One of the PT6 maintenance items you always hear about is the hot section inspection. The hot section inspection is a mid-TBO inspection where we split the engine. At the same time, hot section inspection time requirements may differ by engine model because TBO on engine models may vary. Performance loss can also dictate the need for an inspection. For example, the PT6A-34AG engine TBO (Time Between Overhauls) is 4000 hours, and hot sections are recommended at 2000 hours. For the PT6A-65AG, the primary engine TBO is 3000 hours, and the recommended hot section inspection is 1500 hours. All this information exists for all engine models in various Pratt & Whitney Canada service bulletins. Time constraints are listed in service bulletins, but hot section criteria are defined in each engine model’s maintenance manual.
What exactly is a hot section inspection? The hot section inspection ensures the condition of some of the hardest working parts in your engine can continue to do the job up to the next inspection interval or until TBO. The PT6 engine is all about efficiency, and the hot section inspection is a simple and fast way to confirm the engine is not losing efficiency in this extreme environment. Because of the temperature and forces applied to the parts in the hot section, various factors can change efficiency through regular operation. Fretting, wear, cracking, and rubbing can all exist in the hot section and can negatively affect your engine’s performance.
When the engine is split to perform the hot section inspection, one of the first tasks is to measure the compressor turbine blade tip clearance. Tip clearance is one of the critical areas for efficiency. On a PT6A-34AG, for example, the median average tip clearance is .013”. That is only the thickness of a couple of business cards. That clearance is close for a disk that spins thousands of revolutions per minute in hundreds of degrees. When those tip clearances begin to increase, you lose performance. In the cockpit, that means the engine is not making power because of temperature limitations.
After measuring tip clearances, the turbine disk is removed, and the rest of the hot section parts are checked for deterioration, distress, or other problems. Components are resurfaced and resealed once repairs are made and issues are addressed as needed. The compressor turbine blade tip clearance is reset for maximum efficiency by changing and/or grinding the segments. Then, the hot section is reinstalled. That is a basic overview.
There are additional requirements as part of the inspection. The bleed valve and compressor condition must be checked. The gas generator case has inspection criteria. The fuel nozzles, the power turbine stator and housing, exhaust duct, oil strainers, oil filters and chip detectors are also checked. No one wants to invest time and money into a hot section only to discover that the compressor has issues and the engine must be removed. What if you find out that the gearboxes are making metal? Perhaps distress in your hot section is being caused by a fuel nozzle issue? Proper inspection of all parts and fulfilling all inspection criteria are essential when it comes to hot section inspection.
I touched on some of the things done during your hot section inspection. Just remember it is more than a split, and peek at the parts. We want to ensure the engine is safe for the operator and can be operated fully when needed. Preventative maintenance and inspections are also a way to keep costs down. If problems can be detected early and repaired, it is less expensive than replacing parts like a vane ring or turbine blades.
Keep a few other things in mind: if you operate a PT6A-65AG, a blade inspection needs to be done at 3000 hours. In some other engine models, the PT6A-34AG, for example, requires blade inspection at 5000 hours and includes cutting two blades for metallurgical analysis. Read your maintenance manual so you are aware of any additional inspection criteria that may need attention during the hot section inspection.
Robert Craymer has worked on PT6A engines and PT6A-powered aircraft for the past three decades, including the last 25+ years at Covington Aircraft. As a licensed A&P mechanic, Robert has held every job in an engine overhaul shop and has been an instructor of PT6A Maintenance and Familiarization courses for pilots and mechanics. Robert has been elected to the NAAA board as the Allied-Propulsion Board Member. Robert can be reached at firstname.lastname@example.org or 662-910-9899. Visit us at covingtonaircraft.com.