What do the colors represent and indicate on the refueling boom?
They represent the same thing they usually represent anywhere else (at least in Western culture, I am not that familiar with others):
- Green = good
- Orange = warning
- Red = bad
Now, the more interesting question is not what they represent, but what situation could make it turn green, orange, or red?
The part of the boom that is on the right on the photo is the part that is attached to the tanker. The part on the left is the part that attaches to the receiving aircraft.
The left part slides inside of the right part, to give the two aircraft a little bit of room to maneuver. Remember, the boom is rigid, if this telescoping were not possible, the two aircraft would have to stay in exactly the same relative position, up to the millimeter!
So, if the receiving aircraft speeds up and gets closer to the tanker, the inner pipe gets pushed further into the boom, if the receiving aircraft slows down and gets farther away from the tanker, it pulls out the inner pipe from the boom.
The green part is where you normally want the pipe and boom to be. If you get too close to the tanker and risk crashing into it, you push the pipe in further to the orange part and ultimately to the red part. Usually, once you hit the red part, you abort the refueling, and the receiving aircraft breaks off.
Likewise, if you get too far away, you risk ripping off the boom. So, again, as you get further away, you get first to the orange part and then to the red part, at which point, you abort the fueling process.
The colors correspond with various fore/aft # of feet that define the limits of the extension and compression capacity of the boom. There is also a correlation between the lights on the underside of the KC-135 and the markings on the boom. All are ways of determining the fore/aft position as the receiver pilot during Air-Air Refueling.