The modular design of the PT6 has been documented and discussed in previous articles. We have talked about the rear module and the gas generator. Now let’s turn our attention to the front of the engine, called the power section. The power section does precisely that; it turns the air from the gas generator into power. Now we look closer at how air is turned into energy.
Simply stated, the power section comprises a turbine section and a gearbox. Much like the engine’s modular design, the power section can be looked at in modules itself. The rearmost subassembly or module is the power turbine. It comprises either one or two rotors (power turbine disks), depending on the engine model on a shaft. This module also includes the power turbine vane ring and power turbine stator housing. The shaft is the power turbine or PT shaft, and it runs through a housing where two bearings support it. Moving forward in the engine, the end of the PT shaft opposite the disk(s) is the 1st stage sungear. This is the entry point into the rear reduction gearbox section of the power section. Before looking at the reduction gearbox or RGB, think about how fast the power turbines spin. They take all the hot compressed gases from the gas generator and use them to turn before that gas is exhausted from the engine. The power turbine speeds are in the range of 30,000 to 33,000 RPM. That is the speed that starts its way into the reduction gearbox.
The reduction gearbox contains two sets of gears and is built like our solar system. We have two sun gears in the center, surrounded by two planetary gears. Each gear set is housed in a housing: the rear and front RGB housings. As stated, the 1st stage sungear is attached to the PT shaft. As it turns, it drives the 1st stage planet gears. This set of gears is a group of three gears matched in weight within a certain tolerance. They are contained in a balanced carrier assembly and, on the outside, run inside a ring gear. The 2nd stage sungear is also attached to the first stage carrier. All the 1st stage assembly is contained in the rear RGB housing. This housing also includes the torque piston and cylinder, where the torque indication comes from. This completes the middle module of the power section.
The final module is the front RGB housing. Five planetary gears rotate around the 2nd stage sungear. These are the 2nd stage planet gears, and they are all matched by class and are housed in the 2nd stage carrier. The carrier is attached to the prop shaft. Now we are finally able to turn the prop shaft. Depending on the model, the final gear reduction is 14.5 to 17.5 of power turbine RPM to one prop revolution. Bearings on both ends support the 2nd stage carrier and prop shaft. The #5 bearing supports the carrier end. When you have a live wire or lightning strike, the path of the electricity most commonly taken is prop, prop shaft, and 2nd stage carrier. We typically find arcing damage on both the bearing and the carrier. The prop end of the prop shaft is supported by the #6 and, in large engines, the #7 bearing. Also housed in the front RGB are the drive gears for the prop governor, overspeed governor, and prop tach. All these gears and bearings in the reduction gearbox are why there is also a chip detector in the front housing. I stress to everyone to ensure their chip detector is connected and working, which we check every 100 hours.
Besides the chip detector, the other inspection items on the power section center around the exhaust duct, power turbine blades and temperature sensing system. I encourage everyone to pull a stack and look at the PT blades. If that is not possible, then at least put the borescope in and take a peak. You need to make sure that the blades look as you expect. Twenty years ago, an AD was issued regarding low-quality exhaust duct welds (AD 2002-23-13). I haven’t seen a poor weld in a while, but it is worth mentioning as it is a straightforward, one-time inspection if you have an older small engine.
Finally, we must discuss the temperature sensing system. The ITT or internal turbine temperature system is housed on the power turbine stator housing. There are 8 – 10 temperature sensing probes connected by a bus bar and connected to a wire harness to the outside of the engine. I had many calls about engines that exhibit intermittent turbine indicating problems. We test these components with a Barfield tester. We measure insulation resistance and loop resistance per the engine maintenance manual. I recommend having this done occasionally, especially when you have the power section removed. The insulation breaks down over time, potentially leading to intermittent problems.
Hopefully, you now have a good idea of the construction of the PT6A engine. I always find it fascinating that the engine was developed in the 1960s, and much of the technology developed during that initial engineering is still in use today. The PT6A continues to be one of the world’s most useful and dependable engines.
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 robertc@covingtonaircraft.com or 662-910-9899. Visit us at covingtonaircraft.com.
