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ENGINES


get the “burn”. The burning is continuous, like a gas furnace, barbeque or plumber’s torch. This is a continuous, controlled combustion process. The burning air/fuel mixture expands tremendously into a hot gas at high pressure within the combustion chamber. This high-pressure gas is then directed into the turbine section where the pressure is converted into a combination of drive power (torque) and thrust (exhaust). This is the “blow” part. Since, in this case, the pressure is converted mainly into power to drive a transmission, it is referred to as a turboshaft engine. Obviously, we have oversimplified the operation here, but you should have a feel for what is going on.


accordingly. NR is the speed of the main rotor. We also have inlet turbine temperature (ITT), turbine outlet temperature (TOT), exhaust gas temperature (EGT)and turbine gas temperature (TGT). When working with gas turbine engines, faults (problems) can generally be divided into two types. One type occurs during the engine start cycle and the second type occurs while the engine is running. Here is a listing, in no particular order, of some of the maintenance issues that were found to be the cause of the fault listed. These were found among many different engines and not one in particular.


Gas Turbine Engine Starting Problems


No RPM (N1) During Start Attempt • No electrical power to starter/generator • Starter shaft sheared — starter only spins • Starter shaft spline worn out — starter only spins • N1 indication defective – Is oil pressure rising? Could be defective indicator or sheared shaft on tach drive


• N1 rotor system seized – Remove starter and attempt to turn drive


Honeywell T53 Gas Turbine Engine Cutaway View Getting Power from the Engine


to the Rotor System Now that we have power, we need to get that power to our rotor system. The turboshaft is spinning at many thousands of rpm and the rotor system cannot spin that fast for many reasons. We need to reduce the engine output rpm down to a useable number that is typically measured in hundreds of rpm. This gearing down is accomplished through the main transmission gearbox. In the process of reducing the rpm for the rotor system, the


torque is increased. The transmission drives the mast assembly to which the main rotor blades are attached. Often another drive shaft will come out of the transmission to drive the tail rotor assembly directly. An accessory gearbox mounted on the engine draws a small amount of engine power to drive the oil pump, electrical generator and fuel control for the engine.


Abbreviations If you have worked in aviation for any period of time, you have come to learn that we have a language all our own. (I have even thought about doing a dictionary on aviation terminology, but that is for another time.) To further complicate matters, many of the terms used with regard to gas turbine engines have no industry agreed-upon standard. Working with these engines, we see abbreviations such as N1 (low-speed compressor) and N2 (high-speed compressor). These N speeds indicate what speed the compressor and main rotor blades are spinning as a percentage of maximum allowable rpm. Likewise, we see temperature or pressure readings designated as T1 or P1, respectively, and T3 or P3


20 HelicopterMaintenanceMagazine.com April | May 2014


Insufficient N1 RPM at Start Attempt • Insufficient voltage to starter? – Check the power source and leads. Are the batteries low?


• Check N1 rpm indicating system – The indicator may read low, be hung up or sticking


Delayed Start • Improper start technique – Check the flight manual for the correct procedure


• Fuel manifold adapters are positioned improperly – Adjust to the proper position


• Fuel nozzle flow restriction – Check and remove restriction


• Air in the fuel system – Bleed the system; check on why and where air is entering the system


• Check spark igniters – Replace as necessary


• Check ignition exciter – Check on air start while motoring; replace as necessary


• Check voltage to ignition exciter – Low voltage may be accompanied with low cranking rpm; this can cause a hot start


Engine Fails to Light • Improper start technique – Check the flight manual for the correct procedure


• No fuel to the engine – Check aircraft valve on; check fuel boost pump pressure


• Excessive air in fuel system – Bleed fuel system – Check the reason for air entering the system


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