Headache at 9500 Feet: Heater, Hypoxia or Carbon Monoxide?
On a cold winter afternoon you are cruising a Cessna 172 at 9500 ft over the central North Island. The cabin heater has been on for about 20 minutes. You develop a dull headache, feel nauseated, and notice tingling in your fingertips. Your passenger, normally chatty, says she feels weird and sleepy. You feel mildly dizzy and your vision seems grey at the edges. The engine is running smoothly, the gauges are normal, and the outside air temperature is minus 5 degrees Celsius. There is no pressurisation problem and 9500 ft is below the altitude at which you would expect severe hypoxia. There is no carbon monoxide detector fitted; a small portable oxygen cylinder is on board. You wonder whether to continue for another 25 minutes to the planned destination. You have no history of motion sickness.
1. Explain why carbon monoxide is a credible cause here and why a pulse oximeter could be falsely reassuring.
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Carbon monoxide is credible because the cabin heater in a light aircraft typically draws air over the engine exhaust muff or a heat shroud. A crack or poor seal can admit exhaust gas into the cabin. Carbon monoxide is colourless and odourless, so the absence of a smell proves nothing. The symptoms are a good match: headache, nausea, dizziness, tingling and a passenger who becomes sleepy and withdrawn. The onset after about 20 minutes of heater use is also consistent with a slow accumulation in a small cabin.
A pulse oximeter can be falsely reassuring because it does not reliably distinguish carboxyhaemoglobin from oxyhaemoglobin. It may display a normal or near-normal saturation even when a significant proportion of haemoglobin is bound to carbon monoxide. A normal reading therefore cannot exclude carbon monoxide poisoning. At 9500 ft, mild hypoxia is possible, but it would not usually produce this cluster of symptoms in a healthy pilot and passenger while the engine and cabin environment appear normal. The safer assumption is that the heater is the source until proven otherwise.
2. State your immediate actions and the aeromedical follow-up you would expect after landing.
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Immediate actions should follow a simple priority order:
- Remove the source: turn the heater off and select fresh air. Open vents or a window if it does not cause unacceptable noise or discomfort.
- Protect the occupants: use 100 percent oxygen from the portable cylinder if available, and descend toward a lower altitude.
- Land as soon as practicable: do not continue to the planned destination. Choose the nearest suitable aerodrome, declare urgency if symptoms are significant, and minimise further workload.
- On the ground: shut down, get both occupants into fresh air, and seek medical assessment. Do not fly again until cleared.
The aeromedical follow-up matters because carbon monoxide can cause delayed neurological effects. A blood carboxyhaemoglobin measurement and medical review are appropriate, and the pilot should notify their aviation medical examiner if symptoms were significant. The aircraft should not fly again until an engineer has inspected the heater system, exhaust and heat shroud. A carbon monoxide detector should be fitted and treated as required equipment for future flights.
3. What prevention measures would you recommend for this aircraft and for your personal preflight routine?
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Aircraft measures include a functioning carbon monoxide detector in the cabin, a thorough inspection of the exhaust and heater shroud at the appropriate maintenance intervals, and a preflight check for exhaust stains, cracks or loose connections around the heater components. The detector should be checked before flight, not simply carried.
Personal routine measures include:
- Briefing passengers on the symptoms of carbon monoxide and asking them to report headache, nausea or drowsiness immediately.
- Treating unexplained symptoms in flight as a potential heater problem until proven otherwise, rather than attributing them to turbulence or fatigue.
- Using fresh air and a window when the heater is on, and avoiding prolonged heater use if the aircraft has a known history of exhaust or heater defects.
- Carrying oxygen where appropriate and knowing how to use it.
- Recording any suspected event and reporting it through the appropriate maintenance and safety channels.
The key reasoning is that the danger is insidious: the pilot may attribute early symptoms to cold, fatigue or mild hypoxia and delay action. A detector plus a simple rule, symptoms plus heater equals heater off, descend and land, converts a subtle hazard into a prompt decision.