How Air Starter Unit (ASU) Works and Which Types of Aircraft Require ASU

ASU machine plays a critical role in helping aircraft engines start when the aircraft’s onboard systems are not capable of doing so alone. Air travel depends on a wide range of ground support equipment (GSE) that ensures smooth, safe, and efficient operations. One of the most essential pieces of GSE is the Air Starter Unit. Understanding how an air starter unit works, and knowing which types of aircraft require it, is crucial for aviation professionals, students of aeronautical engineering, and even airline passengers curious about what happens before takeoff. In this article, we will explore in detail what an Air Starter Unit is, how it works, its components, types of air starter unit, and the specific aircraft that rely on it. By the end, you will have a complete technical yet simple understanding of this vital piece of airport equipment.

1. What is an Air Starter Unit (ASU)?

An Air Starter Unit is a ground support machine that provides the necessary compressed air to start the main engines of an aircraft. Aircraft engines, especially large turbofan and turbojet engines, require an initial rotational force to start combustion. Since they cannot self-start like car engines, external assistance is required to spin the compressor blades fast enough to suck in air, compress it, and ignite the fuel-air mixture.

That is where the air starter unit comes in. The air starter unit generates high-pressure, high-volume compressed air and delivers it through a flexible hose connected to the aircraft’s pneumatic start system. Once the engine reaches a sufficient rotational speed (known as N2 or core speed), the ignition and fuel systems engage, and the engine starts running independently.

In simple words:

• Think of the ASU as a “jump starter” for aircraft engines.
• Instead of electricity, it uses compressed air to kick-start the turbine rotation.

2. Why Aircraft Need an Air Starter Unit

Aircraft engines are powerful and complex. They need external support to overcome inertia and compression resistance at the time of start-up. There are three main reasons why ASUs are required:

1. Large Engines Cannot Self-Start:
   Unlike small piston engines in cars, turbofan engines need external rotation before fuel ignition.
2. APU (Auxiliary Power Unit) May Not Be Available:
   While many aircraft have an onboard APU that provides compressed air for starting, there are situations where the APU is inoperative, under maintenance, or not installed. In these cases, an ASU is the only option.
3. Energy Efficiency on Ground:
   Using ASUs reduces fuel consumption and wear on the aircraft’s own systems. Airports often prefer external ASUs to minimize APU usage, especially for environmental and noise reduction purposes.

3. How Air Starter Unit Works – Step by Step

To understand how ASUs work, let’s go through the process:

Connection to Aircraft:

The ASU has a flexible pneumatic hose that is connected to the aircraft’s pneumatic start port.

This connection ensures an airtight seal to transfer compressed air.

Compressed Air Generation:

Inside the ASU, a diesel engine or electric motor powers a compressor.

The compressor pressurizes ambient air, typically up to 30–50 psi (pounds per square inch) or more, depending on the aircraft requirement.

Air Delivery:

The high-pressure air flows through the hose into the aircraft’s starter turbine.

This turbine is linked to the aircraft’s high-pressure compressor spool (N2 shaft).

Engine Rotation:

As the turbine spins, it accelerates the core compressor, achieving the required RPM for fuel injection.

This RPM is usually around 20–25% of the core speed.

Ignition and Fuel Flow:

Once sufficient speed is reached, the aircraft’s ignition system sparks, and fuel is introduced into the combustor.

Combustion begins, and the engine starts self-sustaining.

Disconnection:

After the engine stabilizes and reaches idle RPM, the ASU is disconnected.

The aircraft is now fully powered by its own engines.

4. Main Components of an Air Starter Unit

An ASU is a sophisticated machine consisting of several key components:

1. Compressor: Produces high-pressure air required for starting.
2. Diesel Engine or Electric Motor: Powers the compressor.
3. Air Tank/Reservoir: Stores compressed air before delivery.
4. Control Panel: Allows operators to monitor pressure, temperature, and flow rate.
5. Flexible Hose & Coupling: Transfers air from ASU to aircraft.
6. Cooling System: Prevents overheating during prolonged operation.
7. Filters: Ensure clean air enters the aircraft’s pneumatic system.

5. Types of Air Starter Units

ASUs come in different designs depending on their power source and application.

Diesel Engine Driven ASU:

Most common at airports.

Portable, mounted on a towable cart or truck.

Independent of external power supply.

Electric Motor Driven ASU:

Powered by electricity from airport power grid.

More environmentally friendly and quieter.

Suitable for airports with strong infrastructure.

Fixed ASU Systems:

Permanently installed at airport gates.

Connected directly to jet bridges or underground systems.

Provide air supply without the need for mobile equipment.

6. Which Types of Aircraft Require ASU?

Not all aircraft require an ASU every time, but there are specific situations where it becomes necessary.

1. Aircraft Without an APU

• Many older aircraft or smaller regional aircraft are not equipped with an APU.
• For these aircraft, ASU is mandatory for engine start.
• Example: Some versions of Tupolev Tu-154, Antonov An-24, and older Boeing 707s.

2. Aircraft with Inoperative APU

• If the onboard APU is unserviceable, the aircraft will depend on ground ASU.
• Airlines often rely on ASUs to prevent delays during maintenance.

3. Large Commercial Jets

• Wide-body aircraft like Boeing 747, Boeing 777, Airbus A330, and Airbus A340 often use air starter units when operating at airports with restrictions on APU usage.

4. Military Aircraft

• Many military jets and transport aircraft rely on ground air starter units due to their engine design.
• Example: C-130 Hercules, Sukhoi Su-27, MiG-29 (depending on configuration).

5. Business Jets and Turboprops

• Some business jets and turboprops either lack APUs or require ASUs in specific weather conditions.

7. Advantages of Using ASU

1. Cost Savings: Reduces reliance on onboard APU fuel consumption.
2. Reliability: Provides consistent, stable air supply for starting engines.
3. Eco-Friendly: Electric air starter unit reduce emissions at airports.
4. Operational Flexibility: Can be used on different aircraft types.
5. Reduced Wear: Minimizes stress on aircraft systems.

8. Challenges and Maintenance of ASU

Like any GSE, ASUs require regular maintenance to remain effective. Some common issues include:

• Air Leaks: Can cause pressure drop.
• Overheating Compressors: Especially during continuous use.
• Filter Clogging: Leads to contaminated air supply.
• Engine Problems: In diesel-driven air starter unit, engine oil and fuel system failures can occur.

Maintenance teams regularly check:

• Hose integrity,
• Pressure calibration,
• Oil and coolant levels,
• Filter replacement.

9. Future of Air Starter Units

With the aviation industry moving towards sustainability and reduced emissions, the future of it is leaning towards electric-powered and hybrid systems. Airports are investing in fixed gate air starter unit systems to minimize the need for mobile diesel-driven units. Moreover, advances in battery technology may soon allow battery-powered air starter units, further reducing environmental impact.

10. Conclusion

The Air Starter Unit is an indispensable part of modern aviation. By providing the compressed air necessary to start aircraft engines, air starter unit ensure safe, efficient, and reliable operations at airports worldwide. While many aircraft today are equipped with APUs, air starter units remain critical backup systems, especially for large jets, regional aircraft without APUs, and in cases where APUs are not functioning. From diesel-powered mobile units to advanced electric systems, air starter units continue to evolve with the aviation industry.