What indicates a properly functioning fire warning system during testing?

A properly functioning fire warning system during testing is indicated when both the FIRE and MASTER WARN annunciators illuminate. This illuminates the alert system effectively, signaling to the pilot that the fire detection system has detected a potential fire condition or failure. Illuminating these warning lights is a part of the normal functionality of the system when it is tested, serving to confirm that the annunciators are operational and ready to alert the crew in an emergency situation.

The other options, while they may suggest aspects of the aircraft’s performance or safety systems, do not specifically indicate that the fire warning system is functioning correctly. A lack of sound indication does not confirm system functionality, and if the fire suppression system activates, it may indicate an actual fire rather than just a test condition. Stable temperature readings might imply normal engine operation, but they do not provide direct evidence regarding the fire warning system’s operational status. Thus, the illumination of both the FIRE and MASTER WARN lights is the clear and direct indication of a properly functioning fire warning system.