What does APU stand for in helicopter operation?

The term APU stands for Auxiliary Power Unit in the context of helicopter operations. An Auxiliary Power Unit serves as a secondary power source that can provide electrical power and pneumatic energy needed for engine start-up and other systems when the main engines are not running. This is particularly important for ground operations, as it allows the helicopter to function efficiently without relying on its main engines, ensuring that essential systems remain operational even when the helicopter is on the ground. Additionally, the APU can help with air conditioning and other support functions, making it a crucial component in maintaining aircraft readiness and efficiency during preflight checks and while on the ground.

In contrast, the other options refer to concepts that do not accurately describe the role of an APU in helicopter operations. Automatic Pilot Units relate to flight control systems, while Aerodynamic Performance Units and Air Propulsion Units do not represent standard terms used in the aviation industry for power and energy management. The APU's unique function as a support system distinctly sets it apart from the other choices.