When may a controller issue a safety alert to an aircraft?

Issuing a safety alert to an aircraft is crucial for ensuring the safety of flight operations. A controller may need to alert the pilot when the aircraft is in unsafe proximity to terrain, obstructions, or another aircraft. This scenario is directly related to the immediate safety of the flight, as the proximity to hazards poses a risk of collision or accident.

When aircraft are approaching altitudes or distances that could lead to dangerous situations—such as flying too low near mountains or other structures, or too close to other aircraft—the controller has the responsibility to inform the pilot promptly. This type of alert helps to prevent accidents by providing the pilot with essential information that allows them to take evasive action or adjust their flight path to maintain a safe distance from potential hazards.

The other options, while relevant to safety, do not pertain to immediate threats in the same direct manner as unsafe proximity. For instance, severe thunderstorms and adverse weather conditions can be significant hazards, but the unsafe proximity to terrain or other aircraft represents an urgent and critical situation that necessitates an immediate response from the controller.