Headache at 10,500 Feet

Shortlist, in order of likelihood in this cockpit:

1. Mild hypoxia - 10,500 ft in a healthy non-smoker produces exactly this: headache, tingling, euphoria, degraded judgement. Night flying lowers the threshold, and the pilot's first warning is usually that nothing feels wrong.
2. Carbon monoxide - cabin heat is selected, and a cracked heat muff or leaking firewall fitting feeds exhaust into the cabin. Headache and light-headedness are the classic pair; the tingling fits too.
3. Fatigue and hunger - 13 hours awake, 3.5 h flying, no lunch, night. Fatigue does not cause tingling, but it removes the reserve that would otherwise mask mild hypoxia and it degrades the pilot's ability to notice the problem.
4. Hyperventilation - possible if the pilot has begun breathing fast out of anxiety; tingling and light-headedness fit, but there is no obvious stressor yet.

Discrimination tests:

• Turn the heat off and open fresh-air vents for 3–5 minutes. CO symptoms ease; hypoxia does not.
• Descend 2,000–3,000 ft. Hypoxic symptoms lift quickly and dramatically - often within a minute or two. CO symptoms improve only slowly.
• Check the mixture and engine - a lean misfire is not on the list here because the engine is smooth.
• Ask yourself a hard question out loud: reciting the cruise checks or doing simple mental arithmetic exposes the deficit that euphoria is hiding.

Immediate actions, in order:

1. Cabin heat off, fresh air on. Removes the CO source if that is the cause and costs nothing.
2. Descend now to below 8,000 ft, ideally 6,000–7,000 ft. Hypoxia is the most likely diagnosis and the only reliable treatment is more oxygen partial pressure. Do not wait to see whether it improves - the symptoms already include impaired judgement, so the pilot is not a reliable judge of the trend.
3. Sit on the mixture and engine - lean correctly for the new altitude, watch RPM and EGT.
4. Consider a PAN. With a symptomatic pilot, night conditions and no oxygen, alerting ATC buys priority and someone to talk to if the situation deteriorates.

Continuation. Do not continue at 10,500 ft; the plan was to go higher for smooth air, which is now off the table. Descend, then reassess honestly: if symptoms clear completely below 8,000 ft within a few minutes, the most likely cause was hypoxia and a low-level continuation to a nearby aerodrome is defensible. If the headache persists with the heat off and the descent made no difference, treat it as CO, land at the nearest suitable aerodrome and seek medical advice - CO has a long half-life and mild symptoms can mask a significant carboxyhaemoglobin level.

Flight-specific fixes:

• Oxygen or altitude cap. With no oxygen on board, treat 10,000 ft as a hard ceiling at night. A written personal limit ('night, single pilot, no oxygen: 8,000 ft') is worth more than a vague intention to be careful.
• Heater discipline. Cabin heat in an older light aircraft is a potential CO source. A cheap CO detector in the cabin converts a diagnosis problem into an indication. Use heat in short bursts and prefer warm clothing.
• Scheduling. 13 hours awake, three legs, no food and a night leg is a fatigue profile, not a normal day. The third leg is where fatigue, hunger and mild hypoxia compound; the compounding is not additive but multiplicative.
• Fuel for the body. Eat and hydrate before the last leg, and treat food as part of flight planning, not a comfort item.

The human-factors lesson: the pilot felt slightly euphoric - that is the dangerous part. Impaired judgement is not recognised by the person whose judgement is impaired, and the presence of mild symptoms at 10,500 ft is evidence that they were already operating above their personal threshold. Disciplined pilots decide the limit on the ground, when they can still think straight, because in the air the first thing they lose is the ability to notice what they have lost.