They are used to illuminate the leading edge for ice detection, they can also be used at night whilst on ground to give an indication about the dimensions of the aircraft (same as logo lights). Some airlines put them on and off as standard together with the beacons. I have flown ATR before, on the ATR we used them to indicate the number 2 engine was running in hotel mode (engine running with propeller brake on), if we didn’t used hotel mode we turned them on and off with the beacons.
1 Like