What indicates that the APU is ready to supply electrical power?

The indication that the APU is ready to supply electrical power is represented by a green indicator light. This light typically signifies that the APU has successfully reached operational status and is ready to provide power to systems as needed. The green color universally denotes a safe and operational condition in aviation, thus instilling confidence that the APU is functioning correctly and can be relied upon for electrical supply.

Other indicators, like a flashing red light or a yellow caution light, usually denote malfunction or cautionary states, which would require further attention before considering the APU ready for operation. A blue operational light may indicate some aspect of the APU's functionality but does not universally represent readiness to supply electrical power in the same way that the green indicator light does.