An aircraft shall display an anti-collision light:

• A. while taxiing, but not when it is being towed
• B. outside the daylight-period in flight, but not on the ground when it is being towed
• C. outside the daylight-period at engine-start. During the daylight-period this is not applicable
• D. on the ground when the engines are running

Answer: (D)

Anti-collision lights are used to make the aircraft highly visible to others, especially when the aircraft could be moving or starting up. The safest and most general rule is that these lights should be on when the aircraft is operating on the ground and the engines are running, such as during engine start and taxiing. This helps prevent collisions with other aircraft or ground vehicles as soon as the aircraft begins moving or could start moving.

The other options constrain when the lights must be on in ways that don’t align with this safety purpose—for example, limiting lighting to only taxiing, or to certain daylight or towing situations—whereas the fundamental requirement is to have the anti-collision lights on on the ground whenever the engines are running.