Understanding Aircraft Spoilers and APU Activation in Engine Failures

A question often arises regarding the activation of aircraft spoilers in the event of engine failure: Why don't pilots activate spoilers through the Auxiliary Power Unit (APU)? This article aims to clarify the roles and functionalities of spoilers and the APU in the context of engine failure, addressing common misconceptions and providing a detailed explanation.

Overview of Spoilers and the APU

Moderately complex aircraft, particularly those in the transport category, utilize spoilers to help maintain control and ensure a safe descent. Spoilers function by disrupting the airflow over an airplane's wings, resulting in a reduction of lift and an increase in drag. This is essential during landing or in the event of a single engine failure to decelerate the aircraft effectively. On the other hand, the APU (Auxiliary Power Unit) serves as an independent power source, typically used for starting the main engines and powering auxiliary systems. It is not directly involved in the deployment or actuation of spoilers.

Role of the APU in Engine Failures

During an engine failure, the primary role of the APU is to provide electrical power, as most aircraft generators are linked to the engines. Once the APU kicks in, it can power various systems, including communications, lighting, and the starting systems for the main engines. However, the APU alone does not contribute to making the aircraft descends, as its main functions are to maintain flight systems and provide redundancy. The deployment of spoilers is a hydraulic or manual process and is not directly assisted by the APU.

Hydraulic Systems and Backup Power

Aircraft are equipped with hydraulic systems that are essential for the operation of spoilers, actuators, flaps, and other components. In the event of main engine driven hydraulic pumps failing (due to engine failure), the aircraft is designed with accumulators to provide backup hydraulic power. These accumulators store hydraulic fluid and can maintain system pressure, ensuring the spoilers and other hydraulic systems function correctly until the issue is resolved or the aircraft can land.

Pilot Actions During Engine Failures

Pilots have a series of actions to take during an engine failure to maintain control and ensure a safe landing. Once an engine failure is detected, the pilots will start the APU to ensure electrical power is available. They will then initiate a specific procedure, often known as a "Single Engine Procedure," to manage the aircraft's speed, altitude, and descent rate. Spoilers are used to help the aircraft maintain control and decelerate appropriately. The pilots may deploy spoilers manually through the flight deck controls, even if the APU is activated.

Key Takeaways

The APU is a crucial component for providing electrical power in engine failures, but it does not directly control or activate spoilers. Pilots use spoilers to help manage descent rates, decelerate, and maintain control during engine failures, aerodynamic phenomena, or routine flight conditions. Accumulators store hydraulic power to ensure the continued operation of spoilers and other hydraulic systems when main hydraulic pumps fail.

Conclusion

In summary, the APU and spoilers serve distinct roles in aircraft operations. The APU provides electrical power in the aftermath of an engine failure, while spoilers help manage the aircraft's performance, especially during adverse conditions such as engine failures. Understanding these roles and functionalities is critical for both pilots and passengers to appreciate how aircraft are designed and operated to ensure safety and efficient flight.