Technical report | Improvements to Filter Debris Analysis in Aviation Propulsion Systems
The accurate analysis of metallic wear debris is fundamental to determining the health of aviation propulsion oil-wetted systems. The oil filter is an excellent source of wear debris, however methods for removing and assessing the debris have traditionally involved tedious visual examination of the filter pleats and manual counting of particles. This report describes two enhanced methods for extracting and assessing filter debris: the first method uses a manual extraction and capture process; the second method uses a commercially available instrument for automatic extraction and quantification.
The analysis of wear debris has been shown to be an effective condition monitoring tool for oil-wetted systems and is considered to be a valuable adjunct to existing condition monitoring techniques. The oil filter is a potentially rich source of information about the health of oil-wetted components in aircraft machinery, but is generally under-utilised as a condition monitoring tool in the Australian Defence Force. Historically, the analysis of aircraft oil filter debris was time consuming and was not suited to in-field assessment. The two primary challenges associated with oil filter analysis are extracting the debris in a reliable and controlled manner and interpreting the debris to assess whether maintenance action is required. In particular, the military context (involving regular deployments to remote localities or to sea) presents its own set of challenges for extracting useful information from oil filters. Additionally, the well documented benefits of introducing fine oil filtration has resulted in some of the traditional oil analysis techniques, such as Spectrometric Oil Analysis (SOA), becoming ineffective. In aviation propulsion machinery, this generally leaves the filter and magnetic chip detectors as the prime sources of wear debris information.
This report describes the application of two Defence Science and Technology Organisation (DSTO) initiatives to improve the analysis of oil filter wear debris. The first initiative involves the application of an in-field manual debris extraction kit to the F117-PW-100 engine (powering the C-17A aircraft) oil filters. The kit enables maintenance staff to conveniently extract the filter debris and deposit it on a filter patch for inspection and further analysis if required. The process used for extraction in this instance is a manual method currently used on RAAF PC-9/A aircraft. The previous method of inspecting the filter from this engine involved visual inspection of each filter pleat and manual counting of particles. The advantages of the new method include greater extraction efficiency (i.e. greater recovery of debris compared to the previous method) and a less tedious and laborious task for staff.
The second initiative involves the assessment, trial and introduction of a commercial instrument known as FilterCHECK. This device automatically extracts the filter debris using a combination of reverse fluid flow combined with compressed air pulsations. The resulting slurry is then passed through an inductive sensor to quantify the ferromagnetic and non-ferromagnetic debris. This instrument has been applied to the external scavenge filter fitted to the T56-A-14 and T-56-A-15 engines (powering the P3C and C130-H aircraft respectively). Routine filter debris analysis is conducted at 150 hour intervals on these Royal Australian Air Force (RAAF) engines. The advantages of this technique include less time spent processing the filters, elimination of hazardous solvent exposure to staff and a higher fidelity particle detection method.