The TSB’s investigation concluded that, during the ascent to and continued flight at 15,000 feet without supplementary oxygen, the pilot likely experienced a headache, dizziness, air hunger, and fatigue, as well as gradual degradations in thinking, memory, judgment, and muscular coordination. His reactions would also have slowed down. His hearing capacity may have decreased while his peripheral vision most likely narrowed as the visual field darkened. The pilot may have found it more and more difficult to perform tasks using working memory or to judge airspeed, heading, and orientation. However, because of the slow and gradual onset of the hypoxic symptoms and the probable associated sense of well-being, it is unlikely the pilot noticed the effects.