What Is an Aviation Air Start Unit

In aviation, ground support equipment (GSE) often has a significant influence on aircraft turnaround times and their operational readiness. One of the most notable pieces of GSE is the Aviation Air Start Unit (ASU), designed to assist with engine startup for gas turbine-powered aircraft. In this blog, we will cover all that aviation professionals and enthusiasts alike should know about the design and operation of ASUs.

What Is an ASU?

As gas turbine engines are unable to generate the required rotational speed from a static position, they rely on an external source of compressed air to initiate combustion. The ASU delivers this high-pressure air through an external hose connection, spinning the engine's compressor section until it reaches a speed where sustained ignition can occur.

While the specific design and performance of ASUs may vary across aircraft requirements and manufacturers, models share a consistent set of features:

• Air Pressure Output: ASUs deliver compressed air at approximately 40 to 42 PSI.
• Air Flow Rate: Their airflow capacity typically ranges from 90 to over 450 pounds per minute (PPM), with the rate determined by the size and type of the aircraft engine being serviced.
• Mobility and Mounting: Most of the time, ASUs are mounted on self-propelled trucks, towable trailers, or stationary skid platforms for flexible positioning across the tarmac.
• Connection Interface: Standardized air hose connectors are used to facilitate compatibility with aircraft engine starter ports, supporting safe and secure coupling.

When Are They Used?

• ASUs are primarily used during aircraft turnaround procedures, particularly when the onboard Auxiliary Power Unit (APU) is inoperative, undergoing maintenance, or when environmental or noise restrictions prevent its use.
• ASUs are employed to conserve fuel, since APUs rely on onboard fuel supplies while ASUs operate independently.
• They are also used for wide-body aircraft or cold weather starts, where additional airflow is beneficial.

How Does an Air Start Unit Work?

1. Positioning and Connection

Before startup, the ASU is maneuvered to the aircraft’s designated side. When it is ready to be deployed, it is connected to a high-pressure inlet on the engine, using one to three hoses depending on engine size.

2. Drawing, Compressing, and Supplying Air

The ASU houses an engine, commonly diesel although turbine-driven models exist, that powers an oil-free screw or rotary compressor. This draws in ambient air, takes it through a filtration system, compresses it to the intended pressure, and finally delivers it through the discharge hose to the aircraft’s engine starter.

3. Engine Rotation and Start Sequence

Usually, one engine is started externally with the ASU, after which its bleed air may be used to start additional engines. As compressed air enters the engine's pneumatic starter or air starter motor, the rotation of the compressor or fan spool is taken up to a self-sustaining speed. Once this threshold is reached, the pilot introduces fuel to the combustion chamber and the engine takes over, at which point the ASU is disconnected.

4. Control Systems and Safety Features

To facilitate safe procedures, most modern ASUs are equipped with dual-pressure regulators, demand-throttle control, multiple operating modes, and safety relief valves. Additionally, interfaces with real-time monitoring of pressure, temperature, and fault diagnostics are available so ground crews can operate it with great care and precision.

Find Comprehensive Solutions for Ground Support Operations on AOG Unlimited

As we have illustrated, ASUs stand as a reliable, fuel-efficient method for starting aircraft engines. Because they are such essential tools, it is imperative to invest only in options that meet strict aviation standards and performance needs.

At AOG Unlimited, you can readily purchase ASUs, ground support equipment, and thousands of other top-notch products from reputable manufacturers. Moreover, with competitive pricing, rapid global shipping, and dedicated experts to help and find the perfect solutions at every stage of procurement, you never have to compromise when you shop with us. If you are ready to see how we can assist with your operational requirements, get in contact with us today.

1. stephen smith
2. Posted on October 6, 2022
3. aviation

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