Originally Posted by learsimmer
Sorry to be pedantic, but I want to be quite clear about the operation of these buttons.
Nothing pedantic about that to me.
My description is valid for Concorde, but I don't think the basic principle has changed at all since.
When you write "when a mode is engaged" do you refer to an armed mode becoming active...
Yes.
... or do you refer to pressing the button?
No.
Use glide slope as an example.
Imagine yourself on centerline during the approach, ALT HOLD mode at 3000 ft, but still far enough out to be well under the glideslope.
ALT HOLD engaged and illuminated.
Press the GLIDE SLOPE button.
The G/S mode is now "armed" or "primed". The button does not illuminate, but the "prime" light under it lights up.
When the AFCS detects you're at the right point, G/S mode engages automatically, takes over control and eases you onto the glideslope.
At this point the "prime" light goes out and the GLIDE SLOPE button itself illuminates, indicating the mode is now engaged and controlling the aircraft.
The ALT HOLD mode disengages at the same time, and the ALT HOLD light goes out.
What color(s) do the buttons display?
For more recent aircraft, you'll have to ask somebody else....
On Concorde the buttons themselves are white, with black letters.
The "prime" light is a triangle under the button, but not sure of the colour (it's over 30 years...).