The switch lights have two filaments each. I have seen many times only half of a white bar showing, left or right half. From the above picture you can see LNAV and VNAV and a SPEED mode engaged by looking at the three green symbols at the top of the Primary Flying Display. This is the only real way to see which modes are active in the autopilot system. The lights in the switches themselves are relatively unimportant and could be totally unserviceable without affecting the operation at all. Of more concern in the above picture is the amber caution warning at the top of the centre screen. This would be the focus of my attention but the picture resolution does not enable me to comment further.
If a switch light is pressed and the mode is just armed instead of active, as in the case of the Glide Slope Capture mentioned above, then the symbol/mnemonic on the PFD will show in amber rather than green, but the corresponding switch light will still show white. The switch lights are only on (armed or active) or off (inactive).
Last edited by Oldy; 26th Aug 2006 at 16:58.