During O2 flight mode, what position must the crossover valve be in?

In the context of oxygen systems used in aviation, during O2 flight mode, the crossover valve must be in the vertical position. This positioning is crucial for the proper flow of oxygen to the crew members during flight operations at high altitudes where supplemental oxygen is essential. When the crossover valve is vertical, it allows for uninterrupted oxygen flow from one side of the system to the other, ensuring that all crew members can receive the needed oxygen without restriction.

The other potential positions of the valve, such as horizontal or diagonal, would impede the flow or create potential issues in the oxygen delivery system, which can jeopardize aircrew safety during critical flight operations. Therefore, maintaining the crossover valve in the vertical position is essential for optimal function of the oxygen system during flight.