Understanding the Function and Importance of APU in Airplanes
Have you ever wondered how an airplane is able to maintain its systems and functions, even when the main engines are not running? The Auxiliary Power Unit (APU) plays a crucial role in ensuring that airplanes can operate smoothly and efficiently. This article will explore how APU works in an airplane, its benefits, and some critical incidents related to APU fires.
What is an APU?
APU is an auxiliary power unit, a small engine that generates electricity for various purposes, including air conditioning, lighting, and hydraulic pressure, among others. Unlike the main engines, which are designed primarily to propel the plane, the APU is designed to provide auxiliary power. This makes it possible to run essential systems without the main engines operating.
Functions of APU
APUs have several key functions, especially in commercial and military airplanes:
Electric Power Generation: The APU is responsible for providing electrical power when the main engine is not running. This is often used for powering the in-flight systems, cabin lighting, and other essential components. Hydraulic System Pressure: The APU can also generate hydraulic pressure, which is important for the plane's hydraulic systems, such as the rudder and elevator controls. Starting the Main Engines: In some cases, the APU can be used to start the main engines without the need for external power or large batteries. This is particularly useful in emergency situations where the main engines might be inoperable. Ground Operations Power: On the ground, the APU can power various systems while the main engines are off, such as the air conditioning and electrical systems.Notable Incidents and Reasons for Flight Restrictions
While the APU is a valuable component, it is not without its risks. Several incidents have highlighted the dangers associated with APU fires:
US Airways 1549 Emergency
During the 2009 emergency landing of US Airways Flight 1549, the APU played a crucial role in maintaining the plane's hydraulic pressure and steering capabilities. Despite both engines failing, the APU allowed the plane to make a successful emergency landing on the Hudson River, resulting in no injuries to the passengers and crew. This incident underscores the importance of an APU in maintaining control of the aircraft.
Aircraft Accidents Due to APU Fires
Tragically, APU fires have resulted in several major accidents:
Tu-144 Airliner Crash: In the USSR, a Tu-144 airliner crashed due to a fuel leak ignition by the APU, highlighting the potential for catastrophic failures. B-29 Recovery Operation: In a 1990s attempt to recover an abandoned B-29 bomber from Greenland, an APU fire led to the evacuation of the aircraft, which subsequently burned to the ground.These incidents demonstrate the need for strict safety measures and regular maintenance to prevent APU-related accidents.
Types of APU
There are two main types of APU:
Engine-Based APU
The first type of APU is a tiny jet engine or turbine that powers a generator. This design is often employed for ground operations, where the main engines can be shut down, and power is still required for various systems. Some large commercial aircraft have also utilized generator-based APUs, similar to a home generator, though this is not as common in modern aircraft designs.
Turbine-Based APU
The second type of APU is a small wind turbine-like device with a propeller that drives a generator. This design is more compact and is often used for extended ground operations when the main engines are shut down.
Placement of APU
The APU is typically located at the tail of the airplane, often with its exhaust pipe being the most rearward part of the plane. This placement helps to minimize the impact of APU operations on the overall aerodynamics of the aircraft and also provides better access for maintenance and repairs.
In summary, the APU is a critical component in airplane operations, providing essential auxiliary power and enhancing safety, especially in emergency situations. While APU fires present significant risks, ongoing advancements in technology and maintenance practices continue to reduce these risks, making the APU an invaluable asset in modern air travel.