Which sequence correctly pairs flight levels with their Time of Useful Consciousness?

• A. FL300 — 1-3 minutes; FL350 — 30-60 seconds; FL400 — 15-20 seconds
• B. FL300 — 15-20 seconds; FL350 — 1-3 minutes; FL400 — 30-60 seconds
• C. FL300 — 30-60 seconds; FL350 — 15-20 seconds; FL400 — 1-3 minutes
• D. FL300 — 1-3 minutes; FL350 — 15-20 seconds; FL400 — 30-60 seconds

Answer: (A)

Time of Useful Consciousness is the window you have to think and act before hypoxia makes recovery actions difficult. As you climb higher, ambient pressure drops and the amount of oxygen reaching your brain falls, so this window shrinks. That's why TUC is longest at lower altitudes and becomes progressively shorter at higher ones. The sequence that matches this pattern shows a longer time at FL300 (about 1–3 minutes) and shorter times as altitude increases: roughly 30–60 seconds at FL350 and about 15–20 seconds at FL400. This reflects the steep drop in usable oxygen with altitude. Remember, actual times vary with exertion, individual physiology, and whether supplemental oxygen is used, but the trend remains the same.