Which scenario best describes an accident involving impairment of ventilation?

The scenario that best describes an accident involving impairment of ventilation is characterized by an unplanned roof or rib fall in active workings. In underground mining operations, maintaining proper ventilation is crucial to ensuring the safety and health of workers. An unplanned fall of rock or debris can obstruct the airflow necessary to ventilate the mine adequately. This obstruction can lead to hazardous conditions, such as increased concentrations of harmful gases or inadequate oxygen levels, posing immediate risks to the miners within the affected area.

In contrast, the other options—routine air flow checks, ventilation system upgrades, and planned ventilation inspections—are proactive measures aimed at assessing or improving the ventilation system. These practices contribute to maintaining a safe working environment rather than creating an incident that impairs ventilation. Therefore, the incident described in the first scenario is correctly identified as one that compromises ventilation and creates an unsafe situation.