Which phase of flight is typically associated with the greatest risk of hypoxia?

The climb phase of flight is typically associated with the greatest risk of hypoxia because it involves an increase in altitude where the atmospheric pressure decreases and, consequently, so does the partial pressure of oxygen. As an aircraft ascends, the available oxygen in the environment becomes thinner, which can lead to a deficiency of oxygen available for breathing if the aircraft is not pressurized or if the pilot or passenger is not using supplemental oxygen.

During the climb, pilots can experience an increased demand for oxygen due to physical exertion and stress, which also heightens the risk of hypoxia. This is particularly true for unpressurized aircraft operating at higher altitudes. It's crucial for pilots to be aware of this risk and be prepared to use oxygen if they are climbing to altitudes where the risk increases, such as above 10,000 feet.

Other phases of flight, such as takeoff, descent, and cruise, typically do not present the same level of hypoxia risk, as most aircraft are still at lower altitudes during takeoff and descent, and cruise generally stabilizes at an altitude where supplemental oxygen may not be required depending on the specific altitude and regulations.