In which situation is supplemental oxygen not required for aircraft occupants?

The requirement for supplemental oxygen in aircraft depends largely on the altitude at which the flight operates. At altitudes below 12,500 feet MSL (Mean Sea Level), supplemental oxygen is not mandated for pilots and passengers, as the atmospheric pressure is sufficient to provide adequate oxygen for normal physiological functioning.

However, it is important to note that this is applicable only for unpressurized flights. Above this altitude, the effects of decreased oxygen availability begin to impair cognitive and physical performance, necessitating the use of supplemental oxygen. While flight tests, night flights, or ground taxiing may have specific operational considerations, none of those situations inherently exempt the requirement for oxygen based solely on altitude. Therefore, when considering the altitude factor, the correct choice regarding the absence of the requirement for supplemental oxygen aligns with operations below 12,500 feet MSL.