Headache at 7,500 ft with the cabin heat on
It is a cold winter night and you are pilot-in-command of a piston single cruising at 7,500 ft, unpressurised, with no oxygen on board and none required at that level. The cabin heat has been full on for the last 40 minutes. The aircraft is 40 years old; the heat muff has done 60 hours since the last inspection and the exhaust system 400 hours since new. You have a dull throbbing headache, slight nausea and tingling in your fingertips, and you have put it down to fatigue and the sandwich you ate. Your passenger, who does not normally sleep in the aircraft, is drowsy and says the cabin 'feels fuzzy'. You open a fresh-air vent and the headache eases slightly. There is an aerodrome with a lit runway 15 minutes ahead; your destination is 50 minutes away; ATC is available on the current frequency.
1. What is the most probable cause, and how do you separate it from hypoxia or simple fatigue?
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Carbon monoxide from the cabin heat system is the working diagnosis until it is excluded.
The evidence points one way. Both occupants are affected at the same time - fatigue is personal, a shared environment is not. The symptom cluster (throbbing headache, nausea, tingling extremities, drowsiness, a 'fuzzy' cabin) is the textbook presentation. And the source is right there: a 40-year-old exhaust system feeding a heat muff that is only 60 hours from its last inspection. A cracked muff or a leaking exhaust joint delivers exhaust gas straight into the cabin air.
Why not hypoxia? At 7,500 ft a healthy non-smoker retains well over 90% saturation, no oxygen is required, and the passenger was fine on the ground. Hypoxia is a poor fit for the altitude and for the suddenness.
Why not fatigue? Fatigue does not give tingling fingers, and it does not make the passenger feel drunk.
Two traps. CO is colourless and odourless - the absence of an exhaust smell proves nothing, and exhaust leaks often present with no smell at all. And feeling better after opening a vent is not a diagnosis: CO bound to haemoglobin clears with a half-life of several hours on air, so your symptoms will lag well behind the cabin air quality. The passenger being affected is the tiebreaker; do not explain it away.
2. Set out your actions in order, from the moment you suspect CO.
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1. Stop the source. Cabin heat fully off. All fresh-air vents and windows open, even at 7,500 ft in winter - cold and noisy beats poisoned.
2. Tell ATC now, while you can still speak clearly. Declare urgency: PAN PAN, stating 'suspected carbon monoxide, both occupants affected, request priority to [aerodrome 15 NM ahead]'. If you judge your own capacity is degrading, do not be brave about the wording - MAYDAY is available and no one will criticise it afterwards.
3. Fly the shortest safe route to that aerodrome. Fifteen minutes ahead beats fifty minutes to a familiar destination. Do not attempt a diversion to somewhere you are not sure of; the nearest lit runway is the answer. Ask the passenger for what help they can genuinely give - radio calls, reading the checklist - but assume their judgement is as degraded as yours.
4. Reduce workload and stay visual. No oxygen on board, so your treatment is fresh air, a descent and a landing. Do not troubleshoot the heater in flight.
5. On the ground: shut down, get both of you out into fresh air, and get medical assessment - a hospital can measure carboxyhaemoglobin and provide hyperbaric oxygen if it is severe. Symptom relief in the fresh air is not discharge.
6. Do not fly the aircraft again until the exhaust and heat muff have been inspected and the defect has been certified as rectified. Record it.
3. What made this situation dangerous, and what would you change?
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The danger was never the headache. It was the explanation for the headache.
The onset was gradual and the symptoms were ordinary - tired, end of a long day, night flight, a bit of nausea. Every one of those has a benign cause, and the benign cause was available and flattering. The passenger falling asleep looked like a tired passenger rather than a second data point. Then there is plan continuation: the destination is 50 minutes away and familiar, the diversion is unknown, and the pilot's own capacity to weigh the alternatives is the very thing being degraded. CO does not announce itself; it removes the faculty you would use to notice it.
The 'eases with fresh air' observation is the last trap - it feels like confirmation that nothing serious is wrong, when it is the opposite.
What to change: fit a cheap chemical CO indicator in the cabin, check it before every flight that uses cabin heat; have the exhaust system and heat muff inspected at the recommended intervals and treat any exhaust defect as grounding; use cabin heat sparingly and suspect it whenever anyone in the cabin develops a headache; brief yourself on the standard symptoms before you need them; and make the go/no-go decision on the nearest aerodrome at the first symptom, not the second.