What is a critical step after applying brakes when the tow bar separates?

When a tow bar separates while an aircraft is being towed, one critical step that must be taken is to drop the tail hook. This is vital for safety reasons, as it ensures that the tail of the aircraft is secured and that there is no risk of it inadvertently moving or swinging. Dropping the tail hook serves to stabilize the aircraft and prevent potential damage or accidents that could occur if the aircraft were to roll or shift unexpectedly.

In this context, the other options do not adequately address the immediate concerns related to safety and control of the aircraft. While inspecting the fuel level, turning off the engine, or changing the tow bar may be important maintenance and operational tasks, they do not directly mitigate the risks posed by the separation of the tow bar at that critical moment. The priority following such an incident is to stabilize the aircraft effectively, making the action of dropping the tail hook particularly relevant.