Aircraft equipped with red anti-collision lights may display them:
- outside the daylight-period in flight, but not on the ground when being towed.
- while taxiing, but not when being towed.
- outside the daylight-period at engine-start. During the daylight period this procedure is not applicable.
- on the ground when the engines are running.
Fixed wing aircraft must have at least one anti-collision light which may be either red or white.
If fitted with a white anti-collision light, this may be displayed for engine start but it must be displayed during taxiing. (This rule has possibly come about because a white anti-collision light has a 'strobe-style' effect and could be distracting for the start crew.)
If fitted with a red anti-collision light then this must be displayed for engine start and taxiing.
(Helicopters only have red anti-collision lights!)