Time of Useful Consciousness (TUC) is the time from onset of hypoxia until the pilot can no longer take corrective action (e.g. don oxygen, descend). Not the same as unconsciousness — TUC ends when the pilot is still awake but no longer able to act.
Approximate values (sitting, resting, healthy adult)
| Altitude | TUC (rest) | TUC (with exertion) |
|---|---|---|
| 18 000 ft | 20–30 min | 10–15 min |
| 22 000 ft | 5–10 min | 3–5 min |
| 25 000 ft | 3–5 min | ~2 min |
| 30 000 ft | 1–2 min | 30–60 s |
| 35 000 ft | 30–60 s | 15–30 s |
| 40 000 ft | 15–20 s | 10 s |
| 43 000 ft | 9–12 s | 5 s |
(Source: FAA AIM 8-1-2 and USAF Aircrew Manuals — established aeromedical reference values.)
Factors that shorten TUC
- Physical exertion (~50 % shorter)
- Smoking (raised CO-Hb)
- Alcohol (residual)
- Stress
- Cold
- Anaemia
- Mountain sickness or unaccustomed altitude
Relevance to VFR PPL
A PPL operating below 10 000 ft in a SEP is normally safe from acute hypoxia — but:
- Night vision suffers from ~5 000 ft (see §5 Vision).
- During alpine climbs / turbocharged operation > 10 000 ft → TUC and symptoms become relevant.
- In emergency oxygen scenarios in unpressurised aircraft: in case of sudden pressure loss (rare in SEP) every second counts.
Practical implications
- Pulse oximeter (SpO₂ measurement) in the cockpit as an early warning (battery-powered, commercially available).
- Recognise compensatory and disturbance stages early — on suspicion, descend immediately.