Final answer:
The exact time for when oxygen is required on unpressurized aircraft at altitudes above 13000 feet is not specified in the provided references, but aviation regulations generally mandate immediate oxygen use above 14,000 feet and after 30 minutes of exposure between 12,500 and 14,000 feet.
Step-by-step explanation:
For aircraft operated in an air transport service with an entire period of flight at cabin-pressure altitudes above 13000 feet ASL, all persons on board the aircraft require oxygen for unpressurized aircraft. The answer to the student's question regarding how long it would take at such altitudes for oxygen to be necessary is not directly provided in the reference material. However, according to Federal Aviation Regulations, at cabin pressure altitudes above 12,500 feet (up to and including 14,000 feet), oxygen must be used after 30 minutes of exposure to those altitudes. Above 14,000 feet, oxygen must be used immediately. The Oxygen Requirements for Aircraft may vary based on regulations and aircraft type, so for the most accurate guidelines, one should refer to current aviation regulations or safety guidelines from the FAA or a similar regulatory body in their respective country. It's crucial to maintain proper oxygen levels to prevent hypoxia, which can severely impact a person's health and safety during flight. This physiological concept is critical in situations like the operation of unpressurized aircraft at high altitudes.