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 required whenever the aircraft is operating to make it clearly visible to other traffic. On the ground, when the engines are running, the aircraft is still in an active phase (taxiing, engine start/run-up), so having the lights on provides crucial visibility and reduces the risk of collisions with ground or air traffic. The other scenarios either imply the aircraft is not in an active operating state (e.g., being towed with engines off, or daylight-only periods) or restrict visibility to conditions that don’t align with standard operating safety. Therefore, the correct requirement is to display anti-collision lights on the ground when the engines are running.