ENGINES


MANY MODELS OF GAS TURBINE ENGINES HAVE A HIGHER ALLOWABLE HOT SECTION TEMPERATURE DURING THE FIVE-MINUTE TAKE-OFF TIME PERIOD THAN THE LOWER MAXIMUM CONTINUOUS ALLOWABLE TEMPERATURE.


continuous temperature limits the same. On the other hand, operation at or near the maximum continuous temperature limit signifi cantly shortens hot section life. Figure 1 shows the maximum allowable temperatures for takeoff and maximum continuous operation, and also provides suggested lower cruise temperatures that the experience of several operators has shown will increase hot section longevity. (Note all temperatures are in degrees Celsius). There is an example that illustrates this concept. PT6A-11, PT6A-21 and PT6A-28 engines all have the same compressor, combustor, CT vane ring, CT blades, PT vane ring and PT blades. The maximum allowable inter-turbine temperature on the PT6A-11 and PT6A-21 engines are 700 C and 695 C, respectively. The maximum allowable inter-turbine temperature on the PT6A-28 is 750 C. Hot sections on PT6A-11 and 21 engines frequently operate 2,500 to 3,000 fl ight hours before hot section repair is necessary. Conversely, PT6A-28 engines frequently operate only 1,800 to 2,000 fl ight hours before hot section repair is necessary. This illustrates the benefi t that can be obtained by using lower cruise power settings.


Summary and


Conclusion The older and more experienced engine mechanics are already familiar with the topics in this article. Hopefully these more experienced mechanics will consider this article a helpful refresher course. As for the younger and less experienced engine mechanics, it is hoped that the lessons provided in this article will be used to prevent the kinds of problems that cause prematurely gray hair. This can be said another way. There is an old saying, “pay me now or pay me later.” The choice is yours! Look for part two of this series in our next issue of Helicopter Maintenance magazine.


Ralph Hawkins is the chief engineer for MORE Company, Minden NV. MORE Company holds eight FAA STCs providing


alternative ICAs for PT6A engines. These STCs prescribe an enhanced inspection methodology that allows the operator and their mechanics to recognize engine problems in their early stages and to correct them promptly. In turn, the prompt correction of engine problems allows the engine overhaul interval to be safely extended to 8,000 fl ight hours. These eight MORE STCs include several of the previously-mentioned techniques, as well as several additional techniques that are especially well suited to the PT6A turbo prop engines. The MORE Company STCs apply to PT6A-11 through PT6A-45R and PT6A-110 through PT6A-135A engines. MORE Company began providing the use of their alternative ICAs to operators in the autumn of 1993. As of Jan. 22, 2015, there were 2,243 PT6A engines operating in 50 countries that are now or had been using MORE Company’s STCs. For more information, visit www.morecompany.net


April | May 2015 HelicopterMaintenanceMagazine.com


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