Which command indicates a need to cease engine operation?

The command "Stop engines" clearly indicates the necessity to cease engine operation. This command is definitive and leaves no room for interpretation, directing personnel to immediately halt all engine functions. It is essential for ensuring safety in various situations, such as during emergencies or when instructed by ground crew, and ensures that all personnel understand the importance of stopping the engines as a priority action.

In contrast, other commands like "Remove chocks," "Ready to depart or ready to tow," and "Activate airstart unit" pertain to different operational procedures. Removing chocks is a preparatory step for movement, the readiness commands signal that a departure or tow is forthcoming, and activating an airstart unit is related to starting the engines rather than stopping them. Therefore, the clarity and direct intent of the command "Stop engines" make it the correct response to indicate the need to cease engine operation.