IFOs
Helicopters
Sound travels further after
dark, so even when quite far away, the noise of the rotor blades should be
heard. However if the wind is blowing away from your position, this could make
it harder to hear or even appear silent. Like aeroplanes, helicopters have
distinctive lighting configurations that generally include the colours white, red, blue and green.
At a distance, like aircraft, the lights on helicopters will often blur into one brightly glowing yellowish light. It is only when you get closer that the individual lights of the helicopter can be made out.

