Which oxygen system requires training and is used above 40000 feet?

The pressure-demand oxygen system is specifically designed for high-altitude operations. It requires training due to the complexities of its operation, especially at altitudes above 40,000 feet where the atmospheric pressure is significantly lower and where the need for a reliable oxygen supply becomes critical.

In pressure-demand systems, oxygen is delivered to the user at a pressure greater than the environmental pressure, ensuring that it reaches the lungs effectively despite the thin air. As altitude increases, the need for supplemental oxygen becomes crucial, and the pressure-demand system ensures that the user receives a sufficient volume of oxygen regardless of the external atmospheric conditions.

This system also reduces the risk of hypoxia, which is vital for flight safety at high altitudes. Training is essential to ensure proper understanding and operation of such systems, making it necessary for pilots who operate above 40,000 feet. The other options listed are more suitable for lower altitudes, and while some may not require specific training, the pressure-demand system is a key part of high-altitude safety protocols.