ENGINES


the lowest inter-turbine temperatures on PT6A engines occur at approximately 59 percent compressor speed. This means that as the compressor speed is reduced below 59 percent, the inter-turbine temperature increases. Higher temperatures obviously mean shorter hot section lives. Ground idle should not be set at any value below the recommended speed, but increase the ground idle speed with the throttle(s) to reduce the inter turbine temperature(s) whenever safe and possible. This is especially important when starting the second engine on a twin-engine aircraft. During the start cycle, the pilot’s attention is concentrated on the engine that is being started. However, the engine that has already been started is at ground idle and has a large load applied to its (starter) generator by the starter on the engine that is being started. During this time it is easy to have the generator load draw the idling engine below 52 percent ground idle. If this were to happen, the fuel control will automatically go into the start acceleration schedule and increase fuel flow in an attempt to accelerate the engine up to ground idle. This increase in fuel flow significantly increases inter-turbine temperature, thereby shortening hot section durability. A similar condition can occur on a hot day with the aircraft air conditioning system running at maximum while attempting to cool a hot cabin and passengers. I know of an operator who was operating an engine at ground idle while refilling the Freon in the aircraft air conditioning system. The Freon was introduced fast enough that the ground idle was drawn below flight idle, the increased fuel flow could not accelerate the engine back up to idle, and the engine hung in a sub-idle condition for a long enough period of time to destroy the hot section. Another expensive lesson was learned.


In Flight, Cruise Engine


Temperatures 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. Some models of gas turbine engines (like the PT6A turbo prop engine) have identical maximum allowable hot section temperatures during both takeoff and maximum continuous operation. There is a safety benefit, especially during emergencies, to have the takeoff and maximum


MODEL PT6A-20 PT6A-11 PT6A-21 PT6A-27 PT6A-28 PT6A-34 PT6A-36 PT6A-114 PT6A-114A PT6A-41


PT6A-42 Figure 1


MAX T5 LIMIT 750 700 695 725 750 790 790 805 805 750


800


SUGGESTED CRUISE T5


700 or less 680 or less 680 or less 680 or less 680 or less 740 or less 740 or less 740 or less 740 or less 725 or less 725 or less


April | May 2015 HelicopterMaintenanceMagazine.com


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