Which category is considered the most severe for runway incursions?

Category A incursions are classified as the most severe type of runway incursion because they involve a serious risk of collision between an aircraft and a vehicle or another aircraft. This category is characterized by incidents where an immediate and significant threat to safety is evident, with the potential for severe consequences if action is not taken. Essentially, a Category A incursion typically results from a high probability of collision due to the close proximity of the aircraft or vehicle to an active runway while it is in use.

In contrast, the other categories—B, C, and D—represent less severe incidents that do not carry the same level of imminent danger. They might involve situations where the risk of collision exists but is considered lower, either due to the nature of the incursion or because the circumstances were not as critical as those found in a Category A incident. Understanding these classifications is crucial for maintaining safety in airport operations, ensuring appropriate responses based on the severity of the situation.