is an ideal companion as it is primarily a trending data tool. Oil analysis measures the number of metal particles that have passed through the filter or screen and are suspended in the oil sample. These particles more often indicate wear tendencies, rather than gross failures that send metal shavings or barbs into the filter media. These trends can draw attention to growing problems that can often lead to failures.

Cylinder compression

The ubiquitous “compression test” has been the subject of more debate in engine maintenance articles than almost anything else. Historically, this was the gold standard for measuring engine health during the annual inspection. However, far too many cylinders were removed prematurely due to misinterpretation or over-emphasis of the results and the compression test got a pretty bad reputation for being a poor judge of cylinder health. The reality is that compression testing is a great resource when used as one step in the process and in combination with other diagnostic tools. A compression test provides raw objective data that we can use to direct further investigation. When large deviations from manufacturer limits are found, a compression gauge does a good job of identifying if a problem exists and where to look. This is not a good tool for trending data, as individual results can vary widely from test to test on the same engine.

Endoscopic inspections

Borescope inspections supplement cylinder compression testing with visual data and photographic history of valve, piston and cylinder wall trending condition. The compression test often tells us where to look for problems based on where the air is escaping. The endoscopic inspection then allows us to focus and find the culprit. Using research provided by Adrian Eichhorn, Mike Busch, Dave Pasquale and others, it also allows us to assess exhaust valve heat signatures and monitor them for tendencies towards failure.

Consider this a brief introduction to the five pillars of engine health monitoring. In future articles, we’ll dive deeper into each topic so you can manage your engine health like a pro. Until next time, I hope you and your families stay safe and healthy, and I wish you blue skies.

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