Understanding the Role and Limitations of APU in Airline Operations

Introduction to APU in Aviation

The assumption that airlines fly without an Auxiliary Power Unit (APU) is based on a fundamental misunderstanding of modern aviation technology. In reality, most sizeable airliners are equipped with an APU, which plays a crucial role in various aspects of flight operations. This article aims to clarify the functions of an APU and explore the reasons why an APU might not be used in specific situations.

Primary Functions of the APU

APUs are essential tools in modern aircraft, serving several critical functions:

Ground Operations: On the ground, the APU provides electrical and hydraulic power, as well as air conditioning, replacing the need for external power sources and ground support equipment. This ensures that the aircraft can maintain its systems fully operational even when the main engines are not running. Airborne Backup: In the air, the APU acts as a backup power source if a primary system fails. For example, in the event of a single engine failure, the APU can provide electrical and hydraulic power, ensuring the aircraft can continue to function. Critical Emergencies: In extreme cases such as dual engine failure, the APU becomes the last resort to maintain control of the aircraft. Experienced pilots rely on the APU to initiate emergency procedures and stabilize the plane. A notable example is the SULLY incident, where Captain Chesley Sullenberger used the APU to successfully land an aircraft on the Hudson River.

The Reasons for Not Using the APU

While most modern airliners are equipped with an APU, there are situations where not using the APU is necessary or preferred. Here are some of the reasons:

1. APU Inoperable

In some cases, the APU may be inoperable due to maintenance or servicing issues. When this happens, ground support equipment (GSE) must be used to provide power and amenities.

Electric Power and Ground Heating/cooling: External power sources and ground heating/cooling units are required to compensate for the lack of APU support. Pushback Operations: Before the aircraft can be pushed back, one engine must be started at the gate, followed by the start of the second engine. This increases the risk for ground personnel and requires greater coordination with air traffic control (ATC).

2. Local Airport Regulations

Some airports have regulations that restrict or limit the use of APUs due to considerations such as noise, heat, and pollution. Modern APUs can burn approximately 100 pounds of fuel per hour. These regulations aim to minimize environmental impact and ensure safer flying conditions.

3. Fuel Efficiency and Maintenance Costs

Many airlines adopt strategies to minimize the use of APUs to save fuel and reduce maintenance costs. By reducing reliance on the APU, airlines can see a positive impact on their operational overhauls and fuel expenses in the long term.

Conclusion

The APU is an indispensable part of modern aviation, playing a vital role in ground and air operations. While it is rare for an APU to be inoperable, there are valid reasons why airlines may choose not to use the APU under certain circumstances. Understanding the functions and limitations of the APU is crucial for both pilots and ground support staff to ensure the safe and efficient operation of aircraft.