What Is an Aviation Air Start Unit

When operating aircraft with gas turbine engines, a starter unit is often required to help the engine attain the self-sufficient speeds necessary for standard operations. Start units come in a variety of types, and they are all considered Ground Support Equipment (GSE) as they are operated by airlines and ground handling agencies that utilize the unit during the final boarding procedures before departure. One of the most common types of unit is the air start unit (ASU), that of which may be found at most airports across the globe. In this blog, we will discuss the general design and functionality of the air start unit so that you can better understand its importance.

As passengers begin to embark an aircraft after servicing has been completed, the air start unit will need to be used so that engine operations can be kick-started prior to pushback and taxiing. To begin, a towing tractor will bring the unit over to the aircraft stand, and upon receiving confirmation from the pilot, the unit’s discharge nose will be connected to a high pressure receptacle present on the aircraft itself. ASUs are known for being one of the loudest equipment types, so it is important that proper hearing protection is used during its operation.

With the air start unit operating, the engine fan, air compressor, and turbine blades will all begin to revolve as a result of the power provided. As the rotational speed increases, the pilot will activate the fuel injection system to begin combustion. Once the engine has reached speeds where operations are self-sustaining, the air start unit will be disconnected. The ASU is typically only needed for one engine, as the other can often be started with cross bleed air from the first engine.

While this is the general role of the air start unit, it is important to know how the system provides this functionality. In their most basic form, air start units are simply engine-drive air compressor assemblies that draw in atmospheric air, filter it, compress it, and supply it to the air starter motor of the gas turbine engine. For the sake of mobility, most air start units are placed within protective housings that may be mounted onto a truck. To ensure optimal operations, the air pressure capability of most units range around 42 PSI. As air flow rates are also essential, the right value will need to be chosen based on the size of the aircraft engine itself.

In some instances, an aircraft may have an auxiliary power unit (APU) situated on the fuselage that can provide the compressed air that is necessary for operations, though this is not always the case. Additionally, there are various benefits to using the air start unit by itself, such as minimizing the amount of resources that are spent prior to a flight. APUs utilize fuel, so it can be much better to use an ASU so that as much fuel as possible is reserved for the flight operation itself. With a better understanding of the air start unit, you will be able to see why it is so important.

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