How does the APU control panel indicate that the unit is operational?

The APU control panel uses a green indicator light to signify that the unit is operational. This green light typically means that the APU is providing the necessary power or systems that it is designed to support, such as electrical power or bleed air for engine starting. In aviation systems, green lights generally indicate that systems are functioning properly, making it a standard color for operational status indicators.

In contrast, other colors like red usually indicate a warning or fault condition, while yellow is employed for cautionary situations that require attention but are not urgent. Blue might be used for indicating specific conditions or features, but it does not represent operational status in the context of an APU. Understanding these color codes is critical for operators to quickly assess the functionality and safety of the APU during ground operations and while in flight.