In traditional turbine engine development and manufacturing, engines were certified with a mandatory overhaul interval. This ensures that after a specified period of use, the engine would be completely disassembled and components brought back to blueprint specifications. The process sought to maximize safety and minimize downtime. On the early turbines, it was successful. However, many components were needlessly reworked or replaced, driving the cost of operation much higher than necessary.
Similarly, a Hot Section Inspection (HSI) was conceived to only examine the section of the engine from the combustion point on, since this is typically where the most damage and erosion takes place. HSIs were similarly expensive as the overhaul mentality was maintained in bringing parts back to blueprint specifications and replacing many parts that were still serviceable.
When the TFE731 was being developed, the manufacturer, Garrett, was able to certify the design with a Compressor Zone Inspection (CZI) and a Major Periodic Inspection (MPI). The CZI is a much more in-depth and costly inspection, requiring complete disassembly of the engine. The MPI looks primarily at the turbine section; many of the same areas of the traditional Hot Section. Although these inspections are similar to the traditional Overhaul and Hot Section (and the terms are often interchanged in the industry), they are very different. Garrett, at the time, relied heavily on the IRAN philosophy (Inspect and Repair As Necessary). This eliminated a lot of unnecessary replacement of parts, made maintenance on the engine much less expensive, thus driving down the cost of operation on aircraft on which they were used.
The TFE731 MPI and CZI intervals started out rather low as the engine entered service. As operational experience expanded, these intervals were extended. On most of the original versions of the engine, the current CZI interval is 4200 operating hours and the MPI is 1400 operating hours. The intervals are set so that in normal circumstances, the inspections can be combined every third shop visit. Some of the later models are certified to go 6000 hours between CZIs and 3000 hours between
MPIs. The applicable maintenance manual must be consulted to determine the MPI and CZI interval for any given engine.
The maintenance manual will refer to service bulletins which must be researched in order to determine the inspection interval. As the manufacturer develops improvements to the engine over time, service bulletins are released to incorporate better materials and processes to increase safety and durability. Once these service bulletins have been incorporated, a longer interval between inspections can be set. This ensures maximum safety while minimizing operational expense.
Hopefully, this blog was able to give you a little insight on how Overhauls, Hot Section Inspections, MPIs and CZIs work (and how they differ). We hope you understand the benefits to your aircraft, your safety, and your pocketbook. For more information on TFE731 Turbine Engine services visit https://turbineengine.com/parts-service/