[sunkencity]

absence of signal is not a good indicator of operation. it is the same signal as total failure of your system.

[dingaling]

Interestingly that reflects Airbus vs Boeing cockpit philosophies.

In the early 1980s Airbus adopted Dark Cockpit where the default state of all annunciator lights is off. If one illuminates then it indicates something worthy of attention. The colour of illumination indicates functional state; blue is good, amber is malfunctional.

[andai]

When would something illuminated blue? When it's both working smoothly and worthy of attention?