The PT6A engine is a “Free” turbine engine! How we all wish that was an economic statement instead of a description of a type of compressor / propeller relationship!
This free turbine compressor is designed to provide the necessary compressed air to the Hot Section of a PT6A engine for use in cooling and fuel – air mixture / combustion. The compressor’s speed in Revolutions Per Minute (RPM) is controlled by hot gases flowing (a result of combustion) through the compressor turbine vane ring and power turbine vane ring. These vanes have a specified “Class” number indicating their ability to flow these gases. These class sizes and the condition of the compressor itself determine the speed of the PT6A’s compressor at any given power setting. The class size range for a specific engine is determined by the manufacturer’s engineering. The speed of a gas generator is set or corrected to limits at engine manufacture or during engine overhaul testing. The compressor delivers sufficient air pressure for combustion and cooling to allow the engine to develop the required horse-power/torque.
Since this “Free” turbine compressor is not mechanically connected to the propeller through the power section, what happens when it gets dirty or damaged by foreign objects (FOD)? The damage or contamination reduces the compressor’s efficiency and results in a higher gas generator speed (NG) for the same power setting. The pilot is targeting Torque and RPM when he applies power. The reduced efficiency of the gas generator will result in the engine requiring more fuel to obtain the same torque and RPM. This means higher fuel flow, higher temperature (T5) and higher gas generator speed for any given power setting! All three of these parameters are “Up”. The Maintenance Manual identifies this condition as indicating a “Cold Section (Compressor) Problem”!
Hot weather will result in faster gas generator speeds due to the reduced air density at high temperatures. Suppose your engine’s NG speeds have crept up to just beyond the Red Line and a compressor Performance Recovery Wash did not restore the previous settings (slow it down). Now you are retarding the throttle at take off to keep from over-speeding the gas generator! This results in less power available, not a good circumstance in the middle of an AG spray season. Do you need to “Re-class” the engine by installing different classes of compressor or power turbine vane rings?
The Maintenance Manual cautions to install the same class size vanes when replacing compressor turbine or power turbine vanes! The airflow requirements on these vanes for any given engine are determined and tested (certified to perform) in the test cell. All possibilities for repair should be exhausted before deciding to re-class an engine on-wing because of perceived poor performance. All aircraft Gages, including the gas generator speed indicator should be calibrated prior to any attempt to re-class an engine. We have corrected at least four (4) “performance” problems in the last eighteen (18) months by replacing faulty gas generator gages! In one of the instances an engine had already been pulled and sent to the manufacturer, tested and performance certified before the gage was questioned. Always do the “Cheap” things first when troubleshooting!
Y’all fly safe – Ron Hollis