What Are Air Turbine Starters?

Gas turbine engines are large and robust assemblies, and they utilize high amounts of fuel and advanced components to achieve the propulsive force needed for aircraft flight. In order to initiate the operations of such engines, a system known as an air turbine starter is used. While being a small and lightweight source of energy, air turbine starters can create a large amount of torque that can drive an engine until self-sustaining operations are reached. As a typical air turbine starter is capable of achieving the same starting efficiency and power as an electric starter two to four times its weight, such systems can be a very beneficial addition to your aircraft.

To initiate the operations of a typical gas turbine engine, a large amount of intake air is needed to drive the turbine and compressor assembly until fuel flow and rotational speeds are sufficient for self-sustaining operations. For a standard air turbine starter, a drive coupling is operated by an axial flow turbine through the use of a reduction gear train and starter clutch mechanism. As engines are most often initiated while on a runway, a ground-operated air cart may be used to supply the necessary air for operating the turbine, though APUs or the cross-bleed air of an operating engine may also serve for driving the assembly. To achieve a successful start, the air pressure provided through the ducts of the starter should be at a value between 30 and 50 psi.

As air is pumped into the inlet of the starter, it is first passed through the housing where it is forced against turbine rotor blades with the use of nozzle vanes in order to cause the assembly to begin rotating. The rotational motion of the turbine rotor drives the reduction gear train and clutch assembly in response, that of which contains the planet gears, sprag clutch assembly, rotor pinion, output shaft assembly, and drive coupling.

The sprag clutch assembly remains engaged to the rotor while it is in rotation, though it will quickly disengage upon the drive coupling surpassing the turning speed of the rotor. At that point, the gear train will reach a stop while the drive coupling and output shaft assembly continue to turn with the operation of the engine. Once the air turbine starter has reached its cutout speed, a rotor switch actuator opens a turbine switch so that the start valve can be closed off to prevent continued air supply.

In order to control the airflow through the starter system, a number of components may be used. With a shutoff valve or bleed valve, the air path can be directed from the inlet. The shut off valve or bleed valve is also charged with regulating pressure, and it can switch off the air supply as needed. The valve is composed of two sub-assemblies, those of which are the pressure-regulating valve and valve control. The regulating valve assembly houses a butterfly valve, and the shaft of the valve is attached to a servo piston so that its actuation causes the rotation of the butterfly valve through a cam arrangement. The control assembly, on the other hand, is placed on the valve housing and contains a regulating valve solenoid which may be energized with a starter switch.

Air starter turbines are useful systems for many aircraft engines, and their regular maintenance and the replacement of aging or defective parts can ensure their proper functionality and reliability for long periods of time. When you are ready to begin sourcing the various aircraft components and maintenance tools that you require for your operations, look no further than ASAP Components. As a premier purchasing platform for new, used, obsolete, and hard-to-find components, we leverage competitive pricing and rapid lead-times with our market expertise and purchasing power for the benefit of our customers. Furthermore, our dedication to quality and compliance allows us to proudly operate with AS9120B, ISO 9001:2015, and FAA AC 00-56B certification and accreditation. Experience our unmatched customer service and dedication as you begin the purchasing process with us today.


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April 22, 2021

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