A Lesson On Redundancy: Auxiliary Power Units

Modern air travel is made possible through a series of complex aviation systems and safety mechanisms that are designed to ensure aircraft operate reliably under all circumstances. One such crucial component is the Auxiliary Power Unit (APU), which is a system specifically engineered to bolster engine redundancy and provide power support when it is needed most. While APUs do not contribute directly to propulsion, they play a critical role in maintaining essential aircraft functions during various phases of flight.

This blog will serve as a guide to APUs, exploring their functions, advantages, and operational contingencies. We will also highlight the importance of reliable sourcing for such equipment in the aviation industry, so read on to learn more.

What Is an Auxiliary Power Unit (APU)?

An Auxiliary Power Unit (APU) is a form of compact, self-contained engine system that supplies power for aircraft systems when the main engines are not running. Most often located in the tail cone section of commercial and military aircraft, these apparatuses are essential for maintaining operational continuity and electrical power during ground operations and emergency situations.

Key Characteristics of APUs:

• Non-propulsion Functionality: APUs are used to supply electrical, pneumatic, and hydraulic power for various functions to be carried out without engine operation.
• Location: As stated before, APUs are typically housed at the rear of an aircraft.
• Operational Modes: APUs are generally designed to function while on the ground, generally shutting off once the aircraft is airborne.

Core Functions of APUs in Aviation

1. Engine Startup

APUs are regularly relied on to provide the initial burst of bleed air needed to spin turbines and ignite the main engines of an aircraft. Without an APU, external power sources like a start cart would be required, making operations potentially more complex or time-consuming.

2. Cabin Comfort and Power

APUs are regularly used to power air conditioning and lighting systems before engine startup, ensuring that cleaning crew can safely navigate spaces, customers are comfortable when entering, and systems can be tested as necessary.

3. Emergency Power Supply

In the event of an in-flight electrical failure, an APU can be used as a backup generator. In such situations, the system will provide power to essential flight controls, navigation equipment, and instrumentation.

What Happens If an APU Fails?

While APUs are not essential to keeping an aircraft airborne, their role in redundancy is invaluable. As such, it is important to know what to do in the occurrence that an APU happens to fail.

On the Ground

If an APU fails before engine ignition, external tools like ground start carts will become necessary to deliver bleed air for starting. Flight operations may also have to be aborted if the issue poses a safety risk or is against regulations.

In the Air

If an Auxiliary Power Unit (APU) fails mid-flight, the aircraft can still maintain operability. At this time, one may try to take advantage of engine cross-bleed, where one working engine can be used to restart the other. Otherwise, aircraft may try and attempt to descend to generate sufficient airflow to restart turbines.

Why Redundancy Matters in Aviation

Aircraft systems are designed with multiple backups and redundancies to ensure safety across numerous operational scenarios. APUs serve as an example that enables continuity when standard systems encounter faults. For example, redundant APU systems can provide benefits like:

• Minimizing risk during engine failures or electrical malfunctions
• Allowing flight crews to maintain control and restore systems mid-flight
• Enhancing aircraft self-sufficiency, reducing reliance on ground-based systems

The Testing and Certification of APUs

To ensure reliability, APUs undergo rigorous testing processes that are designed to simulate a range of conditions to assess system performance under various pressures and altitudes. Common testing practices include:

• Environmental Simulation: This includes high and low temperature testing to ensure operability.
• Altitude Testing: Pressure and bleed air output validation at simulated cruise altitudes is important.
• Real-Time Data Monitoring: This practice involves using high-speed data acquisition systems to capture performance metrics.
• Software Correction Tools: Performance correction algorithms help technicians calibrate units for peak output.

Sourcing Reliable APUs and Avionics Connectors

In the demanding aviation industry, dependability and airworthiness starts with sourcing quality parts. That is why ASAP Semiconductor is proud to offer customers access to a wide range of high-quality avionics connectors and related components through its website, Aviation Sourcing Solutions. By working with leading global manufacturers and maintaining a robust inventory, Aviation Sourcing Solutions delivers tailored procurement options that help reduce the complexity of sourcing and fulfillment. With a team that prioritizes hands-on service, customers can meet all operational requirements with confidence on one convenient website. Get in touch with our staff today to discuss the solutions we can provide.