Failure Analysis of an Air Turbine Starter
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CASE HISTORY—PEER-REVIEWED
Failure Analysis of an Air Turbine Starter Swati Biswas • M. D. Ganeshachar B. R. Sridhar • S. Ramachandra
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Submitted: 23 February 2010 / in revised form: 8 April 2010 / Published online: 29 April 2010 Ó ASM International 2010
Abstract Air turbine starters (ATS) are used for the ground start of aircraft engines. This case history describes the failure of an ATS that occurred while starting a developmental gas turbine engine. Fractographic features indicated that few blades of the turbine rotor disc failed in a progressive mode while the remaining blades of the rotor disc indicated failure by overload. The rotor shaft also exhibited features of sudden overload. Analysis of the previous records revealed the observation of sparks developing while the starter was in operation. This indicated possibility of blade tip rubbing which resulted in fatigue crack initiation at the blade root leading to failure. Keywords Gas turbine starter Fatigue Overload Tip rubbing
Introduction The gas turbine engine is basically a machine designed to accelerate a stream of gas that is used to produce the reactive thrust necessary to propel an aircraft. However, for satisfactory initiation of the engine two separate systems are required; (1) a provision for rotating the compressor and the turbine to a speed at which adequate air passes into the combustion system to mix with the fuel from the fuel spray nozzles and (2) a system to ignite the fuel–air mixture in the combustor. During the engine starting, these two systems must operate simultaneously. The purpose of a starter is to produce a high torque and transmit that torque to the engine S. Biswas (&) M. D. Ganeshachar B. R. Sridhar S. Ramachandra Gas Turbine Research Establishment, Bangalore, India e-mail: [email protected]
rotating assembly in a manner that provides smooth acceleration from rest up to a speed at which the gas flow through the engine provides sufficient power for the engine turbine to takeover [1]. Basic types of starters developed for gas turbine engines are air turbine (pneumatic), electric motor, hydraulic, combustion, and cartridge pnuematic. Air turbine starters (ATS) are the most commonly used for ground starts. An air starter motor has a turbine rotor that transmits power through a reduction gear and clutch to the starter output shaft [1] that is connected to the engine (Fig. 1). The starter turbine is rotated by air pressure taken from an external ground supply from an Auxiliary Power Unit (APU). The air supply to the starter is controlled by an electrically operated control and uses a pressure reducing valve that is opened when an engine start is selected. This valve automatically closes at a predetermined starter speed. The clutch also automatically disengages as the engine accelerates up to idling RPM and the rotation of the starter ceases. The efficiency of any turbine, including those in starter engines, is greatly influenced by the clearance between the blade tip and the casing. However, reduction in tip clearan
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