What is the highest flight level that operations may be conducted without the pilot at the controls wearing and using an oxygen mask, while the other pilot is away from the duty station?

Operations may be conducted without the pilot at the controls wearing and using an oxygen mask up to a maximum flight level of 25,000 feet. This is based on regulations that state that at and above 25,000 feet, pilots must have supplemental oxygen available and must use it when the cabin altitude exceeds certain levels, especially when the other pilot is not present in the cockpit.

In practical terms, this regulation is in place to ensure that pilots maintain adequate levels of alertness and performance in the event of cabin depressurization or other emergencies while flying at high altitudes. Therefore, with one pilot away, the remaining pilot must have effectively controlled conditions and be prepared for rapid changes at any moment. Operations beyond this specified limit require both pilots to utilize oxygen masks to ensure safety and compliance with aviation standards.