How can you tell if an aircraft engine is running?

• A. Anti collision lights are flashing
• B. The cockpit radio is on
• C. The engine noise is audible
• D. The fuel gauge shows full

Answer: (A)

When an aircraft is powered, certain systems show it’s energized, and the anti-collision beacon is the clearest signal of that state. These lights flash to indicate electrical power is on, which happens during engine start and while the aircraft is operating. So seeing the beacon flashing is a reliable cue that the aircraft is powered and the engines (or at least the power source like the APU) are active.

The other indicators aren’t as dependable here: the cockpit radio being on could be due to preflight or ground communication at any time, not necessarily because the engines are running; engine noise can be muffled or inaudible from outside or inside the cabin, especially if you’re farther away; and a full fuel gauge has no relation to engine status.