In which scenario is quick-donning oxygen mask most crucial?

The scenario where a quick-donning oxygen mask is most crucial is during a sudden altitude loss. In this situation, there may be a rapid decrease in cabin pressure, leading to a potential hypoxia risk. Quick access to an oxygen mask allows pilots and crew to immediately secure an adequate supply of oxygen to maintain consciousness and cognitive function.

This is vital because a sudden loss of altitude can impair a crew's ability to react appropriately and manage the aircraft safely. The quick-donning feature is specifically designed to enable the crew to put on the mask rapidly, even while seated and working at the controls, which is essential during emergencies where time is critical.

In contrast, normal operations generally do not involve immediate risks that would require urgent use of an oxygen mask, and preparations such as donning masks before landing or during taxiing would not typically reflect the same level of urgency. Therefore, the assessment of the most crucial scenario for using a quick-donning oxygen mask underscores the necessary response to sudden and hazardous changes in altitude.