Final answer:
A FAULT light with an ECAM caution message on the MASTER SW pushbutton indicates a malfunction or fault with the Auxiliary Power Unit on an aircraft, signaling the need for crew attention and potential troubleshooting.
Step-by-step explanation:
A FAULT light on the MASTER SW (switch) pushbutton accompanied by an ECAM caution message generally indicates that there is a malfunction or a fault condition with the Auxiliary Power Unit (APU) on an aircraft. A FAULT light with an ECAM caution message on the MASTER SW pushbutton indicates a malfunction or fault with the Auxiliary Power Unit on an aircraft, signaling the need for crew attention and potential troubleshooting.
The APU is a device used to provide energy for functions other than propulsion, such as powering the electrical systems of the aircraft when the main engines are not running. When this light illuminates, it is advising the crew that there is a system failure or operational issue with the APU that needs to be addressed. The Electronic Centralized Aircraft Monitor (ECAM) will provide a caution message to give the pilot further details on the nature of the fault and appropriate troubleshooting steps or procedures to follow.