Gunsight, is it off or is that the dimmest setting?

[Blackdog_kt]

I think this is covered in the manual, saying that since the light bulbs don't fail like in the real thing there's no need to completely turn it off. In reality they only turned it on for combat to preserve the life of the light bulb, but since this is not modeled in the sim they decided to have it set to its lowest setting by default.

Atl+L switches between 0% and 100% brightness, but if you click and hold on the relevant controls in the cockpit you can adjust it to intermediate values as well (useful if you're flying during early morning/late afternoon, because then 100% is way too bright).